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January 17— "Porphene: an Easily Tunable 2-D Conjugated Organic Polymer"

• Presenter: Josef Michl, Department of Chemistry, University of Colorado Boulder
• Host: Dan Dessau
• Abstract: Extending quantum effects beyond the nanoscale has been a long-standing goal of chemists and physicists. Graphene has provided one very successful avenue, and porphyrin nanostructures have been another. To date, large porphyrin nanostructures have only been realized as onedimensional tapes or rings. In Nat. Commun. 2023, 14, 6308, we described the synthesis of sheets of porphene 1, a 2-D material made of porphyrin macrocycles. If DFT predictions of metalloporphene properties are right, if our ability to reversibly insert metal ions into porphene can be perfected, if we can produce patterns lithographically or by ink printing, and if we can make and stack distinct single crystalline metalloporphene monolayer domains of sufficient size with sufficiently accurate registry, it will be possible to construct flexible monolithic electronic, photonic, and spintronic circuitry analogous to that currently available in silicon with an ultimate resolution of 0.84 nm.

January 24 — "What's new? Isn't fusion energy always 30 years away?"

• Presenter: Scott Parker, University of Colorado Boulder
• Host: Michael Litos
• Abstract: Fusion energy is a promising technology for producing clean, limitless, zero-carbon energy. Recently, there has been a paradigm shift where today, privately funded research dominates over the historic government-funded fusion program. Private research and development paths to fusion have very short timelines, and some future milestones appear speculative. I will discuss plasma and nuclear physics constraints that experiments will face as they progress toward the fusion goal. I will start with an overview of fusion energy and briefly discuss the recent demonstration of a net gain in inertial confinement fusion at the National Ignition Facility. My focus will be magnetic confinement, the space where the most promising devices exist and where the most private equity is invested.

January 31 — "Two tales about time in living (and not-so-living) transport networks"

• Presenter: Eleni Katifori, University of Pennsylvania
• Host: Andrew Lucas
• Abstract: We utilize transport systems daily to commute, e.g. via road networks, or bring energy to our houses through the power grid. Our body needs transport networks, such as the lymphatic, arterial or venous system, to distribute nutrients and remove waste. If the transported quantity is information, for example carried by an electrical signal, then even the internet and the brain can be thought of as members of this broad class of webs. Despite our daily exposure to transport networks, their function and physics can still surprise us. This is exemplified by the Braess paradox, where the addition of an extra road in a network worsens rather than improves traffic contrary to a naïve prediction.
  In this talk we will explore two cases that highlight the importance of time in load transmission in transport networks. In the first problem, we will discuss how short timescale dynamics in the flow alters the topology of the network in longer timescales, and shapes its morphology. We will first present the system of phenomenological adaptation equations that govern the structural evolution of vascular networks. We will then demonstrate how implicit of explicit dynamics in the boundary conditions (the power supply, or heart) can drastically alter the network topology, and discuss the implications for the development and function of human circulation. Moving to a larger system, will provide evidence that the similar dynamical developmental rules to the ones that are thought to control vascular remodeling in humans also shape tidal delta geomorphology.
  In the second problem, we consider stochastic transport in geometrically embedded graphs. Intuition suggests that providing a shortcut between a pair of nodes improves the time it takes to randomly explore the entire graph. Counterintuitively, we find a Braess' paradox analog. For regular diffusion, shortcuts can worsen the overall search efficiency of the network, although they bridge topologically distant nodes. We propose an optimization scheme under which each edge adapts its conductivity to minimize the graph's search time. The optimization reveals a relationship between the structure and diffusion exponent and a crossover from dense to sparse graphs as the exponent increases.

February 7 — "A Matter of Mystery"

• Presenter: Hirohisa Tanaka, SLAC
• Host: Eric Zimmerman
• Abstract: Neutrinos are enigmatic particles. Their properties are rather basic and yet so bizarre and surprising that at times we hardly believe them. We barely notice their presence, and yet they are everywhere and are essential to things as glaring as the sun’s energy production. The minuscule but non-zero mass of a neutrino, nearly a million times smaller than the electron (the next lightest particle), has enormous consequences for our understanding of these particles and their role in shaping the universe. It is possibly an indication of new processes and interactions that we don’t know about and may enable a matter/antimatter imbalance that allows the universe to exist. In this talk, I’ll briefly introduce this perplexing particle and discuss a quantum interference process called neutrino oscillations that allow us to probe its properties. I’ll discuss the challenges in observing and measuring this process, and conclude with where we stand in studying neutrino oscillations and our next steps. 

February 14 — "Exciting 1D gases"

• Presenter: David Weiss, Pennsylvania State University
• Host: Adam Kaufman
• Abstract: 1D gases with point contact interactions are special because they are integrable many-body systems, which means that they have many extra conserved quantities, beyond the usual few (energy, momentum, etc.). I will explain how we make bundles of 1D Bose gases in the lab, the various ways we excite them out of equilibrium, and how we use them as model systems for studying quantum dynamics.

February 21 — "Turbulent Origins of the Sun's Hot Corona and the Solar Wind"

• Presenter: Steven Cranmer, Department of Astrophysical & Planetary Sciences, University of Colorado Boulder
• Host: Dmitri Uzdensky
• Abstract: The solar corona is the hot and ionized outer atmosphere of the Sun.  It traces out the complex solar magnetic field and expands into interplanetary space as the supersonic solar wind.  In 1958, Eugene Parker theorized that the presence of a million-degree corona necessarily requires the outward acceleration of a wind.  However, despite many years of exploration of both phenomena, we still do not have a complete understanding of the processes that heat the coronal plasma to its bizarrely high temperatures.  In this talk, I will discuss some new observations and theoretical concepts that are helping us get closer to an answer to this infamous coronal heating problem.  We will begin by examining super-high-resolution images of the Sun's surface from the Daniel K. Inouye Solar Telescope (DKIST), zoom out to ultraviolet and X-ray images that illustrate how magnetic field lines thread their way through the corona, and then follow the Parker Solar Probe (PSP) spacecraft through its repeated dives into the innermost zones of the solar wind.  I will do my best to make sense of the puzzling physical processes that must be at work in producing and maintaining this massive plasma laboratory that sits about 150 million kilometers away.

February 28 — "Exploring many-body problems with arrays of individual atoms"

• Presenter: Antoine Browaeys, Institut d'Optique 
• Host: Adam Kaufman
• Abstract: Over the last twenty years, physicists have learned to manipulate individual quantum objects: atoms, ions, molecules, quantum circuits, electronic spins... It is now possible to build "atom by atom" a synthetic quantum matter. By controlling the interactions between atoms, one can study the properties of these elementary many-body systems: quantum magnetism, transport of excitations, superconductivity... and thus understand more deeply the N-body problem. More recently, it was realized that these quantum machines may find applications in the industry, such as finding the solution of combinatorial optimization problems. 

  This seminar will present an example of a synthetic quantum system, based on laser-cooled ensembles of individual atoms trapped in microscopic optical tweezer arrays. By exciting the atoms into Rydberg states, we make them interact, even at distances of more than ten micrometers. In this way, we study the magnetic properties of an ensemble of more than a hundred interacting ½ spins, in a regime in which simulations by usual numerical methods are already very challenging. Some aspects of this research led to the creation of a startup, Pasqal. 
   

March 6 — "Collapse and Ejection in the N-body problem and the Formation of Rubble Pile Asteroids"

• Presenter: Daniel Scheeres, Department of Aerospace Engineering
• Host: Michael Ritzwoller
• Abstract: Rubble pile asteroids are thought to form in the aftermath of cataclysmic collisions between proto-planets. The details of how the detritus from such collisions reaccumulate to form these bodies are not well understood, yet can play a fundamental role in the subsequent evolution of these bodies in the solar system. Simple items such as how particle sizes and porosity is distributed within a body can have a significant influence on how they subsequently evolve. Current space missions are just starting to gain limited insight into these fundamental questions, but require a better theoretical understanding to fully explain their observations.
  To that end, this work studies how the initial energy and angular momentum of a random collection of gravitating bodies is partitioned and redistributed between escaping components and bound multiple body systems. A generic initial distribution of N bodies will naturally lose many components due to multi-body dynamical interactions. If the bodies have finite density, some components will also form condensed distributions, becoming single, binary or multiple component rubble pile asteroids. We derive and apply rigorous results from the Full N-body problem to place limits and constraints on how the energy and angular momentum of such systems can evolve, which controls the formation of stable rubble pile asteroids.
  We are able to establish some of our constraints analytically, providing unique insight into this process. Ultimately, however, we require numerical simulations to elucidate certain aspects of the ejection process. As will be shown, these gravitational ejections will always reduce the system energy yet can cause significant fluctuations in the total angular momentum of the remaining bodies. Some possible implications of these trends will be discussed. 

March 13 — "Strategies for Achieving Rigidity, Resilience, and Robustness in Network-like Soft Materials: Insights from Biopolymer Networks and Circadian Colloids"

• Presenter: Moumita Das, Rochester Institute of Technology
• Host: Nuris Figueroa Morales
• Abstract: Living systems exhibit unique emergent properties such as self-assembly, rigidity, resilience, and robustness. In this talk, I will present results from ongoing projects that underscore the importance of understanding these collective properties in network-like soft materials and of addressing key questions in the rational design of biomimetic soft materials: Can we engineer composite soft matter to display life-like emergent properties? How can we enhance the tunability and control of such soft matter systems? And, is it feasible to activate synthetic soft materials using biological processes? I will begin by examining the potential physical mechanisms that underlie robust and resilient mechanical properties in biopolymer networks in cells and tissues. Utilizing rigidity percolation theory, we explore how the composite and heterogeneous composition influence cell and tissue mechanics, informing the creation of artificial constructs with tunable and robust mechanics. Following this, I will discuss the development of colloidal networks using functionalized clock proteins—proteins that regulate biological clocks—to engineer robust self-assembly kinetics and material properties in colloidal systems. Leveraging protein-based reaction networks allows us to endow synthetic systems with life-like properties. Our findings demonstrate how understanding the emergent structure-function properties in biological and bio-hybrid systems can support the development of biomimetic materials that not only mirror the robustness and adaptability of living systems but also offer enhanced control over their physical properties and functions.

March 20 — “Programmable quantum sensing using ultracold atoms in 3D optical lattices”

• Presenter: Murray Holland, JILA, University of Colorado, Boulder
• Host: James K. Thompson
• Abstract: The creation of a matter-wave interferometer can be achieved by loading Bose-Einstein condensed atoms into a crystal of light formed by interfering laser beams. By translating this optical lattice in a specific way, the traditional steps of interferometry can all be implemented, i.e., splitting, propagating, reflecting, and recombining the quantum wavefunction. Using this concept, we have designed and built a compact device to sense inertial signals, including accelerations, rotations, gravity, and gravity gradients. This approach is interesting, since the atoms can be supported against external forces and perturbations, and the system can be completely programmed on-the-fly for a new design goal. I will report on experimental results in which atoms are cooled into a dipole trap and subsequently loaded into an optical lattice. Protocols for obtaining interferometry steps are derived via machine learning and quantum optimal control methods. Implementing these in the lab, I will show our recent demonstrations of a vector accelerometer capable of sensitively deducing the magnitude and direction of an inertial force in a single shot. I will discuss our vision to use this platform for remote sensing of Earth as part of the recently founded NASA Quantum Pathways Institute.

March 27 — No Colloquium, Spring Break 

April 3 — "Waves of Topological Origin in the Fluid Earth System and Beyond"

• Presenter: Brad Marston, Brown University
• Host: Michael Ritzwoller
• Abstract: Symmetries and topology are central to our understanding of physical systems. Topology, for instance, explains the precise quantization of the Hall effect and the protection of surface states in topological insulators against scattering from disorder or bumps. However discrete symmetries and topology have not, until recently, contributed much to our understanding of the fluid dynamics of oceans and atmospheres. In this talk I show that, as a consequence of the rotation of the Earth that breaks time reversal symmetry, equatorial Kelvin and Yanai waves emerge as topologically protected edge modes. The non-trivial topology of the bulk Poincaré
  waves is revealed through their winding number in frequency - wavevector space. Bulk-interface correspondence then guarantees the existence of the two equatorial waves. I discuss our recent direct detection of the winding number in observations of Earth’s stratosphere. Thus the oceans and atmosphere of Earth naturally share basic physics with topological insulators. As equatorially trapped Kelvin waves in the Pacific ocean are an important component of El Niño Southern Oscillation, the largest climate oscillation on time scales of a few years, topology plays a surprising role in Earth’s climate system. We also predict that waves of topological origin will arise in magnetized plasmas. The waves may appear in laboratory plasma experiments, and they may also arise in the solar system and beyond.

April 10 — "Symmetry, topology and electronic phases of matter"

• Presenter: Charles Kane, University of Pennsylvania
• Host: Andrew Lucas
• Abstract: Symmetry and topology are two of the conceptual pillars that underlie our understanding of matter. While both ideas are old, over the past several years a new appreciation of their interplay has led to dramatic progress in our understanding of topological electronic materials. A paradigm that has emerged is that insulating electronic states with an energy gap fall into distinct topological classes. Interfaces between different topological phases exhibit gapless conducting states that are protected and are impossible to get rid of. In this talk we will discuss the application of this idea to the quantum Hall effect, topological insulators, topological semimetals and topological superconductors. The latter case has led to the quest for observing Majorana fermions in condensed matter, which opens the door to proposals for topological quantum computation. We will close by surveying the frontier of topological phases in the presence of strong interactions.

April 17 — "Architectures for scalable trapped ion quantum information processing"

• Presenter: Daniel Slichter, National Institutes of Standards and Technology
• Host: Adam Kaufman
• Abstract: Trapped atomic ions in ultra-high vacuum possess highly coherent controllable quantum degrees of freedom, making them useful as frequency standards, quantum-enhanced sensors, and building blocks of quantum computers and networks. Numerous efforts to build commercial quantum computers based on trapped ions are now underway worldwide. However, some technologies and architectures currently used in such systems may no longer be viable at the scale of thousands of trapped ion qubits. I will give some background on trapped ion quantum computing, and describe efforts in our research group to develop and demonstrate new scalable methods and architectures for high-performance control and readout of trapped ion qubits. A key element is the use of microfabricated planar ion trap chips where structures to enable qubit control and readout are fabricated into the trap chip itself.

April 24 — "Electron fractionalization in topological quantum materials"

• Presenter: Liang Fu, Massachusetts Institute of Technology
• Host: Andrew Lucas
• Abstract: The emergence of quasiparticles with fractional charge and fractional statistics is an essential feature of fractional quantum Hall states, which occur in two-dimensional electron gas under a strong magnetic field. An interesting question is whether fractional electron states can form spontaneously in quantum materials without the external magnetic field. I will describe theoretical ideas for realizing the fractional quantum Hall effect at zero magnetic field and highlight the recent observation of this remarkable phenomenon in moire materials with topological minibands. The prospect of new fractionalized phases without analogs in Landau levels will be also discussed.   

May 1 — "Effective field theories for phases of matter and cosmology"

• Presenter: Alberto Nicolis, Columbia University
• Host: Andrew Lucas
• Abstract: I will review some modern applications of effective field theories outside their traditional particle physics domain. In particular, I will focus on spontaneous symmetry breaking for spacetime symmetries. The effective theories for the associated Goldstone excitations capture the low-energy/long-distance dynamics of a number of physical systems, from ordinary macroscopic media (solids, fluids, superfluids, supersolids) to more exotic cosmological ones.

For more information about colloquia this semester, contact: Andrew Lucas.

August 30 — "Speed limits and locality in many-body quantum dynamics"

• Presenter: Andrew Lucas, University of Colorado, Boulder
• Host: Tobin Munsat
• Abstract: Information cannot travel faster than the speed of light.  Still, in many practical settings (such as listening to sound), emergent speed limits on information can be far slower than the limit set by relativity.  In 1972, Lieb and Robinson proved that quantum correlations and information propagate with a finite velocity in (non-relativistic) quantum many-body spin systems with nearest neighbor interactions.  In practical systems, these speed limits are orders of magnitude smaller than the speed of light.  Five decades later, the Lieb-Robinson Theorem has become one of the most important results in mathematical quantum physics. I will introduce and motivate the Lieb-Robinson Theorem, and overview our recent generalizations of the Lieb-Robinson Theorem to systems with power-law interactions, bosonic models, and dynamics with measurement. These bounds place tight constraints on how fast information can be shuttled through any future quantum computer.  I will then highlight a number of surprising applications of Lieb-Robinson bounds to many-body physics: the computational cost of simulating many-body dynamics on classical and quantum computers, revealing the properties of gapped phases, and proving the existence and metastability of the false vacuum. These results highlight how locality can play a central role in constraining what is possible in quantum systems.

**CANCELED** September 6 — "Benchmarking Quantinuum’s Second-Generation Quantum Processor"

• Presenter: Steven Moses, Quantinuum
• Host: Jun Ye
• Abstract: One of the main challenges facing large-scale quantum computing is scaling systems to more qubits while maintaining high fidelity operations. In this talk, I will describe our efforts at Quantinuum in scaling trapped-ion quantum computers based on the quantum charge-coupled device architecture. We recently released our second-generation machine, which has a race-track shaped ion trap. The new system incorporates several technologies crucial to future scalability, including electrode broadcasting, multi-layer RF routing, and magneto-optical trap loading, while maintaining, and in some cases exceeding, the gate fidelities of our first-generation system. We initially released the system with 32 qubits, but future upgrades will allow for more. I will describe the thorough set of benchmarking experiments we performed to characterize the system, as well as present a selection of recent results of quantum circuits that have been run on the system.

September 13 — "Geometrical approach for designing protected superconducting qubits"

• Presenter: András Gyenis, Department of Electrical, Computer and Energy Engineering, University of Colorado, Boulder
• Host: Andrew Lucas
• Abstract: Quantum-based electronics is an accelerating technology, where information is encoded in the quantum mechanical states of coupled superconducting circuits. To unlock the potential of quantum computers, one of the key challenges that the field has to overcome is to preserve the coherence of a quantum superposition over extended times. Besides implementing quantum error correction schemes, a complementary approach to prolong the coherence of quantum processors is to develop qubits that are intrinsically protected against decoherence. In this talk, we discuss the basic requirements of such protected superconducting qubits, and present a universal method to describe arbitrary quantum circuits based on the connection between symplectic geometry and graph theory. In the second part of the talk, we present preliminary results on how to use disordered superconductors to open the way to building superconducting qubits with protection against information loss. In particular, we focus on elements built from disordered WSi which has shown excellent properties in single photon detectors.

September 20 — "How old are Saturn’s rings?"

• Presenter: Sascha Kempf, University of Colorado, Boulder
• Host: Tobin Munsat
• Abstract: One of the most exciting and controversial aspects of Saturn’s magnificent rings is that they may actually be a recent phenomenon in the solar system – forming long after the Earth, Saturn, and its moon. This possibility has been vigorously debated for nearly 40 years, since the Voyager flybys of Saturn. Over the years, the most powerful support for this hypothesis has turned out to be the puzzle of the rings’ nearly pure water ice composition – unique in the family of planetary rings – in spite of the constant hail of rocky-carbon meteoroids from outside the Saturn system. However, three major uncertainties have left the young-ring hypothesis unproven. Two of these have already been resolved by the Cassini mission: the amount of non-icy material currently in the rings, and the total ring mass. The third main constraint is the mass flux of non-icy meteoroids falling onto the rings. 
  Measuring this mass flux was always a main science goal of the Cassini mission, and could only be achieved by the Cassini Cosmic Dust Analyzer instrument (CDA). In my talk I will report about the determination of the mass flux of non-icy material coming into the Saturn system, which completes the trifecta of constraints that are required to strongly support a youthful ring system. The measurements present a thorough and detailed analysis of the series of unconnected individual particle detections by CDA over Cassini’s entire 13 year mission, converting these detections into the desired mass flux. The CDA detections determine the incident particle orbits, and they come (surprisingly) not from comets as expected, but mostly from Kuiper Belt Objects. This means that most of the particles have low speeds relative to Saturn and are strongly focused gravitationally, such that the flux at the rings is even larger than previously estimated. The derived mass flux implies a ring exposure time of less than 100 to 400 million years, which is in support of recent ring formation scenarios.

September 27 — "The power of plasma: Extending the energy frontier and democratizing X-ray lasers"

• Presenter: Michael Litos, University of Colorado, Boulder
• Host: Tobin Munsat
• Abstract: Plasma-based particle accelerators have transitioned from dream to reality over the past four decades and have approached within striking distance of application readiness. Bunches comprising tens-of-millions of electrons are now regularly accelerated from rest to nearly 10 GeV of energy-per-particle in the distance of mere centimeters. With such accelerating gradients, it should be possible to extend the reach of particle colliders operating at the energy frontier, as well as reduce the size and cost of ultra-high-brightness X-ray laser sources, making them more accessible to researchers throughout the world. One of the next major challenges to the field is the preservation of beam quality before, during, and after the acceleration process to meet the physics-driven demands of the major target applications. In this talk, I will describe efforts being led by my group to address this and other challenges at SLAC National Accelerator Laboratory’s FACET-II plasma wakefield accelerator research facility.

October 4 — "A long quadrucal number--what chromosomes teach us about animal evolution: the legacy of George Gamow"

• Presenter: Daniel Rokhsar, University of California, Berkeley Department of Molecular and Cellular Biology 
• Host: Paul Beale
• NOTE SPECIAL LOCATION: Jennie Smoly Caruthers Biotech Building - Charlie Butcher Auditorium, Room A115
• Time: 4:00 p.m. (Tea and Cookies available beginning 3:30 p.m.)
• Abstract: Soon after the discovery of the double helix, George Gamow speculated about how what he called "the number of the beast" — the long information-rich DNA sequence present in chromosomes — might encode the properties of different forms of life. His playful formulation of this central problem in biology, and the "RNATIE Club" that he founded to explore this question, stimulated early thinking about the genetic code long before DNA sequencing was possible. Fast-forwarding seventy years, we can now compare the "long quadrucal numbers" from a wide diversity of species. This talk will explain how such comparisons illuminate key steps in the early evolution of animals that occured over five hundred million years ago. We show that, with a few notable exceptions, animal chromosomes are remarkably stable and evolve slowly over hundreds of millions of years, and that some gene linkages extend even further back in time. We then use these deeply conserved aspects of genome organization to (1) show that ctenophores rather than sponges are the earliest branching lineage of living animals, which has implications for the evolution of nervous systems, and (2) decipher the history of Paleozoic polyploidy and promiscuity in our vertebrate lineage.

October 11 — "Two tales about time in living (and not-so-living) transport networks"

• Presenter: Eleni Katifori, University of Pennsylvania
• Host: Andrew Lucas
• Abstract: We utilize transport systems daily to commute, e.g. via road networks, or bring energy to our houses through the power grid. Our body needs transport networks, such as the lymphatic, arterial or venous system, to distribute nutrients and remove waste. If the transported quantity is information, for example carried by an electrical signal, then even the internet and the brain can be thought of as members of this broad class of webs. Despite our daily exposure to transport networks, their function and physics can still surprise us. This is exemplified by the Braess paradox, where the addition of an extra road in a network worsens rather than improves traffic contrary to a naïve prediction.

  In this talk we will explore two cases that highlight the importance of time in load transmission in transport networks. In the first problem, we will discuss how short timescale dynamics in the flow alters the topology of the network in longer timescales, and shapes its morphology. We will first present the system of phenomenological adaptation equations that govern the structural evolution of vascular networks. We will then demonstrate how implicit of explicit dynamics in the boundary conditions (the power supply, or heart) can drastically alter the network topology, and discuss the implications for the development and function of human circulation. Moving to a larger system, will provide evidence that the similar dynamical developmental rules to the ones that are thought to control vascular remodeling in humans also shape tidal delta geomorphology. 

October 18 — "Testing Quantumness in the NISQ Era"

• Presenter: Umesh Vazirani, University of California, Berkeley, Dept. of Computer Science
• Host: Rahul Nandkishore
• Abstract: We are well into the NISQ era of Noisy Intermediate Scale Quantum Computers. Four years on from Google's ‘quantum supremacy’ experiment, we have a deeper understanding of the nature of that experiment, the computing power of NISQ and novel techniques for benchmarking such computers and characterizing their error models.
  At a foundational level, quantum supremacy provides a test of quantum mechanics in a new regime. I will also describe how concepts from cryptography have provided novel and counter-intuitive ways of probing quantum systems, and the prospects they hold for the next generation of quantum computers taking on the quantum supremacy challenge.
  Bio: Umesh Vazirani is Strauch Distinguished Professor of Computer Science at UC Berkeley, Director of the Berkeley Quantum Computing Center (BQIC) and Research Director for Quantum Computing at the Simons Institute. His research has focussed on the foundations of Quantum Complexity Theory, Quantum Algorithms, Quantum Hamiltonian Complexity and Interactive classical testing of quantum devices. Vazirani is co-winner of the Fulkerson Prize for the ARV graph partitioning algorithm, a member of the National Academy of Sciences, and co-author of two books: An Introduction to Computational Learning Theory, and Algorithms.

October 25 — "Quantum Simulation - from Dipolar Quantum Solids to Kinetic Nagaoka Ferromagnetism"

• Presenter: Markus Greiner, Harvard University
• Host: Adam Kaufman
• Abstract: Quantum simulations with ultracold atoms in optical lattices enter the next phase, in which we can extend bosonic and fermionic Hubbard models in a wide range of ways. I will present recent results in which we realized strongly correlated dipolar quantum gases and observed quantum-phase transitions to stripe and checkerboard phases. I will then discuss how we observe Nagaoka Ferromagnetism, a surprising realization of kinetic magnetism, emerging as we introduce geometric spin frustration in a triangular Fermi-Hubbard lattice. 

November 1 — "Observation of Pines' Demon in Sr₂RuO₄ with Momentum-Resolved EELS*"

• Presenter: Peter Abbamonte, University of Illinois, Urbana-Champaign 
• Host: Dan Dessau
• Abstract: The characteristic excitation of a metal is its plasmon, which is a quantized sound wave in its valence electron density. In 1965, David Pines predicted that a distinct type of plasmon, which he named a "demon," could exist in multiband metals that contain more than one species of charge carrier. Consisting of electrons in two different bands beating out-of-phase, demons are acoustic excitations, meaning they are “massless,” meaning their energy tends toward zero as the momentum q  0. Demons may therefore play a central role in the low-energy physics of multiband metals. However, demons are neutral excitations that do not couple to light, so they have never been observed experimentally, at least in an equilibrium, 3D material. 
  In this talk I will present the observation of a demon in the multiband metal Sr2RuO4. Formed of electrons in the β and γ bands, the demon is gapless with critical momentum qc = 0.08 reciprocal lattice units and room temperature velocity v = 1.065(120)×105 m/s. This study confirms a 67-year old prediction and suggests that demons may be a widespread feature of multiband metals.
  *A. A. Husain, et al., Nature 621, 66 (2023)

November 8 — "The Physics of Changing Polar Climates"

• Presenter: Paul Kushner, University of Toronto
• Host: Michael Ritzwoller
• Abstract: The Arctic and Antarctic regions of Earth’s climate system have historically received less scientific attention than, and remain poorly observed and understood relative to, other regions of the world. Being critically sensitive to anthropogenic climate change, the polar regions are hotspots of rapid surface warming, cryospheric melt, and profound oceanic change. In this colloquium I’ll talk about the causes and consequences of this polar amplification (predominantly Arctic amplification) of global warming. I’ll discuss how coupled interactions between the atmosphere, ocean, and floating sea ice drive this amplification, and how Arctic change affects the atmosphere’s stratification, storminess, and overturning circulation, with global implications. Our work uses earth system models (developed right here in Boulder CO!) that seek to realistically capture the processes involved. We’ve recently learned that the way we use earth-system models to probe Arctic amplification could be misleading. I’ll discuss attempts to address this shortcoming and prospects for improved experiments. Finally, I’ll highlight plans for a 2025 program at the Kavli Institute for Theoretical Physics where we’ll study polar amplification and other urgent issues around changing polar climates.

November 15 — "Advancing our understanding of the solar corona by engaging over 1400 students in authentic research"

• Presenter: Heather Lewandowski, JILA, University of Colorado, Boulder
• Host: Noah Finkelstein
• Abstract: Participation in undergraduate research experiences (UREs) has been identified as an important way of increasing undergraduate retention, interest, and identity within the sciences. Course-based undergraduate research experiences (CUREs) have been shown to have similar outcomes to UREs, but can reach a larger number of students at one time and are accessible to any student simply through enrollment in a course. One key component of a CURE is that students must participate in authentic scientific discovery in which they answer a question where the answer is initially unknown to both students and the scientific community. Here, we present student experiences with authentic research in the first large-enrollment, introductory physics CURE conducted remotely during the COVID-19 pandemic. The outcomes of this CURE include a new result suggesting the mechanism for why the sun’s corona is so much hotter than the surface of the sun, which is a longstanding puzzle in solar physics. Additionally, we find that the course helped students gain research skills and programming confidence, engage in productive and enjoyable teamwork experiences, and feel motivated and interested in experimental physics research.

November 22 — No Colloquium, Fall Break 

November 29 — "How much of a meritocracy? Untangling the drivers of productivity and prominence among scientists"

• Presenter: Aaron Clauset, University of Colorado, Boulder Department of Computer Science
• Host: Andrew Lucas
• Abstract: Simple measures of the productivity and prominence vary enormously across both individual scientists and across institutions. But, how much do these sometimes enormous inequalities represent genuine meritocratic differences, and how much are they biased by non-meritocratic factors that may limit scientific progress?

  This talk will untangle some of the underlying drivers of differences in scientific productivity and prominence. First, we'll ask, how much of an individual's productivity (how many papers they publish) can be attributed to inherent skill vs. environmental advantages. Second, we'll ask, what drives where a scientist works? Then, we'll ask, what role do social networks and social capital play in shaping an individual scientist's productivity and prominence within their field? In this last section, we'll use two network-based generative models that can untangle these two measures from a scientist's collaboration network, and allow us to evaluate how gendered differences in social networks relate to gendered differences in productivity and prominence, and the degree to which social networks might be inheritable from advisors to advisees. I'll close with a discussion of how knowing the systemic and structural sources of bias in science can help us reconnect with our meritocratic principles, to both diversify the academy and broaden its contributions to society.

  Aaron Clauset is a Professor in the Department of Computer Science and the BioFrontiers Institute at the University of Colorado Boulder, and is External Faculty at the Santa Fe Institute. He received a PhD in Computer Science, with distinction, from the University of New Mexico, a BS in Physics, with honors, from Haverford College, and was an Omidyar Fellow at the prestigious Santa Fe Institute. He was awarded the Erdos-Renyi Prize in Network Science in 2016, and was named a Fellow of the Network Science Society in 2023. Since 2017, he has been a Deputy Editor responsible for the Social, Computing, and Interdisciplinary Sciences at Science Advances.

  Clauset is an internationally recognized expert on network science, data science, and machine learning for complex systems. His research program is around two general themes: identifying fundamental principles of the organization and behavior of complex social and biological systems, and developing approaches for using data and computation to illuminate those ideas. A recent major focus of this work has been on the "science of science," where he studies the shape, origins, and consequences of social and epistemic inequalities on scientific careers, productivity, the spread of ideas, and the composition of the scientific workforce. His research results have appeared in many prestigious scientific venues, including Nature, Science, PNAS, SIAM Review, Science Advances, Nature Communications, and Physical Review Letters. His work has been covered in the popular press by Quanta Magazine, the Wall Street Journal, The Economist, Discover Magazine, Wired, the Boston Globe and The Guardian.

December 6 — "Relativistic Fluid Dynamics: From Particle Colliders to Neutron Star Mergers"

• Presenter: Jorge Noronha, University of Illinois, Urbana-Champaign
• Host: Andrew Lucas
• Abstract: Heavy-ion collision experiments have provided overwhelming evidence that quarks and gluons, the elementary particles within protons and neutrons, can flow as a nearly frictionless, strongly interacting relativistic liquid over distance scales not much larger than the size of a proton. On the other hand, with the dawn of the multi-messenger astronomy era marked by the detection of a binary neutron star merger, it became imperative to understand how extremely dense fluids behave under very strong gravitational fields. Therefore, cutting-edge experimental apparatus in modern science, such as the Relativistic Heavy Ion Collider (RHIC), the Large Hadron Collider (LHC), and the Laser Interferometer Gravitational-Wave Observatory (LIGO), are now taking data whose description requires pushing the boundaries of our current understanding of fluid dynamics. In this talk, I will discuss the new developments that have contributed to redefining the onset of relativistic fluid dynamics and its extension towards the far-from-equilibrium regime. New results involving viscous fluids and their coupling to general relativity will also be presented. These developments pave the way for the inclusion of viscous effects in neutron star merger simulations.

December 13 — "Benchmarking Quantinuum’s Second-Generation Quantum Processor"

• Presenter: Steven Moses, Quantinuum
• Host: Jun Ye
• Abstract: One of the main challenges facing large-scale quantum computing is scaling systems to more qubits while maintaining high fidelity operations. In this talk, I will describe our efforts at Quantinuum in scaling trapped-ion quantum computers based on the quantum charge-coupled device architecture. We recently released our second-generation machine, which has a race-track shaped ion trap. The new system incorporates several technologies crucial to future scalability, including electrode broadcasting, multi-layer RF routing, and magneto-optical trap loading, while maintaining, and in some cases exceeding, the gate fidelities of our first-generation system. We initially released the system with 32 qubits, but future upgrades will allow for more. I will describe the thorough set of benchmarking experiments we performed to characterize the system, as well as present a selection of recent results of quantum circuits that have been run on the system.

For more information about colloquia this semester, contact: Andrew Lucas.

To view recordings of each regular colloquium lecture, please go to the .

To view recordings of each special colloquium lecture, please go to the .

**Canceled** January 18 — "The Fascinating History of Numbers in Ancient Mesopotamia"

• Presenter: Jeanne Nijhowne, University of Colorado, Boulder
• Host: Paul Beale
• Abstract: For tens of thousands of years, humans got along perfectly well without numbers as we understand them. Then around 5000 years ago for reasons that remain unclear, people in the ancient Near East began living in cities with monumental architecture, civil and religious bureaucracies, centralized food distribution systems, and long-distance trading networks. It was simply not possible to run such complex societies without some kind of accounting system or knowledge of mathematical principles. This lecture concentrates on the long, and dare I say “tortured” history of Mesopotamian counting systems. The developments in numbers and mathematics will be discussed in their cultural context in an effort to explain why changes happened as they did and their legacy still influencing us today.
  Jeanne Nijhowne holds a Ph.D. in Anthropology with a specialty in the archaeology of the ancient Near East. She has worked on archaeological excavations in Egypt, Iraq, Turkey, Jordan and Syria ranging in time from the 8th millennium B.C. to the 4th century A.D. She currently works as the Graduate Program Assistant in the Department of Physics and teaches a summer class on Mesopotamian archaeology for the Anthropology Department at CU.

SPECIAL Physics Colloquium — Tuesday, January 24, 2:00 p.m.

• Title: "From Quantum Information to the Black Hole Interior"
• Presenter: Ying Zhao, Kavli Institute for Theoretical Physics, University of California Santa Barbara
• Host: Oliver DeWolfe
• Abstract: Black holes have not just become laboratories for astronomers, but also present some of the deepest unsolved problems in theoretical physics. During the past decade surprising connections have been made between ideas from quantum information and gravitational physics. In this talk I will explain several aspects of these connections. In particular, we will see that by treating a black hole as a quantum computer, we can understand various properties of the black hole interior. I will also show you how the black hole information puzzle can be understood from this point of view. 

January 25 — "The Absurd and Useful Physics of Atomtronics"

• Presenter: Dana Anderson, University of Colorado, Boulder • JILA
• Host: Leo Radzihovsky
• Abstract: Atomtronics, an ultracold atom analog of electronics, provides a practical pathway for utilizing the quantum character of ultracold atoms in a variety of compelling applications involving sensing and signal processing. The atomtronic transistor, for example, is an analog of the electronic transistor that is exceedingly useful for creating amplifiers, oscillators, switches and so on. The understanding of atomtronic circuitry takes us to fundamental depths of field theory applied to a very practical domain. Along with many familiar parallels to Maxwell’s equations and electromagnetics, the physics of atomtronics is also surprising, sometimes seemingly absurd. I will take some time to draw the connection between fundamental physics starting with CU’s Nobel-recognized accomplishments in Bose-Einstein Condensation and the impact on and by the very practical technology developed by CU spinoff ColdQuanta (now Infleqtion), recently honored to have one of Time’s 2022 Invention of the year award for its quantum matter machine “Albert” that is used for developing atomtronic circuits.

SPECIAL Physics Colloquium — Thursday, January 26, 2:00 p.m.

• Title: Quantum Simulation for High-Energy Physics
• Presenter: Federica Maria Surace, California Institute of Technology
• Host: Oliver DeWolfe
• Abstract: Simulating strongly interacting quantum systems is a difficult task: as a prominent example, solving the fundamental equations of the quantum chromodynamics theory of quarks and gluons requires non-perturbative approaches based on computationally-expensive Monte Carlo simulations. The need for alternative solutions that do not rely on Monte Carlo methods has recently motivated an increasing interest in the possible applications of quantum simulation and computation. In this talk, I will review the recent advances in the field of quantum simulation for high-energy physics, with a special focus on analog simulators based on ultra-cold atoms. These controllable quantum systems can be used to simulate a variety of phenomena, including confinement, particle collisions, and false vacuum decay.

SPECIAL Physics Colloquium — Tuesday, January 31, 2:00 p.m.

• Title: Quantum Computational Advantage: Recent Progress and Next Steps
• Presenter: Xun Gao, Harvard University
• Host: Oliver DeWolfe
• Abstract: This talk is motivated by the question: why do we put so much effort and investment into quantum computing? A short answer is that we expect quantum advantages for practical problems. To achieve this goal, it is essential to reexamine existing experiments and propose new protocols for future quantum advantage experiments.
  In 2019, Google published a paper in Nature claiming to have achieved quantum computational advantage, also known as quantum supremacy. In this talk, I will explain how they arrived at their claim and its implications. I will also discuss recent theoretical and numerical developments that challenge this claim and reveal fundamental limitations in their approach.
  Due to these new developments, it is imperative to design the next generation of experiments. I will briefly mention three potential approaches: efficiently verifiable quantum advantage, hardware-efficient fault-tolerance, and quantum algorithms on analog devices for practical problems, including machine learning and combinatorial optimization.

February 1 — "Arrays of Individually-Controlled Molecules for Quantum Science"

• Presenter: Kang-Kuen Ni, Department of Chemistry, Harvard University
• Host: Jun Ye, Ana Maria Rey
• Abstract: Advances in quantum manipulation of molecules bring unique opportunities: the use of molecules to search for new physics; exploring chemical reactions in the ultra-low temperature regime; and harnessing molecular resources for quantum simulation and computation. I will introduce our approaches to building individual ultracold molecules in optical tweezer arrays with full quantum state control. This work expands the usual paradigm of chemical reactions that proceed via stochastic encounters between reactants, to a single controlled reaction of exactly two atoms. The new technique allows us to isolate two molecular rotational states as two-level systems for qubits.  In order to preserve coherence of the qubits, we develop magic-ellipticity polarization trapping. Finally, we are taking advantage of the resonant dipolar interaction of molecules to entangle them with single site addressability. In combination, these ingredients will allow the molecular quantum system to be fully programmable. 

SPECIAL Physics Colloquium — Tuesday, February 7, 2:00 p.m.

• Title: Error Correction in Quantum Computers and Beyond
• Presenter: Aleksander Kubica, Amazon Web Services Center for Quantum Computing and California Institute of Technology
• Host: Oliver DeWolfe
• Abstract: Quantum computers introduce a radically new paradigm of information processing and revolutionize our thinking about the world. However, designing and building quantum computers that operate properly even when some of their components malfunction and cause errors is a heroic endeavor. In this talk, I will explain how quantum error correction and the theory of fault tolerance are indispensable to achieve this task. I will also discuss the profound impact of quantum error correction on our understanding of nature, ranging from new insights into quantum many-body physics to fundamental limitations on sensing and computation.

February 8 — "Waves Affect and Detect Climate"

• Presenter: Baylor Fox-Kemper, Brown University
• Host: Michael Ritzwoller
• Abstract: Ocean surface gravity waves, the ones familiar to surfers and beachgoers, are mostly generated by winds.  Because they are the most direct way that winds affect the ocean, they both affect the climate system and are used in the detection of climate change.  It is difficult to measure the winds everywhere on earth, but detection of ocean waves from satellites provides a near-global view into their continual change.  Over long distances, changes in the waves accumulate the history of the winds that blow over them, so changes in the waves are both an impact (through coastal inundation) and detector of climate change.  Waves also link closely to ocean turbulence, ocean currents, and sea ice breakup.  Nobelist Irving Langmuir first described a key consequence of ocean turbulence driven by waves--windrows where flotsam such as pollution (oil and plastics), seafoam, and seaweed accumulates.  Accounting for this wave-driven turbulence in climate models has improved our ability to project future global warming, and future improvements in modeling waves' effects on sea ice and currents lay ahead.

SPECIAL Physics Colloquium — Thursday, February 9, 2:00 p.m.

• Title: The Black Hole Information Paradox and Quantum Codes
• Presenter: Christopher Akers, Massachusetts Institute of Technology
• Host: Oliver DeWolfe
• Abstract: In 1974, Stephen Hawking argued that black holes destroy information, counter to a fundamental principle in quantum mechanics, calling into question how general relativity and quantum mechanics could ever be unified. Ever since then physicists have wrestled with this question. I will explain the history of this debate, with a focus on modern developments that use ideas from the theory of quantum information and computation to finally offer a sharp resolution.

SPECIAL Physics Colloquium — Tuesday, February 14, 2:00 p.m.

• Title: Tailoring Quantum Error Correction for Structured Noise
• Presenter: Jahan Claes, Yale University
• Host: Oliver DeWolfe
• Abstract: Large-scale quantum computers will require error correction in order to reliably perform computations. However, the hardware overhead for error correction remains dauntingly large, with each logical qubit potentially requiring thousands of physical qubits for reliable operation. One promising approach to reducing the overheads of error correction is to tailor quantum error correcting codes to the dominant noise in the qubit hardware.
  In this talk, I’ll present recent work on tailoring measurement-based quantum computing to biased noise. In the first part of the talk, I’ll explain the basics of measurement-based error correction and cluster states, and show how a cluster state derived from the XZZX surface code, the so-called XZZX cluster state, can effectively correct biased noise. In the second part of my talk, I’ll present two applications of the XZZX cluster state for increasing thresholds in both linear optical qubits and Rydberg atoms. These works demonstrate that carefully considering the dominant hardware noise when designing error correction protocols can drastically increase their effectiveness.

February 15 — "Ab initio approaches to driven quantum matter and quantum information science"

• Presenter: Prineha Narang, University of California, Los Angeles
• Host: Leo Radzihovsky
• Abstract: In this talk, I will present theoretical and computational approaches to describe excited-states in quantum matter, and predicting emergent states created by external drives. Understanding the role of such light-matter interactions in the regime of correlated electronic systems is of paramount importance to fields of study across condensed matter physics, quantum optics, and ultrafast dynamics. The simultaneous contribution of processes that occur on many time and length-scales have remained elusive for state-of-the-art calculations and model Hamiltonian approaches alike, necessitating the development of new methods in computational physics. I will discuss our work at the intersection of ab initio cavity quantum-electrodynamics and electronic structure methods to treat electrons, photons and phonons on the same quantized footing, accessing new observables in strong light-matter coupling. Current approximations in the field almost exclusively focus on electronic excitations, neglecting electron-photon effects, for example, thereby limiting the applicability of conventional methods in the study of polaritonic systems, which requires understanding the coupled dynamics of electronic spins, nuclei, phonons and photons. With our approach we can access correlated electron-photon and photon-phonon dynamics, essential to our latest work on driving quantum materials far out-of-equilibrium to control the coupled electronic and vibrational degrees-of-freedom. In the second part of my talk, I will demonstrate how the same approach can be generalized in the context of control of molecular quantum matter and quantum transduction with implications for scalable quantum architectures.
  Dr. Prineha Narang is a Professor in Physical Sciences at UCLA. Her group works on theoretical and computational quantum materials, non-equilibrium dynamics, and transport in quantum matter. Outside of science, she is an avid triathlete, runner, and starting her mountaineering journey.

SPECIAL Physics Colloquium — Thursday, February 16, 2:00 p.m.

• Title: Long-range Entanglement and Topological States in Quantum Devices: from Hamilton to Galois
• Presenter: Ruben Verresen, Harvard University
• Host: Oliver DeWolfe
• Abstract: One of the most remarkable discoveries in quantum physics is that long-range entangled qubits can give rise to emergent gauge fields and collective excitations exhibiting generalized ('anyonic') exchange statistics. Despite the importance of such 'topological' states for quantum information processing, they are extremely challenging to find in materials. In this talk, we explore how novel 'bottom-up' quantum devices---built atom by atom, qubit by qubit---challenge this status quo. For instance, topological states can emerge as the low-energy description of a many-body Hamiltonian, with experimental data on Rydberg atom tweezer arrays. Alternatively, such long-range entanglement can be obtained from shallow circuits with measurements and feedforward, with experimental data on cold ions. Only the latter route is able to avoid fundamental constraints imposed by locality and unitarity, leading to a surprising connection to the unsolvability of the quintic.

February 22 — "How Much Information Is In A Quantum State?"

• Presenter: Scott Aaronson, Computer Science, University of Texas Austin
• Host: Leo Radzihovsky
• Abstract: To describe an entangled state of n particles, we formally need an amount of classical information that grows exponentially with n.  But given that almost all the information disappears on measurement, in what sense was it "really there"?  In this talk, I'll first survey a career's-worth results showing that, for many purposes, quantum states can effectively be described with vastly less classical information. In the other direction, however, I'll discuss the exponential separation between quantum and randomized communication complexities due to Bar-Yossef, Jayram, and Kerenidis, as well as brand-new work by me, Buhrman, and Kretschmer, which builds on their separation to show that "FBQP/qpoly != FBQP/poly" (unconditionally, there exist relational problems solvable using quantum advice but not classical advice).  I'll end by explaining how this theorem directly suggests a new type of quantum supremacy experiment -- one that, in contrast to the recent supremacy experiments by Google, USTC, and Xanadu, would not depend on any unproved computational hardness assumptions, but would seek a direct "experimental witness for the vastness of n-particle Hilbert space."  I believe this type of experiment is just now becoming technologically feasible.

March 1— "Stacking van der Waals atomic layers: quest for new quantum materials"

• Presenter: Philip Kim, Harvard University
• Host: Leo Radzihovsky
• Abstract: Modern electronics rely heavily on technology that confines electrons in the interface layers of semiconductors. In recent years, scientists have discovered that they can isolate various atomically thin van der Waals (vdW) layered materials. In these atomically thin materials, quantum physics allows electrons to move only in an effective 2-dimensional (2D) space. By stacking these 2D quantum materials, one can also create atomically thin vdW heterostructures with a wide variety of electronic and optical properties. In this talk, we will discuss several research efforts to realize emergent physical phenomena in stacked vdW interfaces between 2D materials.

March 8 — "Lise Meitner, Her Escape from Germany and the Discovery of Fission"

• Presenter: Anthea Coster, Massachusetts Institute of Technology
• Hosts: Allan Franklin and Leo Radzihovsky
• Abstract: Lise Meitner was one of the pioneers of nuclear physics and co-discoverer, with Otto Hahn and Fritz Strassmann, of nuclear fission. Albert Einstein once called her, "the most significant woman scientist of the 20th century". Yet by the 1970's, her name was nearly forgotten. With the publication of the book by Ruth Lewin Sime, "Lise Meitner, A life in physics," to some extent her name has resurfaced. The chronology of the discovery of fission is considerably more complex than the facts, and clouded by events beyond the world of science. The facts are that on January 6, 1939, Hahn and Strassmann reported in Naturwissenschaften their chemical findings for fission. On February 11, 1939, Meitner and Frisch published in Nature the physical interpretation of the process they named fission. In 1944, Otto Hahn alone received the Nobel Prize in Chemistry "for his discovery of the fission of heavy nuclei."

  I became familiar with Lise Meitner and her story when, in 1972, Dr. Sime started writing my father for details about Lise Meitner's escape from Germany. This is because in July 1938, my grandfather, Dirk Coster, was the person who escorted her out of Germany. In Sime's book, Meitner’s escape from Germany reads like a spy novel, except that it is completely based in fact. At age 59, Meitner left Germany forever with 10 marks in her purse, one small suitcase, and a diamond ring given to her by Otto Hahn that he had inherited from his mother.

  This talk will be a combination of facts, excerpts from the film, "Path to Nuclear Fission: The Story of Lise Meitner and Otto Hahn" (a film by Rosemarie Reed), and personal stories heard from my father, aunts, and uncles. Lise Meitner’s early years, her role in the discovery of nuclear fission, her escape from Germany, and the consequences that followed will be covered.

March 15 — "Epidemics, Erdos numbers, and the Internet: The Physics of Networks"

• Presenter: Mark Newman, University of Michigan
• Host: Leo Radzihovsky
• Abstract: There are networks in every part of our lives: the Internet, the road network, networks of friendship or acquaintance, biochemical networks, ecological networks, the power grid, and many others. As large-scale data on these networks has become available in the last few years, a new science of networks has grown up combining observations and theory to shed light on systems ranging from bacteria to the whole of human society.  This talk will give an introduction to this rapidly-growing field, and explain some its best known results and how physics and physical methods can contribute to their understanding.

March 22 — "Programmable control of indistinguishable particles: from sampling to clocks to qubits"

• Presenter: Adam Kaufman, JILA, University of Colorado, Boulder
• Host: Leo Radzihovsky
• Abstract: Quantum information science seeks to exploit the collective behavior of a large quantum system to enable tasks that are impossible (or less possible!) with classical resources alone. While this burgeoning field encompasses a variety of goals, ranging from metrology to computing, most of them rely on the preparation and control of many identical particles or qubits. Meeting this need is a defining challenge of the field. There are several promising platforms that are targeting these capabilities, and I will focus on one such platform — optically-trapped neutral atoms. We have been developing a new suite of tools, based on the use of more exotic atomic species, new trapping architectures, and new control methods. I will provide an overview of these developments and a few specific examples of our recent results, including the use of atoms as indistinguishable bosons for sampling problems, a new kind of atomic clock, and a different kind of qubit.

March 29 — No Colloquium, Spring Break 

April 5 — "Fast jet stream winds get faster under climate change"

• Presenter: Tiffany Shaw, University of Chicago
• Host: Michael Ritzwoller
• Abstract: Earth's upper-level jet streams influence the speed and direction of travel of weather systems and commercial aircraft. The fastest jet stream winds are also linked to severe weather occurrence. Climate change is known to accelerate the average speed of the jet stream. However, little is known about how the fastest jet stream winds will change. Here we show fast jet stream winds get faster under climate change using daily data from climate model projections across a hierarchy of physical complexity. The fastest winds also increase substantially more than the average winds. We give a physical basis for why fast winds get faster and show it follows from the moist get moister response under climate change. The results show moist thermodynamics can explain projected future changes in commercial flight times and clear-air turbulence, including a potential increase in severe weather occurrence under climate change.

April 12 — "From quantum optics to bits and pieces"

• Presenter: Klaus Mølmer, Niels Bohr Institute, University of Copenhagen
• Host: Jun Ye and Ana Maria Rey
• Abstract: In this talk, I shall review the arguments applied by Roy Glauber in the early 1960’s to characterize temporal fluctuations in photo-detection signals.  Such fluctuations can be signatures of non-classical properties, and the theory of photo-detection gave rise to the field of quantum optics with visions to control atomic light emitters to prepare and apply a variety of quantum states of light in experiments. In the past decades, bits and pieces of solid-state materials were manufactured with high purity and precision, enabling observation of similar phenomena with superconducting circuits, microwaves and acoustic waves as had been studied with single atoms and photons in quantum optics. 

  As a theorist I have worked with methods that refine and elaborate on Glauber’s theories to describe the dynamics of open quantum systems, i.e., systems subject to interactions with their environment. I will show how these methods reintroduce, but with a plot twist, Niels Bohr’s quantum jumps in modern quantum physics. They provide the ultimate data processing tools for precision metrology, they offer unique insights into the dynamics of quantum systems, and they imply delightful encounters with the famous discussions between Niels Bohr and Albert Einstein on the interpretation of quantum theory.

April 19 — "The Fascinating History of Numbers in Ancient Mesopotamia"

• Presenter: Jeanne Nijhowne, University of Colorado, Boulder
• Host: Paul Beale
• Abstract: For tens of thousands of years, humans got along perfectly well without numbers as we understand them. Then around 5000 years ago for reasons that remain unclear, people in the ancient Near East began living in cities with monumental architecture, civil and religious bureaucracies, centralized food distribution systems, and long-distance trading networks. It was simply not possible to run such complex societies without some kind of accounting system or knowledge of mathematical principles. This lecture concentrates on the long, and dare I say “tortured” history of Mesopotamian counting systems. The developments in numbers and mathematics will be discussed in their cultural context in an effort to explain why changes happened as they did and their legacy still influencing us today.
  Jeanne Nijhowne holds a Ph.D. in Anthropology with a specialty in the archaeology of the ancient Near East. She has worked on archaeological excavations in Egypt, Iraq, Turkey, Jordan and Syria ranging in time from the 8th millennium B.C. to the 4th century A.D. She currently works as the Graduate Program Assistant in the Department of Physics and teaches a summer class on Mesopotamian archaeology for the Anthropology Department at CU.

April 26 — "A diatomic molecule in a superfluid helium droplet: Is the anomalous Zeeman splitting a signature of a microscopic Einstein - de Hass effect?"

• Presenter: Gary Douberly, University of Georgia
• Host: Heather Lewandowski
• Abstract: The first beam of helium droplets was reported in the 1961 paper Strahlen aus kondensiertem Helium im Hochvakuum by Von E. W. Becker and co-workers [1]. However, molecular spectroscopy of helium-solvated dopants wasn't realized until 30 years later in the laboratories of Scoles and Toennies [2,3].
  It has now been over two decades since this early, seminal work on doped helium droplets, yet the field of helium droplet spectroscopy is still fresh with vast potential. Analogous in many ways to cryogenic matrix isolation spectroscopy, the helium droplet is an ideal environment to spectroscopically probe difficult to prepare molecular species, such as radicals, carbenes and ions. The quantum nature of helium at 0.35 K often results in molecular spectra that are sufficiently resolved to evoke an analysis of line shapes and fine-structure that demands rigorous “effective Hamiltonian” treatments, revealing the microscopic details associated with the interaction between the molecular dopant and the superfluid solvent environment. This lecture will focus on our successful attempts to dope the hydroxyl radical (OH) and hydroxyl-containing molecular complexes into helium droplets [4,5,6]. The properties of these systems have been probed with infrared laser spectroscopy. Anomalous effects such as an unusually large Zeeman splitting are interpreted with a model that accounts for angular momentum transfer between dopant and solvent, which is akin to a microscopic Einstein – de Hass effect.
  [1]  E. W. Becker, R. Klingelhöfer, P. Lohse, Z. Naturforsch. A 16A, 1259 (1961).
  [2]  S. Goyal, D. L. Schutt, G. Scoles, Phys. Rev. Lett. 69, 933 (1992).
  [3]  M. Hartmann, R. E. Miller, J. P. Toennies, A. F. Vilesov, Phys. Rev. Lett. 75, 1566 (1995).
  [4]  Raston, P.L.; Liang, T.; Douberly, G.E., “Anomalous -doubling in the infrared spectrum of the hydroxyl radical in helium nanodroplets” Journal of Physical Chemistry A 117, 8103-8110 (2013).
  [5]  Brice, J.T.; Liang, T.; Raston, P.L.; McCoy, A.B.; Douberly, G.E. “Infrared Stark and Zeeman spectroscopy of OH-CO: The entrance channel complex along the OH + CO →  trans-HOCO reaction pathway” Journal of Chemical Physics 145, 124310 (2016).
  [6]  Raston, P.L; Obi, E.I.; Douberly, G.E. “Infrared Spectroscopy of the Entrance Channel Complex Formed Between the Hydroxyl Radical and Methane in Helium Nanodroplets” Journal of Physical Chemistry A 121, 7597-7602 (2017).

   

May 3 — "Spinning top-ology"

• Presenter: William Irvine, University of Chicago
• Host: Leo Radzihovsky
• Abstract: Geometry, topology and broken symmetry play a powerful role in determining the physics of materials. In this colloquium I will present experimental realizations of activated materials and fluids built out of mechanically spinning components and show how the subtle interplay of structure, time-reversal and parity leads to `odd' solid and fluid mechanics. The self-assembled chiral phases we observe blur the boundary between solids and fluids spontaneously exhibiting novel phenomena both fluid and solid: from chiral surface waves to odd instabilities and motile topological defects. The simple addition of transverse interactions through rotational drive in the 'atomic' constituents proves to be a powerful twist: spontaneously endowing these materials with their active dynamics and exotic responses such as odd viscosity and odd elasticity.

For more information about colloquia this semester, contact: Leo Radzihovsky.

Colloquia were Wednesdays at 4:00 p.m. in the JILA Auditorium. 

Coffee, tea and cookies will be available in G1B31 (across from G1B20) from 3:30 - 3:50 p.m.

August 24 — "Immobile topological quantum matter: fractons"

• Presenter: Leo Radzihovsky, Department of Physics, University of Colorado, Boulder
• Host: Victor Gurarie
• Abstract: I will discuss a bourgeoning field of "fractons" — a class of models where quasi-particles are strictly immobile or display restricted mobility. Focusing on just a corner of this fast-growing subject, I will explain how one class of such theories — symmetric tensor gauge theories surprisingly emerge from seemingly mundane elasticity of a two-dimensional quantum crystal. The disclination and dislocation crystal defects respectively map onto charges and dipoles of the fracton gauge theory. This fracton-elasticity duality leads to predictions of fractonic phases and quantum phase transitions to their descendants, that are duals of the commensurate crystal, supersolid, smectic, and hexatic liquid crystals. Extensions of this duality to generalized elasticity theories provide a route to discovery of new fractonic models and their potential experimental realizations. 

August 31 — "Time-of-flight quantum tomography of single atom motion in an optical tweezer"

• Presenter: Cindy Regal, JILA, Department of Physics, University of Colorado, Boulder
• Host: Konrad Lehnert
• Abstract: Quantum control of mechanical motion has been achieved in a surprising range of platforms in the past decades. These mechanical quantum systems have both piqued the curiosity of physicists, and enabled new approaches to difficult tasks in manipulating quantum information. Trapped particles offer one opportunity to study isolated quantum motion. Laser-cooled ions routinely demonstrate intriguing phonon control, and recent experiments have now brought trapped dielectric nanoparticles to their quantum ground state. In this talk I describe how time of flight combined with trap evolution can map the full quantum state of a trapped neutral particle. I will present this intuitive method through experiments with a single atom in a non-classical motional state in an optical tweezer, and discuss future applicability to larger mass particles. Single atoms in optical tweezers are now a ubiquitous tool for quantum science, and I will also use this opportunity to tell you how these experiments isolate single neutral atoms in the first place.

September 7 — "Planetary dynamos and the dynamics of rotating, electrically conducting fluids"

• Presenter: Michael Calkins, Department of Physics, University of Colorado, Boulder
• Host: Michael Ritzwoller
• Abstract: Planetary magnetic fields are ubiquitous in the Solar System. These fields are generated by the motion of an electrically conducting fluid within the interiors of the planets. For the Earth, turbulence in the liquid iron outer core has sustained the geomagnetic field for at least 4 billion years. Similar turbulent fluid systems are present in most planets, as well as stars. These flows are thought to be strongly influenced by system rotation (i.e. the Coriolis force) and the magnetic fields that they induce, yielding a unique form of turbulence that is both challenging to investigate and dynamically rich. I will give an overview of what we know about natural dynamos, and how computation and theory are being used to better understand their dynamics. 

September 14 — "Geometric frustration, self-assembly, mechanics, and pathways to complexity"

• Presenter: Xiaoming Mao, University of Michigan
• Host: Leo Radzihovsky
• Abstract: Self-organized complex structures in nature, from hierarchical biopolymers to viral capsids and organisms, offer efficiency, adaptability, robustness, and multifunctionality.  How are these structures assembled? Can we understand the fundamental principles behind their formation, and assemble similar structures in the lab using simple inorganic building blocks?  What’s the purpose of these complex structures in nature, and can we utilize similar mechanisms to program new functions in metamaterials?  In this talk, we will start from the perspective of geometric frustration, to explore answers to these questions.  I will discuss our recent work on developing analytic theories based on crystal structures in non-Euclidean space for the self-assembly of nanoparticles into complex structures, mechanical properties of materials in which geometric frustration causes prestress, as well as our ongoing effort in designing topological mechanical metamaterials with and without geometric frustration.

September 21 — "From BEC to CEO"

• Presenter: Chris Myatt, LightDeck Diagnostics
• Host: Leo Radzihovsky
• Abstract: Can a PhD at CU Physics prepare you to start a company? Can a Post-Doc at NIST be the ticket to entrepreneurship? While the subject of research at these institutions may not be directly applicable to industrial problems—in my case, Bose-Einstein Condensation and quantum computing— the skill sets and tools you develop are on the critical path in preparing you to succeed in high-tech industry and even in starting a company. In this personal story presentation, I will take the audience on my personal journey from a JILA lab to my current position as founder of a diagnostics and testing company, making testing equipment for water, veterinary, and diagnostics applications. My first company manufactured high precision optical devices and systems. I will share the lessons learned, the depth of entrepreneurial activity coming out of the CU Physics department, and thoughts on how to encourage more.

  Dr. Myatt is a Founder of LightDeck Diagnostics.  He is an entrepreneur with a proven track record for building technology companies. Prior to LightDeck, Chris successfully founded and led Precision Photonics Corporation, a successful laser and optical technology supplier that is now a division of IDEX. Chris earned bachelors degrees at Southern Methodist University, and a PhD in Atomic Physics at the University of Colorado. Chris studied Bose-Einstein Condensation under Carl Wieman, and quantum information under David Wineland as a National Research Council Postdoctoral Fellow at NIST Boulder. He is now successfully applying many of the tools developed during his academic years to real-world problems in high tech industry.
   

September 28 — "Imaging black holes: accretion physics at the event horizon"

• Presenter: Jason Dexter, JILA, Department of Astrophysics and Planetary Sciences, University of Colorado, Boulder
• Host: Andrew Hamilton
• Abstract: In the past few years, it has become possible to image black holes. Long baseline interferometry experiments operating at millimeter and near-infrared wavelengths can now achieve event horizon scale angular resolution with sufficient sensitivity to detect synchrotron radiation from hot plasma near the Galactic center black hole, Sgr A*, and the supermassive black hole in M87. I will discuss major results from each experiment, and focus on our group's work using the data to study accretion physics and particle acceleration in the immediate vicinity of an event horizon.

October 5 — "Quantum measurements: new, better, easier"

• Presenter: Joshua Combes, Electrical, Computer & Energy Engineering • University of Colorado, Boulder
• Host: Leo Radzihovsky
• Abstract: Quantum measurements are the principal means by which we gain access to and characterize the quantum world. At present the kinds of measurements that are routinely performed in the laboratory are rudimentary and a small subset of the measurements allowed by quantum theory. This talk will describe the past present and future of quantum measurements. Along the way, we will discuss state-of-the-art quantum measurements; the best ways of sensing quantum signals; and a curious connection between measurements and quantum computation.

**Cancelled** October 12 — "Closing the gap between simulating and understanding climate change"

• Presenter: Tiffany Shaw, University of Chicago
• Host: Michael Ritzwoller
• Abstract: Understanding and preparing for the impacts of anthropogenic climate change are some of the most urgent scientific challenges of our time. Remarkable progress has been made in the last half century in our ability to simulate Earth’s climate using state-of-the-art physical models and project future climate changes under specific emission scenarios. However, simulation does not equal understanding. Confidence in climate model projections requires an understanding of the physical mechanisms underlying the modeled responses, which helps to ensure the responses are not the result of structural and parameter uncertainty. In this talk I will discuss progress toward closing the gap between simulating and understanding climate change. I will present examples from the pioneering work of Nobel Prize winning scientist Dr. Syukuro Manabe and from our more recent work on the response of the atmospheric circulation to climate change.

October 19 — "Schrodinger’s 'What is Life?' at 75: The Physical Aspects of the Living Cell Revisited"

• Presenter: Rob Phillips, California Institute of Technology
• Host: Leo Radzihovsky
• Abstract: 2019 marked the 75th anniversary of the publication of Erwin Schrodinger's "What is Life?'', a short book hailed by Roger Penrose as "among the most influential scientific writings of the 20th century'’. In this talk, I will briefly review the long argument made by Schrodinger as he mused on how the laws of physics could help us understand, "the events in space and time which takes place within the spatial boundary of a living organism.'' Though often chronicled for its influence on some of the titans of the rise of molecular biology, my talk takes a different tack. I pivot to the idea of Schrodinger’s classic as a timeless manifesto. Rather than a dated historical curiosity, I argue that "What is Life?"' is full of stylish approaches to understanding the mysterious living world around us and instead, might be viewed as a call to arms to tackle the unfinished business of the scientific study of living matter. To make those points, I will focus on two key case studies from my own laboratory that pursue several of the key threads laid out in Schrodinger’s book: i) how does the control of hereditary information buried within the genome look from the point of view of physics and ii) how do the spatiotemporal structures (mitotic spindle, organelles, tissues, etc.) so familiar in biology arise as the playing out of underlying laws of non equilibrium physics?

October 26 — "Quantum spacetime and quantum information"

• Presenter: Oliver DeWolfe, Department of Physics, University of Colorado, Boulder
• Host: Ethan Neil
• Abstract: How quantum mechanics governs space, time and gravity is a longstanding mystery. Inspired by properties of black holes, recent progress has been made relating quantum spacetime to properties of quantum information and quantum computation such as entanglement entropy, computational complexity and quantum error-correcting codes. I will review some of these developments and discuss some of my own work on geometric realizations of measures of entanglement.

November 2 — "Topological Superconductivity in Superconductor-Semiconductor Hybrids"

• Presenter: Chetan Nayak, Microsoft Research
• Host: Leo Radzihovsky
• Abstract: Topological superconducting nanowires are characterized by Majorana zero modes, which can form the basis of topological qubits. In this talk, I will present some recent theoretical and experimental progress on these systems.

November 9 — "Many-body physics with ultracold gases of atoms and molecules"

• Presenter: Waseem Bakr, Princeton
• Host: Leo Radzihovsky
• Abstract: Understanding emergent behaviors in strongly interacting quantum systems is a frontier area of condensed matter physics. However, simulations of quantum many-body systems on classical computers are not scalable beyond a few dozen particles. This motivates the development of quantum simulators, highly controllable analog quantum computers specifically designed to study certain types of problems in condensed matter physics. I will present an overview of quantum simulation with ultracold gases of atoms and molecules, discussing examples relevant for understanding phenomena that occur in real materials, and others that explore completely novel regimes inaccessible in the solid-state. In particular, I will focus on advances enabled by the introduction of microscopy techniques that probe ultracold gases at the single-particle level and reveal the rich quantum correlations present in these systems. 

November 16 — "Research as a tool to improve the teaching and learning of quantum mechanics"

• Presenter: Gina Passante, Fullerton College
• Host: Steven Pollock
• Abstract: It is well established that people learn better when they are actively engaged with the material they are learning.  In this talk, I will focus on active learning methods used in the upper division, and the research that goes into their development.  In particular, we will discuss the structure of a tutorial to designed to improve student visual understanding of time dependence in quantum mechanics and the assessment of its effectiveness.   We will also discuss whether or not the context of a question can impact student performance.  Finally, we will consider implications for instruction and end with some ideas for incorporating active learning strategies into any upper-division physics class.

November 23 — No Colloquium, Fall Break 

November 30 — "Global Navigation Satellite System origins, unexpected applications, and new directions"

• Presenter: Penina Axelrad, Aerospace Engineering, University of Colorado Boulder
• Host: Leo Radzihovsky
• Abstract: Today we all take for granted the GNSS chips embedded in our phones and the convenience this affords in navigating in unfamiliar places. Scientists both appreciate the physics behind the technology, and have leveraged it in unexpected ways for scientific applications from the measurement of atmospheric conditions to tests of the gravitational redshift. My talk will discuss several of these applications and the key engineering contributions that made them possible. I’ll also talk about how GNSS is evolving today and my understanding of the contributions that quantum clocks and sensors will play in its future. 

December 7 — No Colloquium

For more information about colloquia this semester, contact: Leo Radzihovsky.

Colloquia were Wednesdays at 4:40 p.m. unless otherwise noted and will be in person at DUAN G1B20, as well as online via zoom and Youtube. 

**No Colloquium** January 12

**Online Only - January 19 — "The nature of local dynamic order in CH₃NH₃PbI₃ and CH₃NH₃PbBr₃"

• Presenter: Michael Toney, Chemical and Biological Engineering, University of Colorado Boulder
• Host: Joe Berry
• Special Time: 4:00 p.m.
• YouTube URL: 
• Abstract: Local atomic structure often differs from the global average structure as measured with diffraction and yet the local structure has a profound impact on properties. This structure-function relationship applies in many materials classes, ranging from organics to Li-ion battery cathodes to perovskites. Accurately characterizing this local structure has proven challenging but recent advances in diffuse scattering (“between” Bragg peaks) has enabled local structure determination. In this talk, I will discuss the importance of local structure and how this can be quantified, and will demonstrate this for organic-inorganic hybrid halide perovskites. These materials are a recently re-invigorated class of semiconductor that have demonstrated very high efficiencies for solar cells after just over a decade of research. While the importance of lattice dynamics and dynamical (dis)order have been recognized in these materials, their nature is only poorly known and understood. We used X-ray and neutron diffuse scattering coupled with molecular dynamics to quantify the nature, size, and time scale associated with dynamical local order in CH₃NH₃PbI₃ and CH₃NH₃PbBr₃ perovskites. We observe that the nominally cubic perovskite consists of dynamical, two-dimensional sheets of lower symmetry tetragonal regions of about 3 nm diameter with several picosecond lifetimes. Possible implications on halide perovskite properties will be discussed.

**Online Only - January 26 — "Free range frequency combs – mobilizing frequency comb laser technology for energy and climate applications from atmospheric sensing to wildland fire research"

• Presenter: Greg Rieker, Mechanical Engineering, University of Colorado Boulder
• Host: Scott Diddams
• YouTube URL: 
• Abstract: High precision sensing in complex engineered and natural environments is crucial for resolving challenges in a wide range of applications including detecting greenhouse gas emissions, predicting wildland fire spread, designing hypersonic propulsion systems, and exploring exoplanet atmospheres. In our laboratory, we are developing and harnessing the power of mode-locked frequency comb lasers to address these emerging challenges at the forefront of climate, energy, and environment. Frequency combs emit hundreds of thousands of frequencies of light in exquisitely short, repeatable pulses, which enables broadband measurements of light-matter interaction at high accuracy, precision, and stability. As an example of a real-world intervention strategy, I will talk about the journey of this technology from a laboratory novelty to a leak detection network covering hundreds of square miles of infrastructure in one of the nation’s busiest oil and gas basins.

**Canceled — No Colloquium February 2** 

February 9 Joint Physics/APS Colloquium— "Explosions from magnetized neutron stars”

• Presenter: Andrei Beloborodov, Columbia University
• Host: Dmitri Uzdensky
• YouTube URL: 
• Abstract: Neutron stars are by far the strongest known magnets in the universe. Some of them (called magnetars) generate explosions via sudden dissipation of magnetic energy with power up to 10⁴⁷ erg/s. These magnetic explosions emit giant gamma-ray flares observed in our and neighboring galaxies. Similar explosions in distant galaxies are proposed as the engines of the mysterious fast radio bursts (FRBs). They are many orders of magnitude brighter than pulsars and produce electromagnetic waves with dimensionless strength parameter exceeding unity, resembling nonlinear optics phenomena in laser plasma. Yet more powerful energy release occurs when two neutron stars merge. These cataclysmic events produce gravitational waves, neutrinos, and bright electromagnetic emission. Both mergers and isolated magnetar flares involve sudden heating up to MeV temperatures, creation of copious electron-positron pairs, and launching of ultra-relativistic outflows and shock waves. Recent numerical experiments shed some light on the extreme plasma physics in these events. Observations and simulations of the ultra-powerful bursts also provide insights into neutron star interior. 

February 16 — "Baryogenesis, new Higgs bosons, and what's next"

• Presenter: Corrinne Mills, University of Illinois, Chicago
• Host: John Cumalat and Steve Wagner
• In-Person Location: DUAN G1B20
• YouTube URL: 
• Abstract: Nearly 10 years after its discovery, the Higgs boson remains the subject of intense study at the Large Hadron Collider (LHC) because of its unique properties and potential connection to physics beyond the Standard Model. As our measurements become more precise and more comprehensive measurements, the observed Higgs boson looks ever more like the one predicted by the Standard Model. This is both an experimental triumph and a conundrum. With all of the particles predicted by the SM discovered, there is no single, obvious next target for searches, but most of the questions that motivated the construction of the LHC remain. I will talk about how my research addresses one of the outstanding questions, baryogenesis, through searches for additional Higgs bosons. Looking to the future, I argue that we are entering an experiment-driven era, and will need the best possible multipurpose detectors for the high-luminosity LHC and future colliders. I will talk about one of the tools that will take us there, a silicon semiconductor particle tracker of unprecedented precision and resilience.  

**Online Only** February 23 — "The Black Hole Information Paradox in the Age of Holographic Entanglement Entropy"

• Presenter: Netta Engelhardt, Massachusetts Institute of Technology
• Host: Oliver DeWolfe
• YouTube URL: 
• Abstract: The black hole information paradox — whether information escapes an evaporating black hole or not — remains one of the greatest unsolved mysteries of theoretical physics. The apparent conflict between validity of semiclassical gravity at low energies and unitarity of quantum mechanics has long been expected to find its resolution in the deep quantum gravity regime. Recent developments in the holographic dictionary and in particular its application to entanglement, however, have shown that a semiclassical analysis of gravitational physics has a hallmark feature of unitary evolution. I will describe this recent progress and discuss some potential new avenues for working towards a resolution of the information paradox.

**ONLINE ONLY** March 2 — "Scale and conformal invariance in atomic and nuclear physics"

• Presenter: Dam Thanh Son, University of Chicago
• Host: Andrew Lucas
• NOTE Special Time - 4:00 p.m.
• YouTube URL: 
• Abstract: Scale and conformal invariance plays an important role in field theory and statistical physics. In this talk, we describe a nonrelativistic version of scale invariance and conformal invariance. We review the history of the system called "fermions at unitarity" and show how its thermodynamic and kinetic properties are constrained by scale and conformal invariance. We also show how scale invariance appears at a very different energy scale in the behavior of nuclear reactions involving several neutrons in the final state.

March 9 — "Untangling Feynman's puzzle: Examining the future of computing"

• Presenter: Sadasivan Shankar, SLAC, Stanford and Harvard University
• Host: Sean Shaheen
• YouTube URL: 
• Abstract: Have you wondered as to how nature computes reality? For example, a single grammole of water can have of the order of 1023 molecules. How does each of the molecules compute its own trajectory so that the motion leads to fluid flow? As we go from atoms to materials, we could think of moving from a periodic table of hundred or so usable elements to simple molecules like carbon dioxide, methane to complex macromolecules like polymers or more complex materials (Alvisatos et.al, 1999). Extending this type of thinking to biology, complexity increases in many different ways. A human brain has about eighty billion neurons with about 5000 to 10,000 to synapses per neuron resulting in possibly hundreds of trillion connections. With the advent of machine learning and deep neural networks, it appears that we can simulate the complexity of applications by artificial intelligence-based methods. However, even for training these networks, you need copious amount of data.
  Meanwhile, the computing revolution known by the moniker Moore’s law, rolling past its fifty-year march as one of the most significant advancements of human civilization has been enabled by a confluence of breakthroughs in science and engineering (Moore, 1965). We have had at least about twenty-five doublings of computing ability, with the floating-point instructions per second with 20 doublings, which has enabled simulations of realistic problems. We are on the verge of exa-scale computing, which can compute about 1018 floating point operations per second. No matter we look at this, the computing capabilities and energy needed to simulate natural systems seem exceedingly high even for a small phase space of these systems, given the state of our current knowledge.
  To address these questions, building on older concepts from Turing and von Neumann, and using a new framework, we will help bridge physics, chemistry, and biology with computing. Using these concepts we will get closer to addressing Feynman’s puzzle on computing the physics of a “stinky tiny piece of space-time”.

March 16 — "Representation and Culture as Dimensions of Change in the Physical Sciences: A Work in Progress"

• Presenter: Julie Posselt, University of Southern California
• Host: Noah Finkelstein and Eleanor Hodby
• YouTube URL: 
• Abstract: In this colloquium, Professor Julie Posselt will share findings from her research about trajectories toward diversity, equity, and inclusion among  organizations in the physical sciences. Drawing from insights first proposed by quantum theorists, she will present organizational equity efforts from the standpoint of complex systems, and offer practical insights for those who working to improve representation and culture.

**No Colloquium March 23 — Campus Closed**

**In Person Only** March 30 — "Quantum simulation with ultracold atoms – from Hubbard models to gauge theories"

• Presenter: Monika Aidelsburger, Ludwig-Maximilians-University Munich, Germany
• Time: 4:40 p.m.
• Host: Ana Maria Rey
• Abstract: Well-controlled synthetic quantum systems, such as ultracold atoms in optical lattices, offer intriguing possibilities to study complex many-body problems in regimes that are beyond reach using state-of-the-art classical computations. The basic idea is to construct and use a well-controlled quantum many-body system in order to study its in- and out-of-equilibrium properties and potentially use it to develop more efficient tailored numerical methods that can then be applied to other systems that are not directly accessible with the simulator.
  An important future quest concerns the development of novel experimental techniques that allow us to expand the range of models that can be accessed. I will demonstrate this using the example of topological lattice models, which in general do not naturally  appear in cold-atom experiments. I will show how the technique of periodic driving, also known as Floquet engineering, facilitates their realization and show how charge-neutral atoms in lattices can mimic the behavior of charged particles in the presence of an external magnetic field.
  A key ingredient for quantum simulation is the degree of control one has over the individual particles and the microscopic parameters of the model. We have recently succeeded to not only use the technique of periodic driving to emulate physical systems that we know exist in nature, but to take this idea one step further and realize completely new topological regimes that do not have any static analog. Moreover, we are currently developing a novel hybrid optical lattice platform, where tightly focused optical tweezers are used to locally control the motion of the atoms in the lattice, paving the way towards quantum simulation of simplified lattice gauge theories, which play a fundamental role in a variety of research areas including high-energy physics and topological quantum computation.

**Special Colloquium** Friday, April 1 — "Optimizing plasma confinement: recent advances in stellarator optimization"

• Presenter: Rogerio Jorge, Max-Planck Institute Plasma Physics, Greifswald (Germany)
• Time: 12:15 p.m.
• Location: DUAN G125
• Host: Dmitri Uzdensky
• Abstract: The success of nuclear fusion requires optimal magnetic fields. These should provide the necessary plasma performance to sustain high temperature plasmas for a long enough time period. However, there are still important outstanding issues in magnetic fusion, such as the understanding and control of plasma density, fast particles and heat diffusion. To achieve good confinement, recent stellarator devices such as HSX and W7-X have been designed using optimization based on the calculation of 3D MHD equilibria at each evaluation of the objective function. In here, we show how new design methods have been able to reduce the computational cost of such optimization efforts by orders of magnitude, while also providing new insights into the space of solutions. These methods enable a direct geometrical construction of magnetic fields with extremely good confining properties such as quasisymmetry and quasi-isodynamic fields. Thanks to the reduced computational cost, we are able to achieve good particle confinement to much higher accuracy than reported before and perform wide surveys over parameter space. Furthermore, many figures of merit can be now calculated directly, including MHD stability and all the geometric quantities in the gyrokinetic equation, making the design of a fusion machine optimized for turbulent transport and fast particles a possibility in the near future.

**Special Colloquium** Tuesday, April 5 — "Magnetic pumping as a mechanism for particle energization in space and astrophysical plasmas"

• Presenter: Emily Lichko, University of Arizona Tuscon
• Time: 2:00 p.m.
• Location: DUAN G125
• Host: Dmitri Uzdensky
• Abstract: Energetic particle generation is an important component of a variety of astrophysical systems, from seed particle generation in shocks to the heating of the solar wind. Starting with the kinetic equation, we have derived a magnetic pumping model, where particles are heated by the largest scale turbulent fluctuations. We show that for a spatially uniform flux tube this is an efficient heating mechanism up to the speed of the fluctuations, and naturally produces power-law distributions like those observed in the solar wind, as verified by particle-in-cell simulations. When this model is extended to a spatially varying flux tube, magnetic trapping renders magnetic pumping an effective Fermi heating mechanism for particles moving far faster than the speed of the fluctuations. To test this, we used satellite observations of the strong, compressional magnetic fluctuations near the Earth's bow shock from the Magnetospheric MultiScale (MMS) mission and found strong agreement with our model. Given the ubiquity of such fluctuations in different astrophysical systems this mechanism has the potential to be transformative to our understanding of how the most energetic particles in the universe are generated. 

**Online Only** April 6 — "Physicists do Social Science: The Case of GRE's"

• Presenter: Michael Weissman, University of Illinois Urbana-Champaign
• Host: Rahul Nandkishore
• NOTE SPECIAL TIME: 4:00 p.m.
• YouTube URL: 
• Abstract: A well-funded influential paper concluded that GREs do not help predict whether physics grad students will get Ph.D.s. The paper makes numerous elementary statistics errors, including introduction of unnecessary collider stratification bias, variance inflation by collinearity and range restriction, a peculiar choice of null hypothesis on subgroups, blurring the distinction between failure to reject a null and accepting a null, an obviously biased method for imputing missing data, an extraordinary procedure for radically inflating confidence intervals in a figure, and omission of needed data. I estimate the actual approximate incremental predictive power of GREs using standard methods by scrounging data found in the paper and in equally flawed follow-ups. These papers exhibit exactly the sort of research techniques which we should be teaching students to avoid. Widespread acceptance of such methods raises questions about the culture of physics.

**Special Colloquium** Friday, April 8 — "Particle acceleration and entropy production in relativistic plasma turbulence"

• Presenter: Vladimir Zhdankin, Flatiron Institute, Center for Computational Astrophysics
• Time: 12:15 p.m.
• Location: DUAN G125
• Host: Dmitri Uzdensky
• Abstract: Turbulence has long been considered as a candidate process for accelerating particles to relativistic energies in the collisionless plasmas associated with space and astrophysical systems. There is a long line of stochastic particle acceleration theories spanning back to Enrico Fermi in 1949; these models are widely used for describing cosmic rays and radiative signatures from high-energy astrophysical systems, but have remained essentially unvalidated over the decades. Particle-in-cell (PIC) simulations of relativistic turbulence have recently opened this topic to rigorous, first-principles numerical scrutiny. I will describe the latest progress on understanding relativistic particle energization by turbulence. PIC simulations of turbulence demonstrate efficient particle acceleration and confirm its diffusive nature, while also yielding new insights into the electron-to-ion heating ratio and intermittent kinetic beaming. However, several deep mysteries remain, such as the prevalence and properties of power-law particle energy distributions. I suggest that a complete understanding will require us to confront the age-old problem of entropy production in collisionless plasmas. I will describe new theoretical efforts to model particle acceleration with generalized maximum entropy principles, which explain several numerical results and may extend to phenomena beyond turbulence (such as magnetic reconnection). The next several years promise to bring new, fundamental breakthroughs into these problems.

**Special Colloquium** Tuesday, April 12 — "At the intersection of plasma and quantum"

• Presenter: Yuan Shi, LLNL
• Time: 2:00 p.m.
• Location: DUAN G125
• Host: Dmitri Uzdensky
• Abstract: Plasma physics and quantum physics may seem to have nothing to do with each other. However, in this talk I will show that they are in fact closely connected. I will examine three separately topics and show how insight from one field can help with the other. (1) The first topic is laser-plasma interactions (LPI). Conventionally, LPI is studied by solving classical equations. The equations become very complicated in the presence of a magnetic field, which is recently added to inertial confinement experiments to help achieve fusion ignition. In this talk, I will show how magnetized LPI can be tackled, and how the final result can be derived much more simply using Feynman diagrams. (2) The second topic is quantum electrodynamics (QED) plasma. With high intensity lasers, positrons have been created in plasmas. To describe high-energy events self consistently with collective plasma effects, I will show how QED can be turned into a plasma model. The model contains many new ingredients not well-studied in field theories and may shed light on how dark matter could affect ordinary particles. (3) The final topic is quantum computing (QC). Recent advances of QC capabilities make us wonder whether they can be used for plasma simulations, which are often challenging even for supercomputers. In this talk, I will show how we manage to program a toy problem of LPI on quantum devices. The success makes it hopeful that other plasma problems may also be solvable using the power of quantum mechanics.
   
  Short Bio:
  Dr. Yuan Shi is a Staff Scientist at Lawrence Livermore National Laboratory (LLNL) where he was a Lawrence Postdoctoral Fellow. Prior to LLNL, Dr. Shi earned his MA (2014) and PhD (2018) in Astrophysical Sciences, Program in Plasma Physics, from Princeton University, where he was a recipient of the Carl Oberman Fellowship. His PhD work was awarded the Marshall N. Rosenbluth Outstanding Doctoral Thesis Award by the American Physical Society. Yuan has a BSc from the University of Hong Kong where he was a recipient of the Rosita King Ho Scholarship.

April 13 — "Detecting Axion Dark Matter"

• Presenter: Benjamin Safdi, University of California Berkeley
• Host: Tom DeGrand
• Abstract: The quantum chromodynamics axion is one of the most sought-after beyond the Standard Model particles at present because of its possible connections with the strong-CP problem, dark matter, and ultraviolet physics such as Grand Unification and String Theory. However, axions are notoriously difficult to probe in the laboratory because of their weak couplings to matter. In this talk I will overview multiple recent advances that will help accelerate the search for axions. On the theoretical side, I will present the results of recent simulations of axion dark matter production in the early Universe after the Big Bang that narrow the possible mass range of the axion. I will then describe a novel laboratory detector setup, called ABRACADABRA, that is sensitive to axions connected with Grand Unification. I will show that the up-coming DMRadio project, building off of the successful ABRA-10 cm experiment, will cover some of the most motivated regions of axion parameter space in the coming years. We may soon know if axions exist within the spectrum of fundamental particles. 

**Canceled** April 20

**Online Only** April 27 — "The Return of the LHC"

• Presenter: Nathaniel Craig, University of California Santa Barbara
• Host: Keith Ulmer
• Time: 4:40 p.m.
• YouTube URL: 
• Abstract: The LHC re-awakens this week after a three-year slumber, building towards proton-proton collisions at the highest energies yet attained. While the 2012 discovery of the Higgs boson at the LHC marked the triumphant completion of the Standard Model, the LHC re-awakens to a Standard Model under siege. Tension between the muon’s anomalous magnetic dipole moment and the Standard Model prediction now exceeds four standard deviations, while various decays of bottom quarks into lighter quarks and leptons appear to deviate from Standard Model predictions at the level of three to five standard deviations. Earlier this month, the CDF experiment at the Tevatron announced the world’s most precise measurement of the mass of the W boson, obtaining a result nominally seven standard deviations away from the Standard Model prediction. These growing tensions add to long-standing motivation for physics beyond the Standard Model, ranging from the mechanism of electroweak symmetry breaking to the origin of the masses and mixings of fundamental particles, the predominance of matter over antimatter, and the nature of dark matter. In this talk I’ll survey the LHC's prospects for illuminating both recently-discovered cracks and long-standing mysteries of the Standard Model, as well as the possible connections between the two.

Colloquia are Wednesdays at 4:00 p.m. unless otherwise noted and will be in person at DUAN G1B20, as well as online via zoom and Youtube. 

August 25 — "Honeywell's commercial trapped-ion quantum computer"

• Presenter: Brian Neyenhuis, Honeywell Quantum Solutions
• Host: Ana Maria Rey
• Location: DUAN G1B20
• YouTube URL: 
• Abstract: Noisy Intermediate-Scale Quantum (NISQ) computers are now commercially available for use though cloud access.  I will give an overview of Honeywell Quantum Solutions' first generation of commercial quantum computers. We realize the quantum charge coupled device (QCCD) architecture first proposed at NIST with a fully reconfigurable chain of trapped atomic-ions on a multi-zone surface electrode trap.  I will present the benchmarking methods used to quantify the performance of our computer, along with results from a handful of quantum algorithms. I will also present on our first steps out of the NISQ era with a demonstration of real-time fault-tolerant quantum error correction.

September 1 — "Physical Computation in Insect Swarms" 

• Presenter: Orit Peleg, Biofrontiers Institute, Computer Science, University of Colorado Boulder
• Host: Loren Hough
• YouTube URL: 
• Abstract: Our world is full of living creatures that must share information to survive and reproduce. As humans, we easily forget how hard it is to communicate within natural environments. So how do organisms solve this challenge, using only natural resources? Ideas from computer science, physics and mathematics, such as energetic cost, compression, and detectability, define universal criteria that almost all communication systems must meet. We use insect swarms as a model system for identifying how organisms harness the dynamics of communication signals, perform spatiotemporal integration of these signals, and propagate those signals to neighboring organisms. In this talk I will focus on two types of communication in insect swarms: visual communication, in which fireflies communicate over long distances using light signals, and chemical communication, in which bees serve as signal amplifiers to propagate pheromone-based information about the queen’s location. 

September 8 — "Bosonic Quantum Information Processing with Superconducting Circuits"

• Presenter: Liang Jiang, University of Chicago
• Host: Ana Maria Rey
• YouTube URL: 
• Abstract: Bosonic modes are widely used for quantum communication and information processing. Recent developments in superconducting circuits enable us to control bosonic microwave cavity modes and implement arbitrary operations allowed by quantum mechanics, such as quantum error correction against excitation loss errors. We investigate different bosonic encoding and error correction protocols, and provide a perspective on using bosonic quantum error correction for various applications.

September 15 — "Towards unraveling the puzzling cuprates"

• Presenter: Sriram Shastry, University of California, Santa Cruz
• Host: Rahul Nandkishore
• YouTube URL: 
• Abstract: Our recent results on the resistivity of the single layer cuprate superconductors, and the conditions determining Tc for a d-wave superconducting state in 2-dimensions are summarized. The resistivity found is quasi-linear in T and in quantitative accord with data. The phase diagram for superconductivity has a stretched-dome structure, with a peak Tc of a few hundred K, and a peak density determined by band parameters. The calculations are based on the t-J model in 2-dimension, and the extremely correlated Fermi liquid theory (ECFL) used is non-perturbative in character. In the talk we will go light on technical details, and highlight the physical ideas behind the calculations.

September 22 — "Electron temperature of the solar wind"

• Presenter: Stanislav Boldyrev, University of Wisconsin, Madison
• Host: Dmitri Uzdensky
• YouTube URL: 
• Abstract: The solar wind is a nearly collisionless plasma expanding from the hot solar corona into the interplanetary space. In situ measurements demonstrate that electrons propagating from the sun, have a complex velocity distribution function. In addition to the nearly Maxwellian thermal core, their distribution function contains a narrow beam of suprathermal electrons streaming along the magnetic field lines extending from the corona. The non-Maxwellian component of the distribution function is important for heat conduction, kinetic plasma instabilities, waves, and turbulence, which govern both the dynamics and thermodynamics of the solar wind. In this talk, I will demonstrate that the electron beam plays a significant role in the heating of solar wind electrons. In particular, it helps to explain why the electron temperature declines with the radial distance much slower than it would be predicted by adiabatic law, which is a fundamental long-standing problem of solar wind heating. I will also discuss an intriguing analogy between the problems of electron heating in the solar wind and in the expander region of a plasma mirror machine.

September 29 — "Exploring the Hottest Matter in the Universe with Ultra-Relativistic Heavy Ion Collisions"

• Presenter: Dennis Perepelitsa, University of Colorado Boulder
• Host: Jamie Nagle
• YouTube URL: 
• Abstract: The Relativistic Heavy Ion Collider (RHIC) in New York and the Large Hadron Collider (LHC) in Switzerland accelerate large, fully-ionized nuclei to very near the speed of light. When the nuclei are steered into a head-on collision, the energy density and temperature in the collision zone become so large that the fundamental quark and gluon particles are liberated from their confined state and form a new phase of matter called a Quark-Gluon Plasma (QGP). Although exotic to our everyday experience, QGP is the form which comprised all the matter in our universe during the first few microseconds after the Big Bang. The QGP manifests striking many-body, large-wavelength phenomena which are not obvious from the first-principles of Quantum Chromodynamics (QCD), the theory of the strong nuclear force. In this colloquium, I will discuss my group's research program with the ATLAS detector at the LHC and the future sPHENIX detector at RHIC. Topics include studying the composition and behavior of the QGP with internally-generated probes of partonic "jets" and heavy quarks, understanding the minimum conditions needed to produce QGP, and the exciting prospects for next-generation measurements.

October 6 — "The Perils of Taking Temperatures of Collisionless Plasmas"

• Presenter: Martin Goldman, University of Colorado Boulder
• Host: John Cary
• YouTube URL: 
• Abstract: Among the innovative features of NASA’s Magnetospheric Multiscale (MMS) mission study of magnetic reconnection has been the ability to measure high resolution fully 3-D electron and ion velocity distributions, orders of magnitude faster than ever before. Since Earth’s magnetosphere is highly collisionless these distributions can be far from thermal equilibrium. In order to interpret them, the MMS satellite onboard computers calculated velocity moments of both the electron and the ion distributions. The velocity moments yield “fluid-like” quantities such as the electron flow velocity, density, pressure, temperature, heat flux, etc., and the ion flow velocity, density, pressure, temperature, heat flux, etc. What could possibly be conceptually wrong with this?  And yet...

October 13 — "Choreographing Quantum Spin Dynamics With Light"

• Presenter: Monika Schleier-Smith, Stanford University
• Host: Ana Maria Rey
• YouTube URL: 
• Abstract: The power of quantum information lies in its capacity to be non-local, encoded in correlations among entangled particles. Yet our ability to produce, understand, and exploit such correlations is hampered by the fact that the interactions between particles are ordinarily local. To circumvent this limitation in the laboratory, we let distant atoms “talk” to each other with the aid of photons that act as messengers. By tailoring the spectrum of an optical control field, we program the spin-spin couplings in an array of atomic ensembles, thereby accessing frustrated interaction graphs and exotic geometries and topologies. Such advances in optical control of interactions open new opportunities in areas ranging from quantum technologies to fundamental physics. I will touch on implications for quantum optimization algorithms, quantum-enhanced sensing, and simulating quantum gravity.

October 20 — "Quantum Transport at Infinite Coupling"

• Presenter: Paul Romatschke, University of Colorado Boulder
• Host: Oliver DeWolfe
• YouTube URL: 
• Abstract: The transport properties of a material are the values of its conductivities, diffusion constants and viscosities. Calculating transport properties from first principles is hard at weak coupling, and it's generally considered to be impossible at infinite coupling without invoking unproven conjectures. In this talk, I will discuss results for transport at infinite coupling, finding curious links to experiments ranging from ultracold quantum gases to graphene to relativistic ion collisions at RHIC and the LHC.

October 27 — "From unitary dynamics to statistical mechanics in isolated quantum systems"

• Presenter: Marcos Rigol, Pennsylvania State University
• Host: Ana Maria Rey
• YouTube URL: 
• Abstract: Experiments with ultracold gases have made it possible to study dynamics of (nearly) isolated quantum many-body systems, which has revived theoretical interest on this topic. In generic isolated systems, one expects nonequilibrium dynamics to result in thermalization: a relaxation to states where the values of macroscopic quantities are stationary, universal with respect to widely differing initial conditions, and predictable through the time-tested recipe of statistical mechanics. However, it is not obvious what feature of a many-body system makes quantum thermalization possible, in a sense analogous to that in which dynamical chaos makes classical thermalization possible. Underscoring that new rules could apply in the quantum case, experimental studies in one-dimensional systems have shown that traditional statistical mechanics can provide wrong predictions for the outcomes of relaxation dynamics. We show that isolated "nonintegrable" systems do in fact relax to states in which observables are well-described by statistical mechanics. Moreover, we argue that the time evolution itself only plays an auxiliary role as thermalization occurs at the level of individual eigenstates.

November 3 — "Soft phonons as a window into quantum materials"

• Presenter: Dmitry Reznik, University of Colorado Boulder
• Host: Dan Dessau
• YouTube URL: 
• Abstract: Quantum materials exhibit quantum effects on the macroscopic scale. They have the potential to revolutionize energy technologies, data storage and processing, sensors, and development of “smart” materials that can switch properties “on demand”. However, a lot of basic research is still needed, although great progress is being made. Understanding many quantum materials theoretically is extremely challenging, so a variety of experimental windows into quantum materials are needed to test models and to characterize newly discovered compounds. 
  Soft phonons are atomic lattice vibrations whose frequency is reduced by electron-phonon coupling. After introducing soft phonons in the classic context of charge density wave formation, I will focus on surprising insights into electronic states in quantum materials in and out of thermal equilibrium revealed by measurements of soft phonons. 

November 10 — "Precision measurement and control of atomic hydrogen"

• Presenter: Dylan Yost, Colorado State University
• Host: Jun Ye
• YouTube URL: 
• Abstract: Because of atomic hydrogen’s simplicity, its energy levels can be precisely described by theory. This has made spectroscopy of hydrogen a favorite testbed for bound-stated quantum electrodynamics (QED). In addition, assuming the QED calculations are correct, one can use such measurements to determine the Rydberg constant and the proton-charge radius. A discrepancy of these constants determined through different transitions can indicate new physics.  Unfortunately, spectroscopy of hydrogen is often limited by the temperature of the atomic sample, and laser cooling is very challenging since the wavelength of the radiation required is well into the vacuum ultraviolet (121.57 nm). In this talk, I will present new results where we load a metastable hydrogen beam into a moving optical lattice, which we then decelerate to control the motion of the atoms. This technique offers a robust means to slow atomic hydrogen with visible lasers, which could provide a platform for increased precision. In addition, I will discuss a new measurement of the hydrogen 2S_1/2-8D_5/2 two-photon transition performed in our group, which produces additional tension within the global dataset of precision hydrogen spectroscopy.

November 17 — "Quantum Materials: A View from the Lattice"

• Presenter: Joseph Checkelsky, Massachusetts Institute of Technology
• Host: Rahul Nandkishore
• Will not be broadcast via YouTube
• Abstract: Connecting theoretical models for exotic quantum states to real materials is a key goal in quantum materials science. The structure of the crystalline lattice plays a foundational role in this pursuit in the subfield of quantum material synthesis. We here revisit this long-standing perspective in the context low dimensional emergent electronic phases of matter. In particular, we discuss recent progress in realizing new lattice and superlattice motifs designed to address model superconducting and correlated phenomena. We comment on the perspective for realizing further 2D model systems in complex material structures and connections to further paradigms for programmable quantum matter. 

**No Colloquium November 24 — Fall Break**

December 1 — "First Results from the Muon g-2 Experiment at Fermilab’s Muon Campus"

• Presenter: Chris Polly, Fermilab
• Host: Ethan Neil
• YouTube URL: 
• Abstract: The much anticipated initial results from the Muon g-2 experiment at Fermilab’s Muon Campus were released earlier in the year. The new determination of the muon’s anomalous magnetic moment is in good agreement with the intriguing value obtained at Brookhaven National Laboratory 20 years ago. The Muon Campus at Fermilab is a new accelerator facility capable of delivering the intense beams of muons required for the Muon g-2 and Mu2e experiments. This talk will discuss the Muon Campus physics program with an emphasis on the recent Muon g-2 results.

December 8 — "Four-second optical coherence between different atomic species, and the search for new physics with atomic clocks"

• Presenter: David Leibrandt, NIST, University of Colorado Boulder
• Host: Adam Kaufman
• YouTube URL: 
• Abstract: The extreme precision and accuracy of today’s atomic clocks can be used to look for very small deviations from the predictions of the Standard Model, offering a tool to search for beyond Standard Model (BSM) physics complementary to particle accelerators. These searches are based on measuring the frequency ratio of two atomic transitions that depend differently on interactions with BSM particles or fields. In this talk, I will present frequency ratio measurements between atomic clocks based on Al+, Hg+, Sr, and Yb atoms, and the use of these measurements to constrain the coupling of ultralight scalar dark matter candidates to the Standard Model. The precision of traditional, incoherent frequency ratio measurements and resulting constraints on BSM physics are limited by the coherence time of the lasers used to probe the atomic transitions. We have recently demonstrated a new, coherent frequency ratio measurement technique that removes this limitation and achieved a record for the precision of frequency ratio measurements between different atomic species.

For more information about colloquia this semester, contact: Dmitri Uzdensky.

Colloquia are Wednesdays at 9:00 a.m. unless otherwise noted and will be presented online via zoom. All meetings are password protected. If you are not a student/faculty member in physics, pre-register with Veronica Lingo to attend a lecture.

January 13 — "The Mechanics of Non-Equilibrium Soft Interfaces"

• Presenter: Michael Murrell, Yale University  
• Host: Loren Hough
• Abstract: At small length-scales, capillary effects are significant, and thus the mechanics of soft material interfaces may be dominated by solid surface stresses and liquid surface tensions. The balance between surface and bulk properties is described by an elasto-capillary length-scale, in which equilibrium interfacial energies are constant. However, at small length-scales in biological materials, including living cells and tissues, interfacial energies are not constant, but are actively regulated and driven far from equilibrium. Thus, the balance between surface and bulk properties depends upon the distance from equilibrium, defining a novel material parameter, what we term “active” elasto-capillarity. Here, we model the adhesion and spreading (wetting) of living cell aggregates as ‘active droplets’, with a non-equilibrium surface energy that depends upon internal stress generated by the actomyosin cytoskeleton. Depending upon the extent of activity, the droplet may exhibit both surface stress and surface tension, and each may adapt to the mechanics of their surroundings. The impact of this activity-dependent adaptation challenges contemporary models of interfacial mechanics, including traditional and extensively used models of contact mechanics and wetting. Finally, we show the origin of adaptation is in the breaking of detailed balance at the molecular scale by stochastic binding in the actomyosin cytoskeleton. 

January 20 — "Nature’s optics and our understanding of light"

• Presenter: Sir Michael Berry, University of Bristol
• Host: Rahul Nandkishore
• PLEASE NOTE SPECIAL TIME: 4:00 p.m.
• Abstract: Optical phenomena visible to everyone have been central to the development of, and abundantly illustrate, important concepts in science and mathematics. The phenomena considered include rainbows, sparkling reflections on water, green flashes, earthlight on the moon, glories, daylight, crystals, and the squint moon. The concepts include refraction, caustics (focal singularities of ray optics), wave interference, numerical experiments, mathematical asymptotics, dispersion, complex angular momentum (Regge poles), polarization singularities, Hamilton’s conical intersections of eigenvalues (‘Dirac points’), geometric phases, and visual illusions.

January 27 - No Colloquium

February 3 — "Towards quantitative theories of correlated electron materials"

• Presenter: Garnet Chan, California Institute of Technology
• Host: Rahul Nandkishore
• Abstract: I will talk about two topics. In the first part, I will discuss our efforts in the last decade to elevate the understanding of correlated materials from the level of lattice models to atomistic level details and quantitative precision. In the second part, I will describe a current, partially completed, program of research to build a unified numerical machinery to model realistic many-particle and many-electron problems based on tensor networks.

February 10 — "The XYZ Affair: Tales of the Third (and Fourth) Hadrons"

• Presenter: Richard Lebed, Arizona State University
• Host: Tom DeGrand
• ​Abstract: In the past two decades, many new particles have been discovered that are clearly hadrons (interacting via the strong nuclear force through the theory called QCD), but do not seem to fit into either of the known hadron categories of meson (quark-antiquark) or baryon (3 quarks).  Several species of these “exotic” particles, called X, Y, and Z, are now believed to be tetraquarks, and in July, 2015 researchers at the Large Hadron Collider announced the discovery of pentaquark states, P_c.  We begin by reviewing the basics of QCD (quarks, color, confinement, etc.), and then turn to the question of how conventional mesons are identified, which allows one to distinguish exotics.  After reviewing their experimental discovery, we consider the question of how exotics are assembled.   Several competing physical pictures attempt to describe the structure of exotics: as molecules of known hadrons, as the result of kinematical effects, and others.  I propose that at least some of them arise due to the formation of compact diquarks, a well-known but under-appreciated phenomenon of QCD.  The competing facts of kinematics and diquark confinement create an entirely new kind of bound state: not a molecule with well-defined orbits, but an extended object that lasts only as long as it takes for quantum mechanics to allow the separated quarks and antiquarks to “find” one another, and allow decays to occur.  I will discuss several observed effects that support this picture.

February 17 — "How behavioral and evolutionary constraints sculpt early visual processing"

• Presenter: Stephanie Palmer, University of Chicago
• Host: Loren Hough
• Abstract: An animal eye is only as efficient as the organism’s behavioral constraints demand it to be. While efficient coding has been a successful organizational principle in vision, to make a more general theory, behavioral, mechanistic, and even evolutionary constraints need to be added to this framework. In our work, we use a mix of known computational hurdles and detailed behavioral measurements to add constraints to the notion of optimality in vision. Accurate visual prediction is one such constraint. Prediction is essential for interacting fluidly and accurately with our environment because of the delays inherent to all brain circuits. In order to interact appropriately with a changing environment, the brain must respond not only to the current state of sensory inputs but must also make rapid predictions of the future. In our work, we explore how our visual system makes these predictions, starting as early as the eye. We borrow techniques from statistical physics and information processing to assess how we get terrific, predictive vision from these imperfect (lagged and noisy) component parts. To test whether the visual system performs optimal predictive compression and computation, we compute the past and future stimulus information in populations of retinal ganglion cells, and in the vertical motion sensing system of the fly. In the fly, we anchor our calculations with careful measurements from the Dickinson group on fast evasive flight maneuvers. This survival-critical behavior requires fast and accurate control of flight, which we show can be achieved by visual prediction in the fly vertical sensing system, via a specific wiring motif. Moving on from behavior, developing a general theory of the evolution of computation is a current research direction in our group. We use the repeated evolution of tetra-chromatic color vision in butterflies to test hypotheses about whether neural computations contain shadows of the evolutionary history of the organism.

**SPECIAL Physics/RASEI Colloquium** February 17 — "Metal halide perovskites: an introduction to hybrid semiconductors"

• Presenter: Joseph Berry, NREL
• Host: Michael Ritzwoller
• PLEASE NOTE SPECIAL TIME: 4:00 p.m.
• Abstract: Over the past decade metal halide perovskites (MHPs) have emerged as potentially transformational materials for enabling low-cost high efficiency photovoltaics (PVs).  In the time since the initial reports of PV based on these materials, current state-of-the-art devices have reached lab scale solar cell efficiencies exceeding any other polycrystalline thin film technology.  In addition, MHPs have shown the promise of creating low-cost high efficiency tandems based on both all-perovskite configurations and in combination with incumbent technologies (i.e., Si, CIGS) to target performance beyond single junction PV efficiency limits. This colloquium will provide an introduction to these materials and their physical properties that have made them truly unique as PV absorbers.  Current topics in PV research will be touched upon to indicate some of the motivations for a larger exploration of hybrid organic-inorganic semiconductors more generally.  Selected work in this broader class of hybrid materials will be presented to provide a perspective on the opportunities they offer.  Finally, some outstanding questions regarding how to understand and ultimately manipulate some of the more unique behaviors of these materials will also be highlighted.

February 24 — "Radio Astrophysics from the Moon"

• Presenter: Jack O Burns, CASA, University of Colorado, Boulder
• Host: Michael Ritzwoller
• Abstract: A new era of exploration of the low radio frequency Universe from the Moon will soon be underway with landed payload missions facilitated by NASA’s Commercial Lunar Payload Services (CLPS) program. CLPS landers are scheduled to deliver two radio science experiments, ROLSES to the nearside and LuSEE to the farside, beginning later this year. These instruments will be pathfinders for a 21-cm Global Signal Cosmology telescope (DAPPER) and a 10-km diameter interferometric array, FARSIDE, composed of 128 pairs of dipole antennas. ROLSES and LuSEE, operating at frequencies from ~100 kHz to a few tens of MHz, will investigate the plasma environment above the lunar surface and measure the fidelity of radio spectra on the surface. ROLSES will measure the photoelectron sheath density to better understand the charging of the lunar surface. LuSEE will measure the local magnetic field and exo-ionospheric density, interplanetary radio bursts, and the galactic synchrotron spectrum. DAPPER is designed to open a new window to the cosmos, the Dark Ages and Cosmic Dawn, using the highly redshifted hyperfine line of neutral hydrogen observed from 10-110 MHz. FARSIDE will break new ground by imaging radio emission from Coronal Mass Ejections (CME) beyond two solar radii, and detect auroral radio emission from the magnetic fields of nearby potentially habitable exoplanets.

**SPECIAL Physics/Applied Mathematics Colloquium** February 24 — "Evidence-Based Elections"

• Presenter: Philip Stark, University of California, Berkeley
• Host: Michael Ritzwoller
• NOTE SPECIAL TIME: 4:00 p.m. MST
• Abstract: Elections rely on people, hardware, and software, all of which are fallible and subject to manipulation. Well resourced nation-states continue to attack U.S. elections. Voting equipment is built by private vendors–some foreign, but all using foreign parts. Many states even outsource election results reporting to foreign firms. How can we conduct and check elections in a way that provides evidence that the reported winners really won–despite malfunctions and malfeasance? Evidence-based elections require voter-verified (generally, hand-marked) paper ballots kept demonstrably secure throughout the canvass and manual audits of election results against the trustworthy paper trail. Hand-marked paper ballots are far more trustworthy than machine-marked ballots for a variety of reasons. Two kinds of audits are required to provide affirmative evidence that outcomes are correct: _compliance audits_ to establish whether the paper trail is complete and trustworthy, and _risk-limiting audits_ (RLAs). RLAs test the hypothesis that an accurate manual tabulation of the votes would find that one or more reported winners did not win. To reject that hypothesis means there is convincing evidence that a full hand tally would confirm the reported results. For a broad variety of social choice functions, including plurality, multi-winner plurality, supermajority, proportional representation rules such as D’Hondt, Borda count, approval voting, and instant-runoff voting (aka ranked-choice voting), the hypothesis that one or more outcomes is wrong can be reduced to the hypothesis that the means of one or more lists of nonnegative numbers is not greater than 1/2. Martingale methods for testing such nonparametric hypotheses sequentially are especially practical. RLAs are in law in several states and have been piloted in more than a dozen; there have been roughly 60 pilots in jurisdictions of all sizes, including roughly 10 audits of statewide contests. Open-source software to support RLAs is available.

March 3 — "One-way quantum interactions for fun and profit"

• Presenter: Aashish Clerk, University of Chicago
• Host: Ana Maria Rey
• Abstract: The most common kinds of interactions in physics obey a basic kind of reciprocity: when two systems or particles interact, each one influences the other, and information flows in both directions.  In this talk, I’ll discuss general methods for engineering interactions that break this symmetry, in a fully consistent quantum setting.  These engineered “one-way” quantum interactions open up a host of unusual possibilities, from new methods for manipulating and processing quantum information, to new kinds of topological and many-body physics.  I’ll introduce some of the basic theoretical ideas that underlie these unusual interactions, and connections to recent interest in “non-Hermitian” quantum systems.  I’ll also discuss recent experimental implementations in quantum optomechanical systems and superconducting quantum circuits.  

March 10 — Adaptability, Thinking, and Inclusion – A not so random walk

• Presenter: Patricia Rankin, University of Colorado, Boulder
• Host: Michael Ritzwoller
• Abstract: Physicists at heart are problem solvers. This talk looks at how a precision measurement of the mass and width of the Z⁰ required a variety of problem-solving approaches driven by a need to adapt to changing expectations. Next, I will discuss a more general framework for thinking about the types of problems we encounter and ways to approach them. This framework and the need for us to work on grand challenge problems such as climate change motivates the final part of my talk which will discuss the ways we need to reconsider both how we train the students we have and how we approach the recruiting and retention of physicists in general.   

March 17 — "Using magnetic fields and microgravity to explore the physics of dusty plasmas"

• Presenter: Ed Thomas, Auburn University
• Host: Mihaly Horanyi
• Abstract: Over the last three decades, plasma scientists have learned how to control a new type of plasma system known as a “complex” or “dusty” plasma. These are four-component plasma systems that consist of electrons, ions, neutral atoms, and charged, solid, nanometer- to micrometer-sized particles. The presence of these microparticles allow us to “tune” the plasma to have solid-like, fluid-like, or gas-like properties. This means that dusty plasmas are not just a fourth state of matter – they can take on the properties of all four states of matter. 
• From star-forming regions to planetary rings to fusion experiments, charged microparticles can be found in many naturally occurring and man-made plasma systems. Therefore, understanding the physics of dusty plasmas can provide new insights into a broad range of astrophysical and technological problems. This presentation introduces the physical properties of dusty plasmas – focusing on how the small charge-to-mass ratio of the charged microparticles gives rise to many of the characteristics of the system. In particular, dusty plasmas can be used to study a variety of processes in non-equilibrium or dissipative systems such as self-organization and energy cascade as well as a variety of transport and instability mechanisms. This presentation will discuss results from our studies of dusty plasmas in high (B ≥ 1 T) magnetic fields using the Magnetized Dusty Plasma Experiment (MDPX) device at Auburn University and in microgravity experiments using the Plasmakristall-4 (PK-4) laboratory on the International Space Station.

March 24 — "Creating and measuring the elusive Majorana fermion"

• Presenter: Vidya Madhavan, University of Illinois Urbana-Champaign
• Host: Dan Dessau
• Abstract: In 1937, Ettore Majorana predicted the existence of a special class of fermions where the particle and the anti-particle are identical. However, with the possible exception of neutrinos, there are no known fundamental particles that belong to this class. The possible realization of Majorana fermions as quasiparticle excitations in solids has rekindled interest in these particles, especially since Majorana states in solids may be useful as fault tolerant qubits for quantum information processing. While most studies have focused on Majorana bound states which can serve as topological qubits, more generally, akin to elementary particles, Majorana fermions can propagate and display linear dispersion. This talk is focused on recent work in realizing Majorana modes in condensed matter systems. I will first describe in detail the conditions under which such states can be realized and what their signatures are. I will then show scanning tunneling microscopy data on 1D domain walls and step edges in two different superconductors, which might potentially be the first realizations of dispersing Majorana states in 1D.

March 31 — "On Ising's model of ferromagnetism"

• Presenter: Peter Armitage, Johns Hopkins University
• Host: Rahul Nandkishore
• Abstract: The 1D Ising model is a classical model of great historical significance for both classical and quantum statistical mechanics. Developments in the understanding of the Ising model have fundamentally impacted our knowledge of thermodynamics, critical phenomena, magnetism, conformal quantum field theories, particle physics, and emergence in many-body systems. Despite the theoretical impact of the Ising model there have been very few good 1D realizations of it in actual real material systems. However, it has been pointed out recently, that the material CoNb2O6, has a number of features that may make it the most ideal realization we have of the Ising model in one dimension. In this talk I will discuss the surprisingly complex physics resulting in this simple model and review the history of "Ising's model" from both a scientific AND human perspective. In the modern context I will review recent experiments by my group and others on CoNb2O6. In particular I will show how low frequency light in the THz range gives unique insight into the tremendous zoo of phenomena arising in this simple model system.

April 7 — "A brief history of time crystals"

• Presenter: Shivaji Sondhi, Princeton University
• Host: Rahul Nandkishore
• Abstract: A time crystal should break time translation symmetry in the same sense as a spatial crystal breaks spatial translation symmetry. While this is an intuitively appealing idea, it also runs up against the tendency of macroscopic systems to equilibrate which is why perpetual motion machines are not commonly available. I will discuss the history of this idea and how, in the recent efflorescence of non-equilibrium quantum dynamics a phase with a family resemblance to this idea was discovered. It requires working out of quilibrium, generalizing the idea of a phase and breaking a discrete time translation symmetry - but at the end of it all we have a genuinely new macroscopic and surprisingly robust phenomenon. I will also report on the status of its experimental realization.

April 14 — "Quantum Many-body theory in the Quantum Information era"

• Presenter: Matthew Fisher, University of California, Santa Barbara
• Host: Rahul Nandkishore
• Abstract: Traditionally, quantum many-body theory has focused on ground states and equilibrium properties of spatially extended systems, such as electrons and spins in crystalline solids. In recent years \noisy intermediate scale quantum computers" (NISQ) have emerged, providing new opportunities for controllable non-equilibrium many-body systems. In such dynamical quantum systems the inexorable growth of non-local quantum entanglement is expected, but monitoring such open systems (by making projective measurements) can compete against entanglement growth. In this talk I will describe recent theoretical work exploring the behavior of \hybrid" quantum circuits consisting of both unitary gates and projective measurements. These circuits can be shown to exhibit a novel quantum dynamical phase transition between a weak measurement phase and a quantum Zeno phase. Detailed properties of the weak measurement phase - including relations to quantum error correcting codes - and of the critical properties of this novel quantum entanglement transition will be described.

April 21 — "Element synthesis and neutrinos in neutron star mergers"

• Presenter: Gail McLaughlin, NC State University
• Host: Paul Romatschke
• Abstract: The merging of two neutron stars is a true multimessenger event which includes gravitational waves, an electromagnetic signal and the emission of enormous numbers of neutrinos. In order to understand these signals we need a careful accounting of the microphysics that occurs during and after the merger. I will focus on the elements produced in these objects and the effect of two aspects of this microphysics; nuclear models/reactions and neutrino flavor transformation physics. In particular, I will discuss the importance of new developments in these areas to predictions of kilonova light curves and the astrophysical origin of the r-process.

April 28 — "Superconductivity in the Hubbard Model"

• Presenter: Steve Kivelson, Stanford University
• Host: Rahul Nandkishore
• Abstract: A search on Google Scholar for “Hubbard Model” yields a million results!  Since the discovery of high temperature superconductivity in the cuprates, much of the discussion concerning the mechanism of superconductivity in these (and other) materials has involved stories about the behavior of this paradigmatic model.  Real materials are complicated, with observable behaviors that are sensitive to all sorts of microscopic details, leading to debates over what is essential and what is a herring of one color or another.  However, as will be discussed in this talk, some conditions for the existence of superconductivity in the Hubbard model can be addressed clearly, using theoretical tools that are controlled and unambiguous in appropriate limits.

For more information about colloquia this semester, contact: Rahul Nandkishore.

Colloquia are Wednesdays at 9:00 a.m. via Zoom, unless otherwise noted.

August 19 — "Conformational change in matter and mind"

• Presenter: Danielle Bassett, University of Pennsylvania
• Host: Mike Ritzwoller
• Time: 4:00 P.M. MDT

August 26 — "A Surprisingly Promising Approach to a Fundamental Theory of Physics"

• Presenter: Stephen Wolfram, Wolfram Research
• Host: Rahul Nandkishore
• Time: 4:00 P.M. MDT

September 2 — "Driven Quantum Materials"

• Presenter: Andrea Cavalleri, Max Planck Hamburg
• Host: Dan Dessau
• Time: 9:00 A.M. MDT
• Abstract: I will discuss how coherent electromagnetic radiation at infrared and TeraHertz frequencies can be used to drive collective excitations in complex solids. Important examples involve the nonlinear control of the crystal lattice, to induce atomic-structural deformations often not possible in equilibrium, and have been used by our group to induce interesting collective states at high temperatures, including magnetic order, ferroelectricity and non-equilibrium superconductivity. 

September 9 — "Topological photonics"

• Presenter: Mohammed Hafezi, University of Maryland and JQI
• Host: Ana Maria Rey 
• Time: 9:00 A.M. MDT
• Abstract: There are many intriguing physical phenomena that are associated with topological features -- global properties that are not discernible locally. The best-known examples are quantum Hall effects in electronic systems, where insensitivity to local properties manifests itself as conductance through edge states which are insensitive to defects and disorder. In the talk, we first discuss how similar physics can be explored with photons; specifically, how various topological model can be simulated in various photonics systems, from ring resonators to photonic crystals. We then discuss how the integration of strong optical nonlinearity can lead to intriguing quantum phenomena, such as a topological source of quantum light and chiral quantum optics. These results may enable the development of classical and quantum optical devices with built-in protection for next-generation optoelectronic and quantum technologies. In the end, we discuss an emerging field at the interface of quantum optics and correlated electron systems, with the goal of creating and manipulating many-body states of light-matter hybrids with new functionalities, such as high-Tc superconductors.  

September 16 — "Undecidability in Quantum Physics: Hilbert’s Second and Sixth Problems Meet"

• Presenter: David Perez-Garcia, Universidad Complutense de Madrid
• Host: Rahul Nandkishore
• Time: 9:00 A.M. MDT
• Abstract: The pioneering work of Goedel and Turing in the 30s showed that there exist problems in mathematics and computer science that cannot be solved. They are called undecidable. Since then, several problems in physics have been shown to be undecidable too. In this talk I will show that many interesting properties of a quantum many body system are indeed undecidable. This negative result has, however, a positive side. It predicts the existence of a new effect that we name "size-driven quantum phase transition”. I will present this effect, its characteristic features, as well as our recent ideas to try to observe it.

September 23 — "Fracton order: From quantum hard drive to foliated manifold"

• Presenter: Xie Chen, Caltech
• Host: Rahul Nandkishore
• Time: 9:00 A.M. MDT
• Abstract: One major open problem in quantum information theory is how to build a quantum hard drive, i.e. a quantum mechanical system that can store quantum information reliably for a very long time without active error correction. No completely satisfying solution to this problem has been found, but in the search for possibilities a whole new class of quantum many-body models have been discovered with exotic properties that have never been seen before. This new class of models, dubbed the “fracton” models, host point excitations that cannot move freely, have robust ground state degeneracy that increases with system size, and have unusually slow thermalization dynamics even in the absence of any disorder. In this talk, I will introduce some of the most important fracton models and present a systematic framework for characterizing their universal properties. By realizing that the model properties depend on the foliation structure of the underlying manifold, we are able to reveal the hidden equivalence relation between many seemingly different models and identify the direction of search for new nontrivial features.

September 30 — "Drawing the line between kinematics and dynamics in special relativity and in quantum mechanics"*

• Presenter: Michel Janssen, University of Minnesota
• Host: Allan Franklin
• Time: 9:00 A.M. MDT
• Abstract: The mathematical equivalence of matrix and wave mechanics papers over an important difference in what Heisenberg and Schrödinger took to be their key insights. For Schrödinger it was that atomic physics calls for an underlying wave theory just as geometric optics had in the 19th century. For Heisenberg it was that atomic physics calls for a new general framework for doing physics just as electrodynamics had in the early-20th century. Following Heisenberg rather than Schrödinger, I present a few case studies in special relativity and quantum mechanics in which problems that seem to call for dynamical solutions were solved instead by an appeal to the new kinematical frameworks introduced by these new theories. I use these case studies to bring out some parallels between the standard take on special relativity and a more controversial information-theoretic take on quantum mechanics and then use the former to argue for the latter.    
  *Based on joint work with Michael Janas and Mike Cuffaro

October 7 — "The Higgs Boson as a Tool for Discovery at the LHC"

• Presenter: Keith Ulmer, University of Colorado Boulder
• Host: Mike Ritzwoller
• Time: 9:00 A.M. MDT
• Abstract: The Large Hadron Collider (LHC) at CERN currently provides the highest energy particle collisions ever produced in a laboratory. These collisions were reconstructed and analyzed by the CMS and ATLAS experiments to claim the discovery of the Higgs Boson in 2012, thus completing the Standard Model of particle physics. While incredibly successful in describing observed phenomena in elementary particle interactions, the Standard Model is known to be an incomplete theory. This talk explores recent efforts to use the Higgs Boson itself as a tool for discovering new physics beyond the Standard Model with the aim of understanding dark matter and electroweak symmetry breaking. I will conclude with a discussion of the future discovery potential of the LHC and some of the technological advances that will drive the progress forward in the coming decade.

October 14 — "Quantum Communications and Correlations"

• Presenter: Graeme Smith, University of Colorado Boulder 
• Host: Rahul Nandkishore
• Time: 9:00 A.M. MDT
• Abstract: Sending and receiving quantum signals can boost the performance of communication systems and even let you do things that are impossible with classical signals. For example, a quantum communication channel can be used to transmit quantum information or establish entanglement between distant parties.  Even if the channel is noisy, quantum error correcting codes can allow information to be transmitted essentially perfectly. In this talk, I’ll discuss several questions about the theory of quantum communication: How much noiseless information can a noisy channel transmit? How much noise is too much? Can entanglement enhance the performance of purely classical networks? How can we quantify the correlations in a quantum state? And what does any of this have to do with physics anyway?

October 21 — "Extreme Plasma Astrophysics: A Shining New Frontier"

• Presenter: Dmitri Uzdensky, University of Colorado Boulder
• Host: Mike Ritzwoller
• Time: 9:00 A.M. MDT
• Abstract: Many spectacular phenomena in the high-energy Universe, including bright, rapid gamma-ray flares, are powered by complex collective processes in plasmas around relativistic objects: neutron stars (NSs) and black holes (BHs). While our understanding of such processes has benefitted from knowledge obtained in traditional (e.g., space or magnetic fusion) plasma research, the physical conditions in plasmas around BHs and NSs are so extreme that conventional plasma-physical intuition often fails there and a richer physical framework is required. Extreme astrophysical plasmas are relativistic, interact strongly with radiation, and may be subject to QED (e.g., pair-production and ultra-strong magnetic fields) effects. Understanding how collective plasma processes (e.g., waves, magnetic reconnection, turbulence, shocks) operate in the presence of these “exotic” physics effects is the main charge of Extreme Plasma Astrophysics. Rapid progress in exploring and conquering this exciting new frontier is now being made, stimulated by the growing astrophysical motivation and enabled by concerted, vigorous theoretical efforts and recent computational breakthroughs due to the advent of novel first-principles relativistic kinetic plasma codes incorporating radiation reaction and QED pair creation. Laser plasma experiments may soon also contribute to this revolution.  As I will describe in this talk, our group at 91��ý has been playing a leading role in advancing this burgeoning new field. I will illustrate our recent progress with our kinetic studies of relativistic magnetic reconnection with radiation and pair creation in the context of BH coronae and jets, and of NS magnetospheres, and of radiative relativistic turbulence. I will also outline the future directions of this vibrant Extreme Plasma Astrophysics research program. 

October 28 — "Nonlinear Hall effect and dissipationless thermoelectric Hall effect"

• Presenter: Liang Fu, Massachusetts Institute of Technology
• Host: Rahul Nandkishore
• Time: 9:00 A.M. MDT
• Abstract: Studies of various types of Hall effects have led to great advances in solid state physics. In this talk, I will describe two new Hall phenomena. First, a nonlinear Hall effect at zero magnetic field, whereby an applied electric field induces a transverse current at second order. This effect arises from the Berry curvature of electrons under acceleration in inversion-breaking systems. I will discuss potential application of nonlinear Hall effect in terahertz detection and wireless power transmission. Second,  thermoelectric Hall effect in a quantizing magnetic field, whereby a temperature gradient induces a transverse electrical current. I will show that thermoelectric Hall conductivity in Dirac/Weyl semimetals under quantum limit has a dissipationless origin and is quantized in appropriate units.

November 4 — "The challenge of quantum error correction"

• Presenter: Barbara Terhal, Delft University of Technology
• Host: Graeme Smith
• Time: 9:00 A.M. MDT
• Abstract: We discuss the ideas behind quantum error correction and fault-tolerance and how it differs from classical error correction. We review some experimental progress and challenges for error correction with superconducting qubits, including bosonic error correction with the so-called GKP (Gottesman-Preskill-Kitaev, 2001) code.

November 11 — "Progress and Prospects for the Second Quantum Revolution"

• Presenter: Steve Girvin, Department of Physics & Yale Quantum Institute, Yale University and Director, Co-Design Center for Quantum Advantage, Brookhaven National Laboratory
• Host: Ana Maria Rey
• Time: 9:00 A.M. MDT
• Abstract: The first quantum revolution brought us the great technological advances of the 20th century—the transistor, the laser, the atomic clock and GPS, the global positioning system.  A ‘second quantum revolution’ is now underway based on our relatively new understanding of how information can be stored, manipulated and communicated using strange quantum hardware that is neither fully digital nor fully analog.  We now realize that 20th century hardware does not take advantage of the full power of quantum machines.  This talk will give a gentle introduction to the basic concepts that underlie this quantum information revolution and describe recent remarkable experimental progress in the race to build quantum machines for computing, sensing and communication.

November 18 — "Weighing Neutrinos"

• Presenter: Joe Formaggio, Massachusetts Institute of Technology
• Host: Keith Ulmer
• Time: 9:00 A.M. MDT
• Abstract: Neutrino oscillation experiments performed throughout the latter half of the twentieth century have yielded valuable information about the nature of neutrinos. The data gathered has provided the first positive evidence for neutrinos having mass, the first alteration of the Standard Model. Yet, the mass scale of neutrinos continues to be experimentally elusive. My talk will present the most recent results and ongoing future efforts to measure the mass scale of neutrinos.

November 25 — Fall Break, No Colloquium

December 2 — "String theory, quark-gluon plasma, and the complex life of hydrodynamic modes"

• Presenter: Andrei Starinets, Oxford University
• Host: Paul Romatschke
• Time: 9:00 A.M. MDT
• Abstract: Heavy ion collision programs at RHIC and LHC and future programs at FAIR and NICA instigated a renewal of theoretical interest in relativistic hydrodynamics as an effective theory. The collisions create hot and dense nuclear matter known as strongly coupled quark-gluon plasma: the parameters of the plasma can be measured with precision, but its theoretical study is hindered by the strong non-perturbative effects of thermal QCD. At the same time, some strongly interacting gauge theories at finite temperature and density can be investigated via the gauge theory - string theory duality, connecting them to classical black holes in higher dimensions. In the talk, we review the current status of this line of research, and reflect on recent advances in our understanding of the gauge theory plasmas in the regime of intermediate coupling provided by dual gravity with higher derivative terms, with an ultimate goal of interpolating between the weak coupling kinetic regime and the strong coupling regime. We also discuss the issue of convergence of the series representing collective excitations of a strongly coupled fluid and the related phenomenon of a black hole quasinormal spectrum level-crossing.

December 9 — "Phase transitions in the complexity of simulating random shallow quantum circuits"

• Presenter: Aram Harrow, Massachusetts Institute of Technology
• Host: Rahul Nandkishore
• Time: 9:00 A.M. MDT
• Abstract: The appeal of quantum computing is based on the fact that simulating N quantum systems on a classical computer takes time exponential in N.  This exponential hardness is known to hold even for shallow quantum circuits, meaning unitary dynamics that run for a constant amount of time.  We show that when the quantum circuits are made of random gates on a 2D geometry, they are not always exponentially hard to simulate.  Instead, we give evidence for a phase transition in computational difficulty as the depth and local dimension are varied.  Our evidence consists of (1) fast classical simulations of random circuits on a 400x400 grid of qubits, (2) a mapping to the order/disorder transition in an associated stat mech model, and (3) a proof that some circuit families are easy to simulate approximately but hard to simulate exactly.  Our algorithms are based on tensor network contraction and mapping the 2D random unitary circuit to a 1D process consisting of alternating rounds of random local unitaries and weak measurements.

  This is based on https://arxiv.org/abs/2001.00021 which is joint work with John Napp, Rolando La Placa, Alexander Dalzell, and Fernando Brandao.

For more information about colloquia this semester, contact: Rahul Nandkishore.

Colloquia are Wednesdays at 4:00 p.m. in DUAN G1B20, unless otherwise noted.

Coffee, tea and cookies will be available before regular colloquia beginning at 3:45 p.m. in DUAN G1B31.

January 15 — "Quantum Clocks With Ultracold Molecules"

• Presenter:  Tanya Zelevinsky, Columbia University
• Host: David Leibrandt
• Abstract: As quantum science moves beyond controlling atoms, diatomic molecules can be created and optically trapped at ultracold temperatures.  This allows exquisite control over the quantum states of the molecules.  Using this platform, we can generate long-lived coherent superpositions of far-separated molecular quantum states.  The capability to create these superpositions results in a precise clock that is based on molecular vibrations.  The vibrational molecular clock allows interatomic force measurements with resolution exceeding a part per trillion, with a potential to improve the constraints on new short-range mass-dependent forces.  The full control of the molecular quantum states also helps to shed light on ultracold chemical reactions that proceed as matter waves, and to enable a range of new experiments in fundamental physics and chemistry.

January 22 — "High performance Single Photon Detectors using superconducting devices"

• Presenter: Sae Woo Nam, NIST
• Host: David Leibrandt
• Abstract: Single-photon detectors are increasingly becoming an essential tool for a wide range of applications in physics, chemistry, biology, communications, medicine, and remote sensing. Ideally, a single photon detector generates a measurable signal only when a single photon is absorbed. Furthermore, the ideal detector would have 100% detection efficiency, no false positive (dark counts), and transform-limited timing resolution. Recently, there has been tremendous progress in the development of superconducting devices (Superconducting Nanowire Single Photon Detector (SNSPD or nSSPD) and superconducting Transition-Edge Sensor (TES) that nearly achieve this ideal performance.   Superconducting detectors such as the SNSPD and TES offer unprecedented performance not achievable using photomultiplier tubes and semiconducting devices.  Yet, there is still tremendous room for improvement to these devices to make them more accessible for even more applications.  I will describe recent advances in new materials, new optical packaging, new cryogenic packaging, and new readout techniques should enable megapixel scale arrays of high performance superconducting detectors.

January 29 — "Mapping the Milky Way's Dark Matter Halo with Gaia"

• Presenter: Mariangela Lisanti, Princeton University
• Host: Keith Ulmer
• Abstract: The Gaia mission is in the process of mapping nearly 1% of the Milky Way’s stars. This data set is unprecedented and provides a unique view into the formation history of our Galaxy and its associated dark matter halo. I will review results based on the most recent Gaia data release, demonstrating how the evolution of the Galaxy can be deciphered from the stellar remnants of massive satellite galaxies that merged with the Milky Way early on. The recent advancements in our understanding of the Galaxy's evolution suggest that a component of the local dark matter is not in equilibrium, as typically assumed, and instead exhibits distinctive dynamics. The updated dark matter map built from the Gaia data has ramifications for direct detection experiments, which search for the interactions of these particles in terrestrial targets.

February 5 — "Making Sound Matter"

• Presenter: Konrad Lehnert, JILA, University of Colorado, Boulder
• Host: Rahul Nandkishore
• Abstract: The mechanical vibrations of man-made and macroscopic objects have recently come under quantum control. It is now possible to resolve the individual quantum particles of sound and to measure, prepare, and manipulate the quantum state of acoustical resonators. In my colloquium, I will describe how these fragile objects were tamed and how this new capability might be applied. Although there are some intriguing possibilities for exploiting quantum sound, our primary motivation is simply that sound is different than light. Unlike electrical and optical systems, which are governed by fundamental equations of electromagnetism, acoustical phenomena are described by the equations of elastic waves in solid bodies. They are subject to different limitations and can reach different regimes of behavior. The speed of sound in a solid material is 100,000-fold slower than light, elastic waves do not propagate through vacuum, they are fundamentally a tensor rather than vector phenomenon, and they couple to atom-like defects through strain rather than electrical or magnetic dipole interactions. These facts have consequences for quantum information science that we have yet to fully understand. 

February 12 — "Quantum supremacy using a programmable superconducting processor"

• Presenter: John Martinis, Google and UC Santa Barbara
• Host: Rahul Nandkishore
• Abstract: The promise of quantum computers is that certain computational tasks might be executed exponentially faster on a quantum processor than on a classical processor. A fundamental challenge is to build a high-fidelity processor capable of running quantum algorithms in an exponentially large computational space. Here we report the use of a processor with programmable superconducting qubits to create quantum states on 53 qubits, corresponding to a computational state-space of dimension 2^53  (about 10^16). Measurements from repeated experiments sample the resulting probability distribution, which we verify using classical simulations. Our Sycamore processor takes about 200 seconds to sample one instance of a quantum circuit a million times—our benchmarks currently indicate that the equivalent task for a state-of-the-art classical supercomputer would take approximately 10,000 years. This dramatic increase in speed compared to all known classical algorithms is an experimental realization of quantum supremacy for this specific computational task, heralding a much-anticipated computing paradigm.

February 19 — "Quantum speed limits on spin transport in Fermi gases"

• Presenter: Joseph Thywissen, University of Toronto
• Host: Ana Maria Rey
• Abstract: The world around us is not in equilibrium, but slowly (or quickly) getting there through transport of conserved quantities such as energy, charge, and momentum. However transport is challenging to calculate ab initio, leaving many open questions (such as high-temperature superconductivity) and room for new paradigms (such as holographic duality). Ultracold atoms provides an ideal platform for the study of non-equilibrium quantum physics, since samples are isolated from the environment, and the strength of interactions can be tuned. We have observed a kind of quantum "speed limit" on the rate of spin diffusion in a Fermi gas. I will discuss how this is measured, and its relation to various conjectured bounds. 

February 26 — "Towards an Optical Clock with Highly Charged Ions"

• Presenter: Piet Schmidt, Leibniz Universität Hannover
• Host: David Leibrandt
• Abstract: Highly charged ions (HCI) have many favorable properties for tests of fundamental physics and as potential next-generation optical atomic frequency standards [1]. For example, narrow optical fine-structure transitions have smaller polarizabilities and electric quadrupole moments, but much stronger relativistic, QED and nuclear size contributions to their binding energy compared to their (near) neutral counterparts. Therefore, HCI have been found to be among the most sensitive atomic species to probe for a possible variation of the fine-structure constant or dark matter coupling.
  HCI can readily be produced and stored in an electron beam ion trap (EBIT). There, the most accurate laser spectroscopy on any HCI was performed on the 17 Hz wide fine-structure transition in Ar13+ with 400 MHz resolution, lagging almost twelve orders of magnitude behind state-of-the-art optical clocks. This was primarily limited by Doppler broadening of the megakelvin hot ion plasma in the EBIT [2]. The lack of a suitable optical transition for laser cooling and detection can be overcome through sympathetic cooling with a co-trapped Be+ ion [3]. Techniques developed for quantum information processing with trapped ions can be used to perform quantum logic spectroscopy [4]: A series of laser pulses transfers the internal state information of the Ar13+ ion after spectroscopy onto the Be+ ion for efficient readout.
  We present the first coherent laser spectroscopy of an HCI. Ar13+ are extracted from a compact EBIT [5], charge-to-mass selected and injected into a cryogenic Paul trap containing a crystal of laser-cooled Be+ ions [6]. By removing excess Be+ ions, a crystal composed of a Be+/Ar13+ ion pair is obtained. Results on sympathetic ground state cooling and quantum logic spectroscopy of the Ar13+ P1/2-P3/2 fine-structure transition at 441 nm will be presented, improving the precision of the observed line center by more than eight orders of magnitude. Furthermore, excited state lifetimes and the first high-accuracy measurement of excited state g-factor demonstrate the versatility of the technique to access all relevant atomic parameters [7]. Finally, we have started to perform frequency measurements of this transition, including first estimates of systematic uncertainties.
  [1]    M. G. Kozlov et al., Rev. Mod. Phys. 90, 045005 (2018).
  [2]    I. Draganić et al., Phys. Rev. Lett. 91, (2003).
  [3]    L. Schmöger et al., Science 347, 1233–1236 (2015).
  [4]    P. O. Schmidt et al., Science 309, 749–752 (2005).
  [5]    P. Micke et al., Rev. Sci. Instrum. 89, 063109 (2018).
  [6]    T. Leopold et al., Rev. Sci. Instrum. 90, 073201 (2019).
  [7]    P. Micke et al., Nature, in print.

**Special Physics Colloquium** Friday, February 28 — "Scientific espionage, open exchange, and American competitiveness"

• Note Special Location: DUAN G130
• Presenter: Professor Xiaoxing Xi, Temple University
• Host: Gang Cao
• Abstract: As the federal government warns universities and colleges about the risk of China to academia in the United States, professors, scientists, and students of Chinese ethnic origin as well as those engaging in academic collaborations with Chinese colleagues are under heightened scrutiny. In 2015, I became a casualty of this campaign despite being innocent. This experience gave me insights into the challenges Chinese scientists face and the immediate threat to the open environment in fundamental research on university campuses. Based on my personal experience and the recent events involving Chinese scientists in the US and China’s talent programs, I urge the audience to join an increasing number of scientists, university administrators, and professional societies in speaking up to defend liberty, support reaffirmation of NSDD-189 on open fundamental research, and safeguard America's research enterprise.
• Bio: Xiaoxing Xi is Laura H. Carnell Professor of Physics at Temple University, and former chair of the Physics Department. Prior to joining Temple in 2009, he was Professor of Physics and Materials Science and Engineering at the Pennsylvania State University. He received his B.Sc. in physics from Peking University in 1982, and his PhD degree in physics from Peking University and Institute of Physics, Chinese Academy of Sciences, in 1987. After several years of research at Karlsruhe Nuclear Research Center, Germany, Bell Communication Research/Rutgers University, and University of Maryland, he joined the Physics faculty at Penn State in 1995. His research focuses on the materials physics of oxide, boride, and 2-dimensional dichalcogenide thin films. His notable contributions include early work on epitaxial thin films, heterostructures, and electric-field effect in high temperature superconductors, pioneering work on lattice dynamics in ferroelectric thin films and nanostructures, invention of hybrid physical-chemical vapor deposition for magnesium diboride, and development of atomic layer-by-layer laser molecular beam epitaxy for oxide thin films and interfaces. He is author of over 340 refereed journal articles and 3 U.S. patents in the area of superconductivity. He is a Fellow of the American Physical Society.
  Since 2015, he has actively advocated for open fundamental research and against racial profiling. He was recently awarded the 2020 Andrei Sakharov Prize by the American Physical Society.

March 4 — "Controlling trapped ions without lasers"

• Presenter: Daniel Slichter, NIST
• Host: David Leibrandt
• Abstract: Trapped atomic ions are a leading platform for quantum computing and sensing experiments.  These experiments require manipulation of the ions' quantum states and the creation of entanglement between ions, both of which are typically accomplished using laser beams.  However, there are drawbacks to this approach, including errors due to photon scattering and the complexity of the required laser sources.  Our group performs quantum state control using oscillating radiofrequency and microwave-frequency magnetic and electric fields, and their near-field gradients, instead of laser beams.  A critical element is the use of a microfabricated surface-electrode ion trap, which holds the ions roughly 30 µm above the electrodes generating the control fields.  We use low-power resonant laser beams for cooling, optical pumping, and readout.  I will give a general introduction to trapped ion quantum computing, and then describe several of our recent results, including the creation of high-fidelity entangled states of two ions using only microwave and radio-frequency control fields, and the use of radio-frequency electric fields to generate squeezed states of ion motion for sensing applications and to amplify phonon-mediated ion-ion interactions. 

**Canceled** March 11 

• Presenter: 
• Host: 
• Abstract: 

**Special Physics Colloquium** Thursday, March 12 — "Bacterial swimming: internal clock and fluid environment"

• NOTE SPECIAL LOCATION: DUAN G126
• NOTE SPECIAL TIME: 1:00 P.M.
• Presenter: Nuris Figueroa-Morales, Pennsylvania State University
• Host: Noel Clark
• Abstract: Bacterial suspensions are ubiquitous in naturally occurring and man-made scenarios. Their transport determines how infections spread and microbiota organize in guts or in soils. In addition, bacteria can turn common passive materials into novel active materials with emergent properties. In this talk, I will discuss our latest and ongoing research on bacterial transport, from statistical properties of their run-and-tumble dynamics, to macroscopic migration in microfluidic channels, and swimming in non-isotropic synthetic and biological liquids. Our results unravel bacterial strategies for space exploration and provide insight into the design and control of out-of-equilibrium materials.

**Special Physics Colloquium** Monday, March 16 — "Polymers and Parkinson’s: Elucidating Protein Function through Soft Matter Paradigms and Techniques"

• ONLINE LECTURE
• NOTE SPECIAL TIME: 2:00 P.M.
• Presenter: Dr. Peter J. Chung, University of Chicago 
• Host: Noel Clark
• Abstract: Despite being unequivocally linked to Parkinson’s disease, the function of a-synuclein remains unclear beyond transiently binding to the lipid membrane of synaptic vesicles (organelles filled with neurotransmitters). This is due, in part, to its intrinsically disordered nature; a-synuclein does not fold into a globular structure and instead behaves much like a biopolymer. While precluding traditional characterization methods, this makes a-synuclein incredibly amenable to investigation via a polymer physics framework. First, through purpose-designed membrane nanoparticles and advanced synchrotron X-ray methods I will demonstrate that a-synuclein binds to and collectively interacts to sterically-stabilize membrane surfaces, a biological manifestation of polyelectrolyte-stabilized colloids. I will then reconcile observed transient binding to synaptic vesicles by establishing that a-synuclein preferentially binds to osmotically-stressed membranes (a proxy for neurotransmitter-filled synaptic vesicles), a newly discovered biophysical function by which a-synuclein interrogates organelle contents. Utilizing these insights, I will contextualize a-synuclein as a guidepost that spatiotemporally directs non-equilibrium synaptic vesicles, a conferred function uniquely possible through its polymeric properties. 

**Special Physics Colloquium** Monday, March 16 — "Searching for the invisible: how dark forces shape our Universe"

• NOTE SPECIAL LOCATION: DUAN G125
• Time: 4:00 p.m.
• ONLINE LECTURE
• Presenter: Dr. Katelin Schutz, Massachusetts Institute of Technology 
• Host: Ed Kinney
• Abstract: How different would our Universe look with the addition of extra particles and forces beyond what we know? We already have ample gravitational evidence for at least one invisible new particle that has properties unlike any particle we have previously discovered. It is possible that this dark matter is made of many different kinds of particles that experience forces unlike the ones we are familiar with in day-to-day life. If these forces only act on dark particles, it may be difficult to discover them and learn more about what is happening in this dark sector. However, dark matter and visible matter do interact gravitationally at the very minimum, and this fact alone might be a good reason not to lose hope. If there are dark forces affecting the distribution of dark matter in our Universe, then that distribution will gravitationally affect the visible matter that we can see. I will discuss how the gravitational portal between dark and visible matter can constrain dark matter theories where dark matter can dissipate energy, can scatter with itself (elastically or inelastically), or can be born non-thermally in the moments after the Big Bang. I will demonstrate this constraining power by harnessing synergies between astrophysical systems including the local Milky Way, nearby dwarf galaxies, galaxy clusters, and large-scale cosmological structures. 

**Canceled** - March 18

• Presenter: 
• Host: 
• Abstract: 

**Special Physics Colloquium** Thursday, March 19 — "Dynamics and rheology of 2-d membranes and soft complex interfaces"

• NOTE SPECIAL TIME: 12:30 p.m.
• ONLINE LECTURE
• Presenter: Dr. Mehdi Molaei, University of Pennsylvania 
• Host: Noel Clark
• Abstract: Soft interfaces and membranes control many biological processes. They form the complex membranes of living cells, and they are hot spots to adsorb biomolecules and active colloids. In the first part of my talk, I discuss how we characterize the dynamics and mechanics of a 2-d model lipid membrane and the complex plasma membrane of liver cells. The plasma membrane displays time-dependent undulations, which are controlled by its bending modulus, excess area, and membrane tension. Together, these physical characteristics couple to and modulate vesicle trafficking, membrane adhesion, and actin-based motility. While the complex physics of lipid membranes is relatively well understood in reconstituted form, the understanding of the dynamics and mechanics of plasma membranes in living cells remains at an early stage. We have developed a new nanoscale rheology approach, based on angle resolved dark field microscopy, to probe interfacial rheology, tension, and curvature fluctuation of the plasma membrane. The goal of this study is to develop a high throughput assay for identification of cancer cells. In the second part of my talk, I present the new approach to study the mechanics of complex fluid interfaces. Adsorbed surfactant, proteins, and macromolecules on interfaces form thin layers with complex structures and with wide range of viscoelasticities. We have developed a new approach for manipulating fluid interfacial systems that enables simultaneous tensiometry to measure surface pressure and high-performance interfacial shear microrheology. We study an adsorbed layer of protein on the air-water interfaces as a model system. The power law frequency dependent shear modulus at high surface pressure reveals the soft glassy response of protein-laden interfaces. To understand complex flow fields established on fluid interfaces, which depend on interface viscoelasticity, (in)compressibility, and hydrodynamic coupling with bulk fluids, we measure displacement fields induced by the thermal motion of passive particles in interfacial films. 

March 25 — Spring Break, No Colloquium

**Special Physics Colloquium** Monday, March 30 — "Bridging multi-scale biology with the physics of solids, fluids and information"

• NOTE SPECIAL TIME: 12:30 p.m.
• ONLINE LECTURE
• Presenter: Dr. Arnold JTM Mathijssen, Stanford University
• Host: Noel Clark
• Abstract: Biological systems flourish through collective functionality, by self-assembling into cells, tissues, flocks and parliaments. Understanding this multi-scale organization also lies at the heart of modern engineering and medicine: Pathologies can arise from deficiencies in collective functionality, while active and adaptive materials can be designed from controlling systems out of equilibrium. In this talk, I will overview our recent work on building first-principle theories, numerical tools, and experiments for studying the fascinating physics of life. We will first focus on reliable communication in ultra-fastbiology, exemplified by the discovery of ‘hydrodynamic trigger waves’ [1]. Second, we will discuss bacterial contamination dynamics, which is enhanced by the ability of cells to swim against flows [2]. Third, we consider the role of topology in biofunctionality, especially in ‘active carpets’ like ciliary arrays [3]. These insights open up exciting new avenues towards unravelling synthetic and biological active matter, through collective functionality, together.
  [1.] Mathijssen AJTM, Culver J, Bhamla MS, Prakash M, “Collective intercellular communication through ultra-fast hydrodynamic trigger waves,” Nature 571, 560-564 (2019)
  [2.] Mathijssen AJTM, Figueroa-Morales N, Junot G, Lindner A, Clement E, Zöttl A, “Oscillatory surface rheotaxis of swimming E. coli bacteria,” Nat. Comm. 10, 3434 (2019)
  [3.] Ramirez-San Juan G, Mathijssen AJTM, He M, Jan L, Marshall WF, Prakash M, “Multi-scale heterogeneity enhances mucus clearance in mouse airways”, Nat. Phys., accepted (2020)

**Special Physics Colloquium** Monday, March 30 — "Quantum Error Correction with Schrödinger Cats"

• NOTE SPECIAL TIME: 4:00 p.m.
• ONLINE LECTURE
• Presenter: Dr. Shruti Puri, Yale University
• Host: Ed Kinney
• Abstract: We are currently in the midst of a second quantum revolution where the fundamental laws of quantum mechanics are being applied to develop computers for solving otherwise intractable problems. Perhaps more remarkable than quantum computing itself is the concept of quantum error correction (QEC). In fact, with fault-tolerant QEC it is possible to implement an arbitrarily long quantum algorithm accurately even with a sequence of faulty operations on imperfect quantum hardware. However, fault-tolerant protocols come at the cost of dauntingly large overheads, pushing practical realization of useful quantum computers out of reach. In this talk I will present an elementary introduction to QEC. I will then introduce a new type of qubit called the Schrödinger cat qubit which is realized in a driven non-linear oscillator. The Schrödinger cat states are superpositions of two opposite phase coherent states. The driven-cat qubit has some in-built protection against errors in hardware and software. Finally, I will discuss how the inherent noise protection in this qubit can be exploited to significantly reduce the overheads for fault-tolerant QEC.

**Canceled** April 1

• Presenter: 
• Host: 
• Abstract: 

**Special Physics Colloquium** Thursday, April 2 — "Ultrafast laser pulses drive novel non-equilibrium dynamics in strongly-correlated materials"

• NOTE SPECIAL TIME: 10:00 a.m.
• ONLINE LECTURE
• Presenter: Dr. Phoebe Tengdin, École polytechnique fédérale de Lausanne
• Host: Ed Kinney
• Abstract: The interactions between electrons, spins, and phonons in magnets, superconductors and other strongly correlated electron materials hold the key to their emergent properties and provide a route to manipulating their phases. While the fundamental length- and time-scales for these phenomena are nanometers (exchange length) and femtoseconds (exchange splitting), tools that enable the exploration of their interactions and functional properties have only recently become available. Using a combination of ultrafast lasers, electron and x-ray pulses, we can go beyond merely observing the static properties, to manipulating a material as it undergoes a light-induced phase transition. In this talk I will describe advances in ultrafast X-ray and electron beam spectroscopy and imaging that allow us to directly observe and manipulate new phases of matter in magnetic and other strongly correlated electron materials. Specifically, I will show examples of how we can drive nonequilibrium phase transitions and observe new material physics in a half-metallic heusler alloy Co₂MnGe, a skyrmion hosting ferromagnetic insulator Cu₂OSeO₃, and in a high-temperature superconductor MgB₂.

**Special Physics Colloquium** Thursday, April 2 — "Dynamic Plasmonic and Biophotonic Devices enabled through Liquid Crystal Materials"

• NOTE SPECIAL TIME: 12:30 p.m.
• ONLINE LECTURE
• Presenter: Dr. Daniel Franklin, Northwestern University
• Host: Noel Clark
• Abstract: The next generation of displays, detectors and biosensors will be enabled by highly functional nanomaterials. Within this talk, I will summarize the fundamental and applied aspects of highly tunable plasmonic-liquid crystalline systems as applied to the visible and infrared regimes. For the visible, this results in dynamic color-changing surfaces capable of covering the entire RGB color space and which is compatible with active addressing schemes. I will then show the application of these concepts to infrared absorbers with liquid crystal and phase change materials. The later of these devices can find use in infrared data/image encoding, thermal management and camouflage. Lastly, I will demonstrate a transient/biodegradable liquid crystal laser for remote temperature sensing. Using biologically derived cholesteryl esters, I will introduce encapsulation strategies, cytotoxicity and system limits stemming from theoretical considerations. This project aims to create a new class of biocompatible NIR imaging probes with dynamic temperature sensing with additional applications including photothermal therapy with active temperature feedback. Together, these works explore the limits of liquid crystal tunable systems and the novel photonic devices they might lead to.

**Canceled** April 8

• Presenter: 
• Host:
• Abstract: 

**Special Physics Colloquium** Thursday, April 9 — "Topologically Frustrated Dynamics of Charged Macromolecules in Crowded Media"

• NOTE SPECIAL TIME: 12:30 p.m.
• ONLINE LECTURE
• Presenter: Dr. Di Jia, University of Massachusetts
• Host: Noel Clark
• Abstract: Movement of very long electrically charged macromolecules, such as DNA, dispersed in aqueous media, is a ubiquitous phenomenon relevant to life processes and formulations of new materials. The facts that such molecules are topologically correlated due to their chain connectivity and that the charges are correlated over large distances through Coulomb forces contribute to rich phenomenology which is difficult to fully understand. In spite of complications from long-range topological and electrostatic correlations, Einstein’s law of diffusion is operative and these large molecules are able to diffuse even when they are in a crowded solution or under strong confinement. As an example, a tagged chain inside a collection of fully entangled long flexible chains diffuses like a drunken snake in a tube-like environment (de Gennes’s reptation model). In contrast with expectations from the Einstein’s law, I will present a new dynamical regime of non-diffusive topologically frustrated state by experimentally eliciting very long-lived metastable states, under intermediate confinements. Why does the molecule diffuse under strong confinements (reptation) but not under weaker confinements? The origin of the new topologically frustrated dynamical state lies in the emergence of multiple correlated entropic barriers, which are so prohibitive for the diffusion of the macromolecule which is localized. I will also address how such localization effects are modulated by enthalpic interactions between guest macromolecules and their oppositely charged host gel matrix. The experimental conditions (essentially water and ambient temperatures) for realizing the new dynamical state are very similar to many biological environments in human body, indicating implications for a better understanding of biological and biologically relevant processes.

**Canceled** April 15 

April 22 — "A Ferroelectric Nematic Liquid Crystal"

• Presenter: Noel Clark, University of Colorado, Boulder
• Host: Rahul Nandkishore
• Abstract: Water is a prime example of a "polar" liquid, a classification created by Peter Debye based on his study of the dielectric response of materials made from molecules that have permanent internal electric dipole moments. Liquid water does not exhibit bulk polarity because its dipoles are oriented in  random directions, but, as Debye noted, is "polar" in the sense that the molecules have a tendency to orient with their dipoles parallel to the direction of an applied electric field. This field-induced alignment results in the strong dielectric response of water, nearly two orders of magnitude larger than that of the vacuum.  Thus water is not polar, but readily "polarizable”.
  We have recently found a three-dimensional fluid that is actually polar (RM734), its dipoles spontaneously adopting a mutual alignment parallel to one another to form a liquid in which there is bulk polarization density [1]. This polarization can be described as a 3D continuum vector field that gives the local average dipolar, and therefore molecular orientation, making this fluid a form of uniaxial nematic liquid crystal. The magnitudes of the polarization density and molecular dipole moment combine to show that, remarkably, this polar molecular alignment is nearly complete (polar order parameter ~0.93) at the lower temperatures in its nematic range (T ~ 90 ºC). As a result the molecular orientation responds strongly and in collective elastic fashion to applied field, exhibiting "giant" field-induced electro-optical and flow effects. This reorientability of the polarization and its spontaneous appearance make this phase a ferroelectric liquid. 
  First-Principles Experimental Demonstration of Ferroelectricity in a Thermotropic Nematic Liquid Crystal: Spontaneous Polar Domains and Striking Electro-Optics,
  X. Chen, E. Korblova, D. Dong, X. Wei, R.F. Shao, L.H. Radzihovsky, M.A. Glaser,  J.E. Maclennan, D. Bedrov , D.M. Walba, N.A. Clark, http://arxiv.org/abs/2003.03020, PNAS (to appear).  

**Canceled** April 29

• Presenter: 
• Host: 
• Abstract: 

For more information about colloquia this semester, contact: Rahul Nandkishore.

Colloquia are Wednesdays at 4:00 p.m. in DUAN G1B20, unless otherwise noted.

Coffee, tea and cookies will be available before regular colloquia beginning at 3:45 p.m. in DUAN G1B31.

August 28 — "New frontiers for localization"

• Presenter:  Rahul Nandkishore, University of Colorado, Boulder
• Host: 
• Abstract: Localization is a central paradigm for non-equilibrium quantum physics. I will explain what localization is, and why it is interesting, highlighting in particular some of the new phases that can arise in the localized regime, which have no equilibrium counterparts. I will then present some of my recent work demonstrating that localization can arise in settings where it was previously believed to be impossible, such as systems with long range interactions. I will conclude with a discussion of some recent developments, their implications, and an outlook for the field. 

September 4 — "Breaking Bias"

• Presenter: Stefanie Johnson, Leeds School of Business, University of Colorado, Boulder
• Host: Eric Cornell
• Abstract: In this talk, Johnson explains the biases that impede diversity initiatives and provides ways of overcoming those biases to effectively promote diversity in organizations. She specifically will talk about one example of a diversity intervention at the Hubble Space Telescope.  Johnson then covers strategies to interrupt unconscious biases though different cognitive mechanisms. 

September 11 — "In search of new scales of compositeness"

• Presenter: Ethan Neil, University of Colorado, Boulder
• Host:
• Abstract:  The study of particle physics has a long history of uncovering compositeness - new substructures within particles that were at first believed to be fundamental.  Are the current set of "fundamental" particles contained in the Standard Model hiding even smaller structures?  In this talk, I will explore the physics of composite particles, the current state of experimental searches for compositeness, and distinctive signatures of undiscovered strongly-coupled composite particles such as composite dark matter.

September 18 — "New Detectors for Probing Our Universe"

• Presenter: Peter Graham, Stanford University 
• Host: James K. Thompson
• Abstract: High precision technology offers a powerful new approach for particle physics and cosmology. In recent years there has been a surge of interest in using technologies such as atom interferometry, nuclear magnetic resonance, and high precision magnetometry in addition to the more traditional particle detection techniques. Excitingly, such technologies can allow the discovery of new physics which is otherwise completely undetectable by conventional techniques. For example the axion is one of the most strongly-motivated dark matter candidates, however to date only a small fraction of its parameter space has been explored. I will discuss several new experimental approaches to searching for this type of “ultralight” dark matter. Interestingly, these approaches are similar in many respects to gravitational wave detection. I will also discuss the use of atom interferometry for gravitational wave and dark matter detection and the new MAGIS detector. Such precision experiments will open new avenues for probing the origin and composition of the universe.

September 25 — "Disorder as a driver of biological filtration"

• Presenter: Loren Hough, University of Colorado, Boulder
• Abstract: Intrinsically disordered proteins are flexible polymers that play a wide variety of vital cellular roles. Their disruption leads to a wide range of diseases, from cancer to neurodegeneration. Disordered proteins present a fascinating enigma; how they can be so important for cell function while remaining flexible and highly dynamic rather than forming well-defined structures? One role of intrinsically disordered proteins is to form the primary filter of the nuclear pore complex.  This remarkable filter allows the selective passage of some macromolecules while inhibiting the passage of others. We combine nuclear magnetic resonance spectroscopy within living cells, bench-top experiments and analytical models to show that binding to flexible filaments can give rise to unexpected diffusive properties that contribute to motion through biological filters.

October 2 — "Network architectures supporting learnability"

• Presenter: Danielle Bassett, University of Pennsylvania
• Host: Loren Hough
• Abstract: Human learners acquire not only disconnected bits of information, but complex interconnected networks of relational knowledge. The capacity for such learning naturally depends on the architecture of the knowledge network itself. I will describe recent work assessing network constraints on the learnability of relational knowledge, and theories from statistical physics that offer an explanatory model for such constraints. I will then broaden the discussion to the generic manner in which humans communicate using systems of interconnected stimuli or concepts, from language and music, to literature and science. I will describe an analytical framework to study the information generated by a system as perceived by a biased human observer, and provide experimental evidence that this perceived information depends critically on a system's network topology. Applying the framework to several real networks, we find that they communicate a large amount of information (having high entropy) and do so efficiently (maintaining low divergence from human expectations). Moreover, we also find that such efficient communication arises in networks that are simultaneously heterogeneous, with high-degree hubs, and clustered, with tightly-connected modules -- the two defining features of hierarchical organization. Together, these results suggest that many real networks are constrained by the pressures of information transmission to biased human observers, and that these pressures select for specific structural features.

October 9 — "Discovering Neutrino Properties with Long-Baseline Beams"

• Presenter: Alysia Marino, University of Colorado, Boulder
• Host: John Cumalat
• Abstract: Neutrinos are fundamental particles with no electric charge and as-yet-unmeasured masses, allowing them to travel unimpeded through enormous amounts of material.  Despite their elusiveness, a lot of compelling evidence shows that neutrinos have non-zero masses and change from one flavor to another. Intense neutrino beams generated by particle accelerators are now being used in order to more precisely probe the physics of neutrino masses and mixing.
  This talk will briefly review the experimental evidence and the framework that describes neutrino oscillations. As an example of a man-made neutrino beam, it will focus on the Tokai-to-Kamioka (T2K) experiment, which creates a beam of muon neutrinos at J-PARC on the east coast of Japan.  With two neutrino detectors, one located near the origin of the beam, and another located 295 km away, T2K has seen the disappearance of muon neutrinos and the appearance of electron neutrinos. The talk will conclude with a brief discussion of future long-baseline neutrino experiments, especially the efforts in the US to send a high-intensity beam of neutrinos from Fermilab to a former gold mine in South Dakota.

October 16 — "Controlling magnetism in a Mott insulator by optical pumping"

• Presenter: David Hsieh, California Institute of Technology
• Host: Rahul Nandkishore
• Abstract: Controlling magnetism in an antiferromagnetic Mott insulator with ultrashort optical pulses can lead to both advances in our fundamental understanding of out-of-equilibrium interacting quantum matter as well as to novel high-speed information storage and processing technologies. However, optically manipulating antiferromagnetic order and detecting its out-of-equilibrium behaviors have proven difficult owing to various factors such as the absence of net magnetization and the ultrafast timescales involved. In this talk, I will describe a novel time-resolved nonlinear optical polarimetry technique that is capable of measuring ultrafast changes in magnetic symmetry. I will then describe how we have deployed this technique to reveal an unusual out-of-equilibrium critical behavior of the magnetic order in an optically pumped Mott insulator that circumvents the laws of equilibrium thermodynamics.

October 23 — "Frontiers of topological quantum matter and beyond"

• Presenter: Michael Hermele, University of Colorado, Boulder
• Host: John Cumalat
• Abstract: The last ten to fifteen years have witnessed extraordinary progress in the theory of topological quantum matter.  This includes the discovery of new families of quantum phases of matter, going hand-in-hand with unprecedented advances in the systematic understanding of what phases are possible in principle.  Progress has been particularly pronounced within a certain context that includes many physically interesting systems, but also leaves a vast frontier to explore.  In this talk, I will describe two recent lines of work, one of which lies at this frontier, while the second crosses into ``non-topological’’ territory.
  First, I will discuss crystalline topological phases, those where the geometrical symmetries of crystalline solids play an important role.  Before the last few years, little was understood about such phases for strongly interacting systems, and obtaining such understanding was believed to be a hard problem.  Surprisingly, due to previously hidden structure that my work uncovered, obtaining a systematic classification of crystalline topological phases turns out to be easier than for their non-crystalline counterparts.
  Second, I will discuss so-called fracton phases of matter.  While fracton phases share some features in common with topological phases, they are not topological.  I will describe my work drawing connections between fracton phases and more familiar topological phases, and my ongoing contributions to the development of a theoretical framework for fracton matter.

October 30 — "Bringing Down the Cost of Fusion Power"

• Presenter: Steven Cowley, Princeton Plasma Physics Laboratory 
• Host: Dmitri Uzdensky
• Abstract: The scientific demonstration of a self-sustained fusion burn in the international experiment ITER — is within sight. However, commercial fusion power will require further innovation to bring down the cost and complexity of fusion systems. I will describe recent advances in the science, design and technology of three-dimensional confinement devices (stellarators) that promise simpler and cheaper fusion reactors.  

November 6 — "Making microwaves with light at the quantum limit and beyond"

• Presenter: Thomas Schibli, University of Colorado, Boulder
• Host: John Cumalat
• Abstract: Precision measurements enabled by exquisitely stable optical sources had a profound impact on Boulder’s current scientific enterprise. This talk will start with an overview of fundamental noise processes in oscillators, and highlight some of the major differences between electronic and optical sources. I will then discuss what levels of stability can be reached directly from a laser to date, and how to occasionally dip far below the quantum limit for specific applications. 
  As a practical application, I will elaborate on the generation of laser-driven, ultra-low noise microwaves, now readily surpassing the best room-temperature microwave sources at a fraction of their size, weight and power.

November 13 — "Quantum metrology at the 19^th decimal place"

• Presenter: David Leibrandt, NIST, University of Colorado Boulder
• Host: John Cumalat
• Abstract: The tools of trapped-ion quantum logic can be used to enable and enhance precision measurements, with applications in time and frequency metrology and the search for physics beyond the standard model.  In this talk, I will describe optical atomic clocks based on Al⁺ which operate at this fertile intersection of fields.  These clocks use quantum-logic gates with a co-trapped second ion species for preparation and readout of the Al⁺ state, and offer the tantalizing prospect of Heisenberg-limited measurements with entangled ions.  Recent progress, including an improved ion trap design and sympathetic laser cooling to the 3D ground state, has enabled total fractional systematic uncertainty below 10^-18.  We have performed frequency ratio measurements between Al⁺, Sr, and Yb clocks with uncertainty below 10^-17, which can be used to place constraints on possible temporal variations of fundamental constants and models of ultralight dark matter.

November 20 — "Does an isolated quantum system relax?"

• Presenter: Joerg Schmiedmayer, Vienna Center for Quantum Science and Technology (VCQ) Atominstitut – Institute of Atomic and Subatomic Physics Technische Universität
• Host: Ana Maria Rey
• Abstract: The evolution of an isolated quantum system is unitary.  This is simple to probe for small systems consisting of few particles.  But what happens if the system becomes large and its constituents interact?
  In a first set of experiments, we study a weak quench introducing quantum noise. The coherence created by coherent splitting of a 1d quantum gas degrades by coupling to the many internal degrees of freedom available [1].  The system relaxes to a pre-thermalisatized quasi steady state [2] described by a generalized Gibbs ensemble [3] which emerges through a light cone like spreading of ’de-coherence’ [4]. By engineering the Quasiparticles we can get coherence back by many body quantum revivals [5].  We conjecture that our experiments point to a universal way through which relaxation in isolated many body quantum systems proceeds if the low energy dynamics is dominated by scrambling of the eigenmodes of long lived excitations [6].
  In a second set of experiments we study a strong cooling quench and demonstrate universal scaling in time and space, associated with the approach of a non-thermal fixed point [7]. The time evolution within the scaling period is described by a single universal function and scaling exponent, independent of the species of the initial state which constitutes a crucial step in the verification of universality far from equilibrium. If successful, this will lead to a comprehensive classification of systems far from equilibrium based on their universal properties similar to the universality classes in phase transitions.  and be the basis for a new type of quantum simulation that let us explore a large variety of systems at different scales.
  Work performed in collaboration with the groups of E. Demler (Harvard), Th. Gasenzer und J. Berges (Heidelberg). Supported by the Wittgenstein Prize, the FWF SFB FoQuS, DFG-FWF: SFB ISOQUANT: and the EU: ERC-AdG QuantumRelax
  [1] S. Hofferberth et al. Nature, 449, 324 (2007).
  [2] M. Gring et al., Science, 337, 1318 (2012); D. Adu Smith et al. NJP, 15, 075011 (2013).
  [3] T. Langen et al., Science 348 207-211 (2015).
  [4] T. Langen et al., Nature Physics, 9, 640–643 (2013).
  [5] B. Rauer et al. Science 360, 307–310 (2018).
  [6] T. Langen, T. Gasenzer, J. Schmiedmayer, J. Stat. Mech.  064009 (2016)
  [7] S. Erne et al. Nature 253, 225 (2018).

November 27 — Fall Break, No Colloquium

December 4 — "Evaluating the Iran Nuclear Deal"

• Presenter: Ron Soltz, Lawrence Livermore National Laboratory
• Host: Jamie Nagle
• Abstract: The Iran Nuclear Deal evokes strong reactions.  It has been called "The Worst Deal Ever" as well as "The best option for preventing Iran from obtaining a nuclear weapon.  Otherwise known as the "Joint Comprehensive Plan of Action", the JCPOA has led to much debate, even if little of it has been substantive.  Put into effect in 2015, the JCPOA continues to influence the behavior of the EU, China, and Russia, while the U.S. formally withdrew in May 2018 and Iran has recently begun to violate the agreement in stages.
  The JCOPOA is unique in the history of international agreements, but as it stands at the intersection of science and policy, it is also a valuable teaching tool for the role that science can play in formulating good policy, while also providing an opportunity to review a few basic concepts in nuclear physics.  I will review the basic components of the JCPOA and explain their origin.  The agreement will also be placed in the context of past nuclear arms agreements and U.S. nuclear non-proliferation policy.  I will review criticisms and flaws of the agreement. Future implications for Iran and global non-proliferation efforts will also be discussed.

December 11 — "The Challenge of a nuclear optical clock: recent progress and perspectives"

• Presenter: Lars von der Wense, Ludwig-Maximilians-University of Munich
• Host: Jun Ye
• Abstract: A nuclear optical clock based on a single 229Th ion is expected to achieve a higher accuracy than the best atomic clocks operational today [1]. Although already proposed back in 2003 [2], such a nuclear frequency standard has not yet become reality. The main obstacle that has so far hindered the development of a nuclear clock is an imprecise knowledge of the energy value of a nuclear excited state of the 229Th nucleus, generally known as the 229Th isomer. This metastable nuclear excited state is the one of lowest energy in the whole nuclear landscape and - with an energy of less than 10 eV - offers the potential for nuclear laser spectroscopy, which poses a central requirement for the development of a nuclear clock. In the past few years significant progress toward the development of a nuclear frequency standard has been made: Starting with a first direct detection of the 229Th isomer in 2016 based on its internal conversion decay channel [3], the isomeric lifetime could be determined in 2017 [4], followed by a first laser-spectroscopic characterization in 2018 [5]. Most recently, a first energy determination based on the isomer’s direct detection was successful [6]. This new knowledge provides, in combination with an achieved drastically enhancement of XUV-frequency comb intensity [7], the basis for improved efforts toward the laser-based search for the nuclear transition [8, 9], which can ultimately lead to the development of a nuclear optical clock. In this presentation I will give an overview over the current status of the nuclear clock development, with a particular focus on the most recent progress. Also the next required steps will be detailed and future perspectives will be given.
• References
  [1] C.J. Campbell et al., Phys. Rev. Lett. 108, 120802 (2012).
  [2] E. Peik and C. Tamm, Eur. Phys. Lett. 61, 181 (2003).
  [3] L. von der Wense et al., Nature 533, 47 (2016).
  [4] B. Seiferle et al., Phys. Rev. Lett. 118, 042501 (2017).
  [5] J. Thielking et al., Nature 556, 321 (2018).
  [6] B. Seiferle et al., Nature 573, 243 (2019).
  [7] G. Porat et al., Nature Photonics 12, 387 (2018).
  [8] L. von der Wense, Phys. Rev. Lett. 119, 132503 (2017).
  [9] L. von der Wense and C. Zhang, arXiv:1905.08060 (2019).

For more information about colloquia this semester, contact: Rahul Nandkishore.

Colloquia are Wednesdays at 4:00 p.m. in DUAN G1B20, unless otherwise noted.

Coffee, tea and cookies will be available before regular colloquia beginning at 3:45 p.m. in DUAN G1B31.

January 16 — "Unleashing Liquid Argon Time Projection Chambers for Neutrino Physics: MicroBooNE, SBN, and DUNE"

• Presenter: Michael Mooney, Colorado State University
• Host: Eric Zimmerman
• Abstract: The neutrino is the most abundant massive particle in our universe, originating from the Sun, the Earth’s core and atmosphere, supernovae, the Big Bang, and man-made sources such as nuclear reactors and particle accelerators. Despite its omnipresence, it remains the least understood known fundamental particle due to its weak interactions with other particles (and thus particle detectors). One promising detector technology that can be used to study the neutrino in great detail is the liquid argon time projection chamber (LArTPC), an imaging detector that can be used to "photograph" neutrino-nucleus interaction events. Three LArTPC neutrino experiments in the US, MicroBooNE (Micro Booster Neutrino Experiment), the SBN (Short-Baseline Neutrino) Program, and DUNE (Deep Underground Neutrino Experiment) are discussed, including the status of each experiment and recent results.

January 23 — "Observations and Discoveries of Our Heliosphere’s Interstellar Interaction"

• Presenter: Dave McComas, Princeton University
• Host: Mihaly Horanyi
• Abstract: The solar wind and its embedded magnetic field flow outward from the sun in all directions, inflating a bubble in the local interstellar medium called the heliosphere. Prior to 2004, there were very few direct observations of the interaction of the heliosphere and local interstellar medium and our knowledge of these regions was largely theoretical. Then, 2004 and 2007 the Voyager 1 and 2 spacecraft crossed the heliosphere’s termination shock and in 2012 and 2018, each went on to cross the heliopause and entered interstellar space. IBEX – the Interstellar Boundary Explorer – launched in 2008, and has been returning 3-D global observations of ion distributions in the heliosheath and beyond via charge exchange Energetic Neutral Atoms (ENAs), continuously since then. These sets of observations are highly complementary with the Voyagers providing detailed in situ measurements along their two trajectories and IBEX returning all-sky maps of ENAs with energies from <0.1 to ~6 keV. Over the past decade and a half, these observations have led to numerous discoveries and “firsts” and a true scientific revolution in our understanding of the outer heliosphere and its interstellar interaction. With the continuation of the Voyagers and IBEX, and NASA’s recent selection of the Interstellar Mapping and Acceleration Probe (IMAP) to launch in 2024, the heliophysics community can look forward to many more years of outstanding new observations and innovative science. This seminar is adapted from the Parker Lecture recently presented at the 2018 Fall AGU meeting, includes connections to planned contributions to IMAP from CU/LASP, summarizes some of the biggest discoveries and most intriguing mysteries of the fascinating region that surrounds our Sun and forms our home in the galaxy.

**Special Colloquium: Thursday, January 24** — "Synthetic Quantum Matter in Superconducting Circuits"

• Presenter: Alex Ruichao Ma, University of Chicago
• NOTE SPECIAL LOCATION: JILA Auditorium
• Host: Konrad Lehnert
• Abstract: Superconducting circuits have emerged as a competitive platform for quantum computation, satisfying the challenges of controllability, long coherence and strong interactions. Here we apply this toolbox to the exploration of strongly correlated quantum materials made of microwave photons. We develop a versatile recipe that uses engineered dissipation to stabilize many-body phases, protecting them against intrinsic photon losses. We build a strongly interacting Bose-Hubbard lattice in circuits and applied our dissipative stabilization method to create a Mott insulator of photons. Site- and time-resolved microscopy provides insights into the thermalization processes through the dynamics of defects in the Mott phase. In another experiment, we realize a superconducting Chern insulator constructed from tunnel-coupled, time-reversal broken microwave cavities and study its topologically protected edge states. Our work demonstrates the power of superconducting circuits for studying synthetic quantum matter in both coherent and driven-dissipative settings. I will briefly discuss future prospects including microscopic studies of strongly interacting topological phases and quantum thermodynamics.

**Special Colloquium: Monday, January 28** — "Quantized States, Berry Phases, and Quantum-Hall Wedding-Cake structures in Graphene Quantum Dots"

• Presenter: Fereshte Ghahari, NIST
• NOTE SPECIAL LOCATION: DUAN G130
• Host: Konrad Lehnert
• Abstract: Recent progress in creating graphene quantum dots (QDs) with fixed build-in potentials has offered a new platform to visualize and probe the confined electronic states. In this talk, I describe scanning tunneling spectroscopy measurements of the energy spectrum of graphene QDs as a function of energy, spatial position, and magnetic field. In zero field, the charge carriers are confined by oblique Klein scattering at the p-n junction boundary giving rise to a series of quasi-bound single particle states. Applying a weak magnetic field, we observe a giant and discontinuous change in the energy of time-reversed angular-momentum states, which manifests itself as the appearance of “new” resonances in the tunneling density of states. This behavior corresponds to the on/off switching of a π- Berry phase when a weak critical magnetic field is reached.  With increased applied magnetic field, the QD states can be confined even further as they condense into highly degenerate Landau levels providing the first spatial visualization of the interplay between spatial and magnetic confinement. This is observed as formation of the seminal wedding-cake structures of concentric compressible and incompressible density rings in strong magnetic fields.

January 30 — "Laboratory Plasma Astrophysics: from Angular Momentum Transport to Magnetic Reconnection"

• Presenter: Hantao Ji, Princeton University
• Host: Dmitri Uzdensky
• Abstract: Studying astrophysical plasma processes in the lab becomes increasingly possible and exciting, complementing numerical simulations. In this talk, I will describe two examples of experimental efforts with which I am closely involved. The first one is about the mechanisms of rapid angular momentum transport required to explain the observed fast accretion, e.g,, enabling star formation and powering quasars. By carefully isolating effects due to artificial boundaries, key astrophysical questions regarding hydrodynamic and magnetohydrodynamic (MHD) instabilities can be uniquely studied in the laboratory. The second example is about the mechanisms of fast magnetic reconnection, considered to be at the core of the observed flares from various astrophysical objects including our Sun. Plasma physics beyond MHD has been identified as a key to fast reconnection in effectively small plasmas, while understanding how fast reconnection works in effectively large astrophysical plasmas is at the current frontier of research. Future prospects of this growing field of laboratory plasma astrophysics will be discussed.

**Special Colloquium: Thursday, January 31** — "Ultracold Molecules: From Quantum Chemistry to Quantum Computing"

• Presenter: Alan Jamison, Massachusetts Institute of Technology
• NOTE SPECIAL LOCATION: JILA Auditorium
• Host: Konrad Lehnert
• Abstract: Cooling atomic gases to quantum degeneracy opened the new field of quantum simulation. Here the precise tools of atomic physics can be used to study exotic models from condensed matter or nuclear physics with unique tunability and control. Ultracold molecules bring many new possibilities to quantum simulation. I will review the physics of ultracold molecules, including our recent production of stable, ultracold triplet molecules and what they can add to quantum simulation. I will also discuss our recent work using ultracold molecules to study chemical reactions with complete quantum state control. All of these tools and ideas come together in a proposal to use ultracold molecules as a new platform for quantum computing.

**Special Colloquium: Monday, February 4** — "Quantum nanophotonics: engineering atom-photon interactions on a chip"

• Presenter: Shuo Sun, Stanford University
• NOTE SPECIAL LOCATION: DUAN G130
• Host: Konrad Lehnert
• Abstract: The ability to engineer controllable atom-photon interactions is at the heart of quantum optics and quantum information processing. In this talk, I will introduce a nanophotonic platform for engineering strong atom-photon interactions on a semiconductor chip. I will first discuss an experimental demonstration of a spin-photon quantum transistor [1], a fundamental building block for quantum repeaters and quantum networks. The device allows a single spin trapped inside a semiconductor quantum dot to switch a single photon, and vice versa, a single photon to flip the spin. I will discuss how the spin-photon quantum transistor realizes optical nonlinearity at the fundamental single quantum level, where a single photon could switch the transmission of multiple subsequent photons [2]. I will next discuss the promise of realizing photon-mediated many-body interactions in an alternative solid-state platform based on a more homogeneous quantum emitter, silicon-vacancy (SiV) color centers in diamond. I will introduce our efforts in creating strong light-matter interactions through photonic crystal cavities fabricated in diamond [3], and the use of cavity-stimulated Raman emission to overcome the remaining frequency inhomogeneity of the emitters [4]. Finally, I will outline the exciting prospects of applying inverse designed nanophotonic structures into quantum optics, and their potential applications in engineering photon-mediated atom-atom interactions.

  References
  [1] S. Sun et al., Nature Nanotech.11, 539–544 (2016).
  [2] S. Sun et al., Science361, 57-60 (2018).
  [3] J. L. Zhang* and S. Sun* et al., Nano Lett.18, 1360–1365 (2018).
  [4] S. Sun et al., Phys. Rev. Lett. 121, 083601 (2018)

February 6 — "Correlations in moire flat bands: topological order, symmetry breaking, and superconductivity"

• Presenter: Andrea Young, UC Santa Barbara
• Host: Adam Kaufman
• Abstract: Van der Waals heterostructures are constructed by layering atomically thin crystals such as graphene, with interlayer bonding provided by the van der Waals force. When neighboring crystal lattices are slightly mismatched, a moire pattern forms from the beating of two slightly mismatched lattices.  Moire patterns can be used to generate artificial lattices for electrons, providing a versatile platform for engineering electronic structure.  Of particular interest is the possibility of engineering flat electronic bands where correlations dominate in determining the electronic ground state.  I will describe how these artificial lattices can be used to realize several exotic states of matter, including states where electrons appear to 'break up'--localizing a fractional of a charge on each lattice site--as well as states where electrons pair up to form a superconductor, all realized in atomically thin sheets of carbon. 

**Special Colloquium: Monday, February 11** — "Measurement of the fine-structure constant as a test of the Standard Model"

• Presenter: Richard Parker, University of California, Berkeley
• NOTE SPECIAL LOCATION: DUAN G130
• Host: Konrad Lehnert
• Abstract: Measurements of the fine-structure constant alpha require methods from several subfields and are thus powerful tests of the consistency of theory and experiment in physics. Using the recoil frequency of cesium-133 atoms in a matter-wave interferometer, we recorded the most accurate measurement of the fine-structure constant to date: alpha = 1/137.035999046(27) at 2.0 x 10^-10 accuracy. Using multiphoton interactions (Bragg diffraction and Bloch oscillations), we demonstrate the largest phase (12 million radians) of any Ramsey-Borde interferometer and control systematic effects at a level of 0.12 parts per billion. Comparison with Penning trap measurements of the electron gyromagnetic anomaly ge-2 via the Standard Model of particle physics is now limited by the uncertainty in ge-2; a 2.5 sigma tension rejects dark photons as the reason for the unexplained part of the muon's magnetic moment at a 99 percent confidence level. Implications for dark-sector candidates and electron substructure may be a sign of physics beyond the Standard Model that warrants further investigation.

February 13 — "X-ray sources from laser-plasma acceleration: development and applications for high energy density sciences"

• Presenter: Félicie Albert, Lawrence Livermore National Laboratory
• Host: John Cary and Michael Litos
• Abstract: Bright sources of x-rays, such as synchrotrons and x-ray free electron lasers (XFEL) are transformational tools for many fields of science. They are used for biology, material science, medicine, or industry. Such sources rely on conventional particle accelerators, where electrons are accelerated to gigaelectronvolts (GeV) energies. The accelerating particles are also wiggled in magnetic structures to emit x-ray radiation that is commonly used for molecular crystallography, fluorescence studies, chemical analysis, medical imaging, and many other applications.  One of the drawbacks of synchrotrons and XFELs is their size and cost, because electric field gradients are limited to about a few 10s of MeV/M in conventional accelerators.
  This seminar will review particle acceleration in laser-driven plasmas as an alternative to generate x-rays. A plasma is an ionized medium that can sustain electrical fields many orders of magnitude higher than that in conventional radiofrequency accelerator structures. When short, intense laser pulses are focused into a gas, it produces electron plasma waves in which electrons can be trapped and accelerated to GeV energies. This process, laser-wakefield acceleration (LWFA), is analogous to a surfer being propelled by an ocean wave. Betatron x-ray radiation, driven by electrons from laser-wakefield acceleration, has unique properties that are analogous to synchrotron radiation, with a 1000-fold shorter pulse. This source is produced when relativistic electrons oscillate during the LWFA process.
  An important use of x-rays from laser plasma accelerators we will discuss is in High Energy Density (HED) science. This field uses large laser and x-ray free electron laser facilities to create in the laboratory extreme conditions of temperatures and pressures that are usually found in the interiors of stars and planets. To diagnose such extreme states of matter, the development of efficient, versatile and fast (sub-picosecond scale) x-ray probes has become essential. In these experiments, x-ray photons can pass through dense material, and absorption of the x-rays can be directly measured, via spectroscopy or imaging, to inform scientists about the temperature and density of the targets being studied. 
  Work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344, supported by the LLNL LDRD program under tracking code 13-LW-076, 16-ERD-024, 16-ERD-041, supported by the DOE Office of Fusion Energy Sciences under SCW 1476 and SCW 1569, and by the DOE Office of Science Early Career Research Program under SCW 1575

February 20 — "Effective Field Theories & Modifying Gravity: The View from Below"

• Presenter: Cliff Burgess, Perimeter Institute and MacMaster University 
• Host: Shanta DeAlwis
• Abstract: We live at a time of contradictory messages about how successfully we understand gravity. General Relativity seems to work very well in the Earth’s immediate neighbourhood, but arguments abound that it needs modification at very small and/or very large distances. This talk tries to put this discussion into the broader context of similar situations in other areas of physics, and summarizes some of the lessons which our good understanding of gravity in the solar system has for proponents for its modification over very long and very short distances. The main message is that effective theories (in the technical sense of effective) provide the natural (and arguably only known) precise language for framing proposals. Its framework is also useful, inasmuch as it makes some modifications seem more plausible than others, though there are also some surprises. Amongst the surprises is evidence that in some ways gravity behaves more like condensed matter physics or optics than particle physics, raising the possibility that tools from these areas may play a useful role in understanding puzzles with cosmology and black holes.

**Special Colloquium: Thursday, February 21** — "Engineering Trapped-Ion Systems for Large Scale Quantum Simulation"

• Presenter: Guido Pagano, University of Maryland and NIST
• NOTE SPECIAL LOCATION: JILA Auditorium
• Host: Konrad Lehnert
• Abstract: Laser cooled trapped ions offer unprecedented control over both internal and external degrees of freedom at the single-particle level. They are considered among the foremost candidates for realizing quantum simulation and computation platforms that can outperform classical computers at specific tasks. In this talk I will show how linear arrays of trapped 171Yb+ ions can be used as a versatile platform for studying quantum dynamics of strongly correlated many-body quantum systems. In particular I will describe how to realize time-crystalline phases in a Floquet setting, where the spin system exhibits persistent time-correlations under many-bodylocalized dynamics [1]. I will also present our observation of a new type of out-of equilibrium dynamical phase transition in a spin system with over 50 spins [2]. Moreover I will show our latest efforts towards scaling up the trapped-ion quantum simulator [3] using a cryo-pumped vacuum chamber where we can trap more than 100 ions indefinitely. The reliable production and lifetime of large linear ion chains enabled us to investigate quasi-particle excitations showing confinement in the quench dynamics [4] and the implementation of Quantum Approximate Optimization Algorithms (QAOA) [5].
  References:
  [1] J. Zhang et al., Nature, 543, 217 (2017)
  [2] J. Zhang, G. Pagano, et al., Nature, 551, 601 (2017)
  [3] G. Pagano et al., Quantum Sci. Technol., 4, 014004 (2019)
  [4] F. Liu, et al., arXiv 1810.02365 (2018)
  [5] G.Pagano, et al., (in preparation 2019)

February 27 — "Machine learning the quantum many-body problem"

• Presenter: Roger Melko, University of Waterloo
• Host: Leo Radzihovsky
• Abstract: The quantum wavefunction presents the ultimate "big data" problem in physics.  When many quantum particles interact in a low-temperature material or a quantum computer, the complexity of the quantum state presents a daunting challenge for any classical simulation strategy.  Recently, a new computational toolbox based on modern machine learning techniques has been rapidly adopted into the field of condensed matter and quantum information physics. Standard tools like feed-forward and convolutional neural networks are being repurposed for training on "images" of microscopic configurations.  Unsupervised and reinforcement learning are making headway in improving standard algorithms such as quantum Monte Carlo.  In this talk, I will discuss recent progress, focussing on how generative modelling with stochastic neural networks can be used to combat the complexity of the quantum wavefunction, with applications in materials science, atomic physics, and the design of future quantum computers.

March 6 — "Quantum Logic Control of a Single Molecular Ion"

• Presenter: Dietrich Leibfried, Ion Storage Group, NIST
• Host: Ana Maria Rey
• Abstract: An amazing level of quantum control is routinely reached in modern experiments with atoms, but similar control over molecules has been an elusive goal. We recently proposed a method based on quantum logic spectroscopy [1] to address this problem for a wide class of molecular ions [2]. We have now realized the basic elements of this proposal.
  In our demonstration, we trap a calcium ion together with a calcium hydride ion (CaH+) that is a convenient stand-in for more general molecular ions. We cool the two-ion crystal to its motional ground state and then drive motional "sidebands" of Raman transitions in the molecular ion, meaning that a transition in the molecule is accompanied by a single quantum of excitation in the motion of the ion pair. We can efficiently detect this single quantum with the calcium ion, which projects the molecule into the final state of the sideband transition, a known, pure quantum state.
  The molecule can be coherently manipulated after the projection, and its resulting state read out by another quantum logic state detection [3]. We demonstrate this by driving Rabi oscillations between rotational states. All transitions we address in the molecule are either driven by a single, far off-resonant continuous-wave laser or by a far-off-resonant frequency comb. This makes our approach applicable to control and precision measurement of a large class of molecular ions.
  [1] P.O. Schmidt, et al. Science 309, 749 (2005)
  [2] D. Leibfried, New J. Phys. 14, 023029 (2012)
  [3] C.-W. Chou, et al. Nature 545, 203 (2017)

March 13 — "Engaging students in Modeling Instruction: developing and studying students as scientists"

• Presenter: Eric Brewe, Drexel University
• Host: Noah Finkelstein
• Abstract: Physics Education Research is both about improving instruction and understanding the fundamentals of what learning is and how learning manifests in its many forms. In this talk I describe the development of Modeling Instruction (MI) for University Physics as a research endeavor into improving instruction. Modeling is built on the idea that all science proceeds through an iterative process of model development, evaluation, deployment, and revision. Accordingly, effective science instruction should promote the development of modeling skills by engaging students in the practices of modeling. I describe research within the context of MI classes as the basis for understanding learning broadly. Over the course of this talk I will summarize the theoretical foundations for MI, and describe research that translates the theory into practice in the MI classroom. Drawing on the MI classroom as a context for research, I will report on findings including: improved conceptual understanding, positive attitudinal shifts, the growth of student networks, and even changes to the neurobiology that underpins physics reasoning. Finally, I will describe how these research findings drive further questions and understanding of learning generally.

March 20 — "New results from the NOvA neutrino oscillation experiment"

• Presenter: Patricia Vahle, College of William & Mary
• Host: Alysia Marino
• Abstract: Neutrino oscillations provide the first hints at physics beyond the standard model of particle physics. Current and future neutrino experiments aim to further refine our understanding of neutrino mixing and reveal the remaining unknowns in the process. Precision measurements in long-baseline accelerator experiments could help answer profound questions about the origin and evolution of our universe, including the assymetry of matter over antimatter. The NOvA experiment at Fermilab uses a beam of neutrinos and two detectors separated by an 810 km baseline to observe muon neutrino disappearance and electron neutrino appearance. These measurements have the potential to resolve the ordering of the neutrino masses, called the hierarchy, determine whether the mixing angle theta_23 is maximal, and if not in which octant it lies, and perhaps even hint at the violation of CP in the neutrino sector. In this talk, I'll describe the current status of accelerator oscillation experiments seeking to answer these questions, and in particular present new results from the NOvA experiment.

March 27 — Spring Break; No Colloquium

April 3 — "Topological Insulators to Weyl Fermions and Beyond"

• Presenter: M. Zahid Hasan, Princeton University
• Host: Margaret Murnane and Dan Dessau
• Abstract: Electrons in solids organize in ways to give rise to distinct phases of matter such as insulators, metals, magnets or superconductors. In the last ten years or so, it has become increasingly clear that in addition to the symmetry-based classification of matter, topological consideration of electronic wavefunctions plays a key role in determining distinct phases of matter [see, for an introduction, Hasan & Kane, Reviews of Modern Physics 82, 3045 (2010)].

  In this talk, I introduce these concepts in the context of their experimental realizations in real materials leading to recent developments. As an example, I present how tuning a 3D topological insulator whose surface hosts an unpaired Dirac fermion can give rise to topological superconductors with helical Cooper pairing leading to novel Majorana platforms, Weyl fermion semimetals with “fractional” surface Fermi surfaces, and other topological nodal and magnetic states of matter. These topological materials harbor novel properties that may lead to the development of next generation quantum technologies accelerating the second quantum revolution.

April 10 — "New opportunities in dark matter at accelerators"

• Presenter: Nhan Tran, Fermilab
• Host: Keith Ulmer
• Abstract: The program to search for dark matter in the past couple of decades has mostly focused on the WIMP (weakly interacting massive particle) at the GeV - TeV scale. It has made impressive strides in sensitivity but has yet to unearth the particle nature of dark matter. Recently there have been many new initiatives to broaden the search for dark matter, many of them smaller scale experiments. Recently, within the US, there has been an effort to organize and contrast various experimental techniques and their sensitivity. One of the main thrusts of this dark matter initiative is to extend searches for dark matter below a GeV using accelerator techniques. I will discuss the status of this dark matter initiative and focus on the opportunities for dark matter at accelerators. I will lay out the various accelerator approaches to look for sub-GeV dark matter including beam dump and fixed target missing momentum techniques. As a specific example, I will go into detail on the missing momentum technique effort in which I am involved. 

April 17 — "Quantum Magnetism from the Iron Age to Today"

• Presenter: Dan Arovas, UCSD
• Host: Leo Radzihovsky
• Abstract: The quantum theory of magnetism has provided many durable paradigms for quantum phases of matter, including intrinsically quantum disordered states, symmetry-protected topological phases, and quantum spin liquids.  In this lecture, I will review some of the history and highlights of this very rich field.

April 24 — "Magic Angle Graphene: a New Platform for Strongly Correlated Physics"

• Presenter: Pablo Jarillo-Herrero, MIT
• Host: Dan Dessau
• Abstract: The understanding of strongly-correlated quantum matter has challenged physicists for decades. Such difficulties have stimulated new research paradigms, such as ultra-cold atom lattices for simulating quantum materials. In this talk I will present a new platform to investigate strongly correlated physics, based on graphene moiré superlattices. In particular, I will show that when two graphene sheets are twisted by an angle close to the theoretically predicted ‘magic angle’, the resulting flat band structure near the Dirac point gives rise to a strongly-correlated electronic system. These flat bands exhibit half-filling insulating phases at zero magnetic field, which we show to be a correlated insulator arising from electrons localized in the moiré superlattice. Moreover, upon doping, we find electrically tunable superconductivity in this system, with many characteristics similar to high-temperature cuprates superconductivity. These unique properties of magic-angle twisted bilayer graphene open up a new playground for exotic many-body quantum phases in a 2D platform made of pure carbon and without magnetic field. The easy accessibility of the flat bands, the electrical tunability, and the bandwidth tunability though twist angle may pave the way towards more exotic correlated systems, such as quantum spin liquids or correlated topological insulators.

**CANCELLED** May 1 — "Understanding and Predicting Turbulent Transport in Tokamak Plasmas"

• Presenter: Anne White, MIT
• Host: Scott Parker
• Abstract: In magnetically confined fusion plasmas like tokamaks, transport of heat and particles is dominated by turbulence. Turbulent transport models can be validated using experimental data, using a rigorous methodology and direct comparisons with turbulence measurements.  While the transport models capture details of the turbulence very well, and can be used to predict steady-state temperature profiles for ITER and SPARC and other future tokamaks, there remain several outstanding questions. A long-standing enigma in plasma transport consists of the observation that controlled edge cooling of fusion plasmas triggers core electron temperature increases on time scales faster than an energy confinement time, which has long been interpreted as strong evidence of nonlocal transport. A novel integrated modeling tool, that we call PRIMA, leverages the new trapped gyro-landau fluid transport (TGLF) model that includes multi-scale physics. This modeling tool has been used to interpret data from C-Mod and develop predictions for new experiments at DIII-D. The interpretive analysis at C-Mod shows that the steady-state profiles, the cold-pulse rise time, and the disappearance at higher density measured in these experiments are well matched by the new TGLF model. This provides new evidence that the existence of nonlocal transport phenomena is not necessary for explaining cold-pulse experiments in tokamak plasmas. Predictive analysis was used to design a new experiment to leverage the new Laser Blow-Off (LBO) system at DIII-D, to test whether or not cold pulse inversion will occur on DIII-D, and if it does occur, to test whether the model can accurately predict the plasma conditions where it occurs. Detailed interpretive and predictive analysis from the C-Mod and DIII-D tokamaks will be presented, to make the case that the existence of nonlocal transport phenomena is not necessary for explaining the behavior and time scales of cold-pulse experiments in tokamak plasmas. This work has helped improve confidence in predictive capabilities for ITER, SPARC and other future tokamak experiments.

For more information about colloquia this semester, contact: Mihaly Horanyi.

Colloquia are Wednesdays at 4:00 p.m. in DUAN G1B20, unless otherwise noted.

Coffee, tea and cookies will be available before regular colloquia beginning at 3:45 p.m. in DUAN G1B31.

August 29 — "Tales from the Cold: New Science from Superconducting Sensors"

• Presenter: Joel Ullom, NIST Boulder
• Host: Scott Diddams
• Abstract: In the last decade, superconducting sensors operating at milliKelvin temperatures have evolved from laboratory curiosities to powerful tools for performing science.  These sensors are now essential for fields of science such as studies of the cosmic microwave background, and they are beginning to impact research areas previously thought unsuitable such as beamline science at large x-ray lightsources.  In this seminar, I will cover three interrelated topics: (1) the growing scope of application for superconducting sensors, (2) recent advances in performance enabled by better fundamental understanding of their behavior, and (3) some examples of recent science enabled by these devices, including demonstrations of tabletop ultrafast x-ray emission and absorption spectroscopy.  Timely background information is available in ref. 1.

  [1] K. Morgan, Physics Today 71, 8, 28 (2018); doi: 10.1063/PT.3.3995

September 5 — "Whispering-gallery-mode microresonators: fundamentals and applications"

• Presenter: Lan Yang, Washington University
• Host: Juliet Gopinath
• Abstract: Light-matter interactions are the fundamental basis for many phenomena and processes in optical devices. Whispering-Gallery-Mode (WGM) optical resonators trap light in a manner similar to a phenomenon found in the gallery spaces of St. Paul’s Cathedral dome in London, where a single whisper (i.e., a sound wave) can be heard along the circular boundary of the architecture. Ultra-high-quality WGM optical micro-resonators provide unprecedented capability to trap light in a highly confined volume smaller than a strand of human hair; a light beam can travel around the boundary of a WGM resonator over 10⁶ times, significantly enhancing light-matter interactions, creating the potential for a wealth of new scientific discoveries and technological breakthroughs difficult to achieve by other devices. They have shown a great promise for a variety of fields of science, spanning from optomechanics to communication, non-Hermitian physics, sensing and metrology. In this talk, I will report the recent research discoveries from my group in this exciting field. I will present a few cases demonstrating the great potentials of high-Q WGM microresonators and microlasers for both fundamental science and engineering applications. Specifically, I will discuss ultra-high-Q microresonators and microlasers for ultra-sensitive detection of nanoscale objects. I will explain a self-referencing sensing scheme for detection and sizing of single virion, dielectric and metallic nanoparticles. These recent advancements in WGM microresonators will enable a new class of ultra-sensitive and low-power sensors for investigating the properties and kinetic behaviors of nanomaterials, nanostructures, and nanoscale phenomena. Afterwards, I will discuss our recent exploration of fundamental physics, such as parity-time symmetry (PT-symmetry) and light-matter interactions around exceptional points (EPs) in high-quality WGM resonators, which can be used to achieve a new generation of optical system enabling unconventional control of light flow. Examples including nonreciprocal light transmission, loss engineering in a lasing system, directional lasing emission, and EPs enhanced sensing, will be introduced. A non-Hermtian phonon laser tuned in the vicinity of EPs will be discussed briefly. In the end, I will present a new generic and hand-held microresonator platform transformed from a table-top setup, which will help release the power of high-Q WGM resonator technologies.

  Short bio: Professor Lan Yang is the Edwin H. and Florence G. Skinner professor in the Preston M. Green Department of Electrical and Systems Engineering at Washington University, St. Louis, MO, USA. She received B.S. from the University of Science and Technology of China and received her Ph.D. in applied physics from Caltech in 2005. Her research interests have been focusing on the fundamental understanding of high-quality photonic whispering-gallery-mode (WGM) resonators and their applications for sensing, lasing, light harvesting, and communications. Recently, her research interests expanded to parity-time-symmetry and non-Hermitian physics in high-quality WGM resonators, which have led to a series of new discoveries for unconventional control of light transport in photonic structures. She received NSF CAREER Award in 2010 for her work on single nanoparticle detection and sizing using an on-chip optical resonator. She is also the recipient of the 2010 Presidential Early Career Award for Scientists and Engineers (PECASE). She is a fellow of the Optical Society of America (OSA).

September 12 — "Picoastronomy: an electron microscopist's view of the history of the Solar System"

• Presenter: Rhonda Stroud, U.S. Naval Research Laboratory
• Host: Mihaly Horanyi
• Abstract: A wide range of astrophysical processes, from condensation of dust particles in circumstellar envelopes to space weathering on airless bodies, are inherently pico-to-nanoscale phenomena. Thus, an electron microscope, used for direct observation of planetary materials in the laboratory, can be as much of an astronomical tool as a telescope pointed at the sky. The energy resolution of state-of-the-art monochromated scanning transmission electron microscopes (STEMs), as low as 10 meV, makes it possible to directly observe the infra-red optical properties of individual cosmic dust grains in the 2 to 25 um range. Thus, distinguishing the 10-um and 18-um features of individual bonafide astrosilicates is now possible. Furthermore, the spatial resolution and sensitivity of the STEM enables imaging and at spectroscopy at scales down to the single atom. Samples of nanomaterials from diverse classes of stars from red giants to supernovae, from asteroids and comets, and the surface of the Moon are all available for laboratory studies. Results of these studies can help aid in the understanding of the formation and evolution of solar systems, and even provide clues for advancing the development of technologically important materials, such as doped nanodiamonds, SiC and graphene.

September 19 — "Magnetism and spin in quantum materials"

• Presenter: Minhyea Lee, University of Colorado Boulder
• Host: John Cumalat
• Abstract: Quantum materials are crystalline solids with exotic physical properties arising from the quantum mechanical nature and interactions of electrons.  Quantum phenomena such as fluctuations, topology, tunneling and interference are at the heart of exciting developments in the fundamental science of quantum materials. At the same time, these phenomena have the potential to play a key role in future technologies.

  In this colloquium I will give some examples from my research on magnetism, illustrating the discovery of novel phenomena in quantum materials, and the development of frameworks that can guide the search for new materials and devices with advanced functionality.

September 26 — "Quantum Nanophotonics from Ultrathin Metallic Junctions"

• Presenter: Maiken Mikkelsen, Duke University
• Host: John Bohn
• Abstract: Tiny gaps between metals enables extreme field enhancements and strongly modified light-matter interactions promising for ultrafast optoelectronics, energy applications and on-chip components for quantum information processing. We use creative nanofabrication techniques at the interface between chemistry and physics to realize nanostructures with critical dimensions on the atomic- and molecular-scale (~1-10 nm), together with advanced, ultrafast optical techniques to probe the emerging phenomena. Here, I will provide an overview of our recent research demonstrating tailored light-matter interactions by leveraging ultra-small plasmonic cavities fabricated with bottom-up techniques. Examples of our demonstrations include 1,000-fold Purcell enhancements [Nature Photonics 8, 835 (2014)], ultrafast single photon sources [Nano Letters 16, 270 (2016)], tailored emission from two-dimensional semiconductor materials [Nano Letters 15, 3578 (2015), ACS Photonics 5, 552 (2018)], perfect absorbers and combinatorial plasmonic colors [Advanced Materials 27, 7897 (2015), Advanced Materials 29, 1602971 (2017)].    

  Bio:

  Maiken H. Mikkelsen is the James N. and Elizabeth H. Barton Associate Professor at Duke University in the Departments of Physics, Electrical & Computer Engineering, and Mechanical Engineering & Materials Science. Currently, she is a Visiting Associate Professor at Stanford University in the Department of Materials Science & Engineering. She received her B.S. from the University of Copenhagen in 2004, her Ph.D. in Physics from the University of California, Santa Barbara in 2009 and was a postdoctoral fellow at the University of California, Berkeley. Her research focuses on nanophotonics and quantum materials to enable transformative breakthroughs for optoelectronics, the environment and human health. Her awards include the Maria Goeppert Mayer Award from the American Physical Society, the Early Career Achievement Award from SPIE, the NSF CAREER award, the Cottrell Scholar Award from the Research Corporation for Science Advancement and Young Investigator Program Awards from the ONR, ARO and AFOSR.

October 3 — "The physics, biology, and technology of resonance energy transfer"

• Presenter: Philip Nelson, University of Pennsylvania
• Host: Loren Hough
• Abstract: Resonance energy transfer has become an indispensable experimental tool for single-molecule and single-cell biophysics, and a conceptual tool to understand bioluminescence and photosynthesis. Its physical underpinnings, however, are subtle: It involves a discrete jump of excitation from one molecule to another, and so we regard it as a strongly quantum-mechanical process. And yet its first-order kinetics differ from what many of us were taught about two-state quantum systems; quantum superpositions of the states do not seem to arise; and so on. The key step involves acknowledging quantum decoherence.

  Ref: P C Nelson, Biophys J 115: 167(2018) http://doi.org/10.1016/j.bpj.2018.01.010

October 10 — "Front and center: physics and the large size of whales"

• Presenter: Jean Potvin, Saint Louis University
• Host: Tom DeGrand
• Abstract: Large baleen whales such as the humpback and blue whales have become the shining stars of a great number of popular nature documentaries. Paradoxically, much of their physiology remains poorly understood. Generally one must look into a complex web of relationships between morphology, physiology, ecology and bio-mechanics to explain the whys and hows of, for example, body architecture. In this presentation I show that the blue whales and their baleen-bearing cousins represent shining examples of the central role played by physics in shaping their evolution into the largest and most massive life forms to have ever lived on Earth. The peculiarities of their feeding strategies permit the use of simple formulations of the work-energy and momentum conservation principles to estimate swimming metabolic energetic expenditures over size and demonstrate how body gigantism can not only become an advantage but also a disadvantage. This presentation will also show the results of an ongoing NSF-funded and inter-disciplinary study aimed at collecting kinematic and video data on humpback and blue whales foraging off the coast of California, with the goals of further understanding their feeding behavior as well as providing data for physical modeling validation.

October 17 — "Observing quantum materials in real time by pump-probe Raman Scattering"

• Presenter: Dmitry Reznik, University of Colorado Boulder
• Host: Mihaly Horanyi
• Abstract: Pump-probe experiments open a completely new way to study and control quantum materials. Their behavior away from thermal equilibrium, generation of metastable phases, photoexcited states, quantum computing-related phenomena, chemical reactions, energy dissipation, etc. can now be measured in real time with unprecedented precision due to rapid advances in ultrafast lasers and x-ray sources. In this talk I will give an overview of several representative pump-probe techniques and then focus on time-resolved Raman scattering. I will then discuss a demonstration project elucidating how energy from hot electrons cascades through different vibrational modes on the way to the thermal bath in graphite. Finally, to illustrate the diversity of problems that can be addressed, I will show how pump-probe Raman scattering is used to probe relaxation of the magnetic state in an antiferromagnetic Mott insulator.

October 24 — "From the Big Bang to Signs of Alien Life, with the James Webb and Future Telescopes"

• Presenter: John Mather, Goddard Space Flight Center
• Host: Jason Glenn
• Abstract: Planned for launch in 2021 on an Ariane 5 from French Guiana, the James Webb Space Telescope (JWST) will observe at wavelengths from 0.6 to 28 µm with a full suite of imagers, spectrometers, and coronagraphs. JWST will extend the discoveries of the Hubble and Spitzer observatories in all areas from cosmology, galaxies, stars, and exoplanets to our own Solar System. With a 6.5 m primary mirror it has a collecting area 7 times that of Hubble and 50 times that of Spitzer. Inventions were required ranging from deployment and in-flight focusing of its segmented telescope, to greatly improved infrared detectors, to a 6 Kelvin refrigerator for one of the instruments.  I will outline the planned observing program and the major scientific challenges being addressed. What were the first objects that formed in the expanding universe? How do the galaxies grow? How are black holes made, ranging from stellar mass to supermassive, over a billion solar masses, and what is their effect on the neighborhood? How are stars and planetary system formed? What governs the evolution of planetary systems, with the possibility of life? How did the Earth become so special? But the most important discoveries will be those we have not even imagined today.

  I will also describe the new telescopes being built on the ground and proposed for space, ranging from far infrared to X-rays.  And now for something completely different, I am developing a radical idea to observe exoplanets with ground-based telescopes and extreme adaptive optics, using an orbiting starshade. Since it does not require a space telescope, it could reveal an Earth twin with signs of life within the next 15 years. Not easy, but not impossible!

October 31 — "Searching beyond the Standard Model at the LHC"

• Presenter: Kevin Stenson, University of Colorado Boulder
• Host: John Cumalat
• Abstract: With the discovery of the Higgs boson in 2012 by the CMS and ATLAS collaborations, all of the fundamental particles in the standard model of particle physics have been observed.  Nevertheless, the many open questions in particle physics make it clear that the Standard Model is not a complete theory, and the main goal now is to discover what lies beyond the Standard Model.  I will describe some existing searches from the CMS experiment at the LHC and how the accelerator and detectors will be upgraded over the next decade to increase our sensitivity to new phenomena.

**CANCELLED** November 7

November 14 — "Active Matter: from colloids to living cells"

• Presenter: M. Cristina Marchetti, University of California, Santa Barbara
• Host: Leo Radzihovsky
• Abstract: Collections of self-propelled entities, from living cells to engineered microswimmers, organize in a rich variety of active fluid and solid states, with unusual properties. For instance, active fluids can flow with no externally applied driving forces and active gases do not fill their container. In this talk I will describe the behavior of such “active materials” and highlight two examples of active phase transitions. The first is the formation of cohesive matter with no cohesive forces in collections of purely repulsive active colloids. The second is a model of epithelial tissues that exhibit a liquid-solid transition at constant density driven by cell motility, contractility, and cell-cell adhesion.

November 21 — Fall Break; No Colloquium

November 28 — "Nanoscale Lasing: A Conundrum?"

• Presenter: Teri Odom, Northwestern University
• Host: Scott Diddams
• Abstract: Metal nanostructures concentrate optical fields into highly confined, nanoscale volumes that can be exploited in a wide range of applications. However, the use of plasmonic structures as cavities for generating coherent emission seems counter-intuitive based on conventional designs of macroscopic lasers. This talk will describe how arrays of nanoparticles can support a unique open-cavity architecture that can be used to interrogate the mechanisms of energy transfer processes and plasmon amplification in confined systems. First, we will describe how single band-edge lattice plasmons in metal nanoparticle arrays can contribute to single-mode lasing at room-temperature with directional emission. Second, we will discuss how ultra-narrow resonances from superlattice plasmons, collective excitations in hierarchical nanoparticle arrays, can support multi-modal nanolasing. Finally, we will describe challenges in and approaches to differentiating among competing energy transfer in the lasing action based on coherence, cavity size, and ultra-fast characteristics.
• Biography: Teri W. Odom is Charles E. and Emma H. Morrison Professor of Chemistry and Chair of the Chemistry Department at Northwestern University. She is an expert in designing structured nanoscale materials that exhibit extraordinary size and shape-dependent optical properties. Odom has pioneered a suite of multi-scale nanofabrication tools that has resulted in flat optics that can manipulate light at the nanoscale and beat the diffraction limit, plasmon-based nanoscale lasers that exhibit tunable color, and hierarchical substrates that show controlled wetting and super-hydrophobicity. She has also invented a class of biological nanoconstructs that are facilitating unique insight into nanoparticle-cell interactions and that show superior imaging and therapeutic properties because of their gold nanostar shape. 

  Odom is a Fellow of the American Physical Society (APS), the American Chemical Society (ACS), the Materials Research Society (MRS), the Optical Society of America (OSA), the Royal Society of Chemistry (RSC), and is an OSA Senior Member. She has received numerous other honors and awards, including a Research Corporation TREE Award; a U.S. Department of Defense Vannevar Bush Faculty Fellowship; the Associated Student Government Faculty Honor Roll; the Carol Tyler Award from the International Precious Metals Institute; a Blavatnik Young Scientist Finalist in Chemistry and Physical Sciences and Engineering; a Radcliffe Institute for Advanced Study Fellowship at Harvard University; the ACS Akron Section Award; an National Institutes of Health (NIH) Director's Pioneer Award; the MRS Outstanding Young Investigator Award; the National Fresenius Award from Phi Lambda Upsilon and the ACS; the Rohm and Haas New Faculty Award; an Alfred P. Sloan Research Fellowship; a DuPont Young Investigator Grant; a NSF CAREER Award; the ExxonMobil Solid State Chemistry Faculty Fellowship; and a David and Lucile Packard Fellowship in Science and Engineering. Odom was founding Chair of the Noble Metal Nanoparticles Gordon Research Conference (2010) and founding Vice-Chair of Lasers in Micro, Nano, Bio Systems (2018). She is on the Editorial Advisory Boards of ACS Nano, Materials Horizons, Annual Reviews of Physical Chemistry, ChemNanoMat, Chemical Society Reviews, Bioconjugate Chemistry, and Nano Letters. She was founding Associate Editor for Chemical Science (2009-2013) and serves as founding Executive Editor of ACS Photonics (2013 - ). Odom’s was featured by ACS Publications.

**Special Colloquium: Monday, December 3** — "An Introduction to Quantitative Finance from the Viewpoint of an Experimental Physicist"

• Presenter: Joseph Mitchell, former researcher at Renaissance Technologies
• NOTE SPECIAL LOCATION: DUAN G125
• Host: Jerry Peterson
• Abstract: During the past several decades it has become common for Wall Street firms to hire workers trained in Physics, Mathematics and other technical fields. Given this, it is perhaps interesting for physicist to learn a bit about quantitative finance. We will discuss some properties of financial data and the very basics of risk and cost models. Along the way we will contrast some aspects of financial data analysis with the data analysis of physics experiments. Finally, time permitting, we will discuss a textbook example of a trading strategy.

December 5 — "Watching Chemical Reactions Happen One Molecule at a Time"

• Presenter: Heather Lewandowski, JILA, Department of Physics, University of Colorado Boulder
• Host: John Cumalat
• Abstract: Reactions between ions and radical molecules play an important role in the chemistry that drives dynamics in the interstellar medium and during combustion of hydrocarbons. Unfortunately, experimental measurements of these reactions are very challenging, and thus very rare. We use tools borrowed from the cold atom community to measure ion-molecule reactions in a well-controlled environment. Here, we can study reactions between atoms and molecules in single quantum states at low temperatures. Our high sensitivity allows us to study reactions where the reaction rate can be as low as one reaction per minute. I will present the capabilities of this cold ion-molecule reaction apparatus and some example reactions we have been able to study using this new system.

December 12 — "Mixed Conduction in Polymeric Materials: Electrochemical devices for Biosensing and Neuromorphic Computing"

• Presenter: Alberto Salleo, Stanford University
• Host: Sean Shaheen
• Abstract: Organic semiconductors have been traditionally developed for making low-cost and flexible transistors, solar cells and light-emitting diodes. In the last few years, emerging applications in health case and bioelectronics have been proposed. A particularly interesting class of materials in this application area takes advantage of mixed ionic and electronic conduction in certain semiconducting polymers. Indeed, the ability to transduce ionic fluxes into electrical currents is useful when interacting with living matter or bodily fluids. My presentation will first discuss the fundamental aspects of how mixed conduction works in polymeric materials and then focus on two families of devices made with such materials: electrochemical transistors and artificial synapses.
  1- Biosensing using electrochemical transistors: The continuous monitoring of human health can greatly benefit from devices that can be worn comfortably or seamlessly integrated in household objects, constituting “health-centered” home automation aka "domotics". I will describe electrochemical transistors that detect ionic species either directly present in body fluids or resulting from a selective enzymatic reaction (e.g. ammonia from creatinine) at physiological levels. Additionally, I will show that non-charged molecules can be detected by making use of custom-processed polymer membranes that act as “synthetic enzymes”. Using these membranes in conjunction with electrochemical transistors we demonstrate that we are able to measure physiological levels of cortisol in real human sweat. Finally, I will show a more biomimetic approach where the sensing layer is a lipid membrane stabilized at a liquid-liquid interface, which we use to detect antimicrobial compounds. The same basic device that we use for sensing can also be used for computing.
  2- Polymer-based artificial synapses: The brain can perform massively parallel information processing while consuming only ~1 - 100 fJ per synaptic event. I will describe a novel electrochemical neuromorphic device that switches at record-low energy (<0.1 fJ projected, <10 pJ measured) and voltage (< 1 mV, measured), displays >500 distinct, non-volatile conductance states within a ~1 V operating range. Furthermore, it achieves record classification accuracy when implemented in neural network simulations. Our organic neuromorphic device works by combining ionic (protonic) and electronic conduction and is essentially similar to a concentration battery. The main advantage of this device is that the barrier for state retention is decoupled from the barrier for changing states, allowing for the extremely low switching voltages while maintaining non-volatility. I will show that the device can rival commercial flash memory in terms of endurance and possibly switching time. When accessed with an appropriate switching device it exhibits excellent linearity, which is an important consideration for neural networks that learn with blind updates.
  Speaker Bio: Alberto Salleo is currently an Associate Professor of Materials Science at Stanford University. Alberto Salleo holds a Laurea degree in Chemistry from La Sapienza and graduated as a Fulbright Fellow with a PhD in Materials Science from UC Berkeley in 2001. From 2001 to 2005 Salleo was first post-doctoral research fellow and successively member of research staff at Xerox Palo Alto Research Center. In 2005 Salleo joined the Materials Science and Engineering Department at Stanford as an Assistant Professor and was promoted to Associate Professor in 2013. Salleo is a Principal Editor of MRS Communications since 2011.While at Stanford, Salleo won the NSF Career Award, the 3M Untenured Faculty Award, the SPIE Early Career Award, the Tau Beta Pi Excellence in Undergraduate Teaching Award, and the Gores Award for Excellence in Teaching, Stanford’s highest teaching award. He has been a Thomson Reuters Highly Cited Researcher since 2015, recognizing that he ranks in the top 1% cited researchers in his field.

For more information about colloquia this semester, contact: Mihaly Horanyi.

Colloquia are Wednesdays at 4:00 p.m. in DUAN G1B20, unless otherwise noted.

Coffee, tea and cookies will be available before regular colloquia beginning at 3:45 p.m. in DUAN G1B31.

January 17 — "Can Evolutionary Dynamics Be Understood Quantitatively?"

• Presenter: Daniel Fisher, Stanford University
• Host: Leo Radzihovsky
• Abstract: The basic laws of evolution have been known for more than a century and there is overwhelming evidence for the facts of evolution. Yet little is understood quantitatively about the dynamical processes that drive evolution: by physicists' standards the theory of evolution is far from fully-fledged. Huge advances in DNA sequencing technology and laboratory experiments have enabled direct observations of evolution in action and, together with theoretical developments, opened up great opportunities for dramatically advancing our understanding. This talk will focus on framing questions and on recent progress addressing some of these.    

January 24 — "Zen and the Art of Atomic Physics"

• Presenter: Eric Hudson, University of California, Los Angeles
• Host: John Bohn
• Abstract: In the Ten Bulls tradition, the fifth stage of Zen is reached when the practitioner has caught and tamed the wild bull. This is roughly the state of atomic physics. The atom has been tamed. It can be prepared in a single quantum state, its evolution coherently controlled, and its entanglement generated at will.  As such, many new quantum-assisted technologies and fundamental physics measurements have resulted and a quantum revolution is well underway that promises to touch almost every aspect of our lives. By the seventh stage of Zen, the Bull has transcended and is no more. This again may be paralleled in atomic physics as the atom is being replaced with richer quantum systems such as molecules, whose internal degrees of freedom provide new opportunities for quantum science and technology.
  In this talk, I will briefly review the science and technology that have come from atom taming and then discuss our efforts to tame other bulls: polar molecular ions and atomic nuclei.

January 31 — "Exploring the Ocean Worlds of the Outer Solar System for Life"

• Presenter: Jonathan Lunine, Cornell University
• Host: Sascha Kempf
• Abstract: Planetary exploration of the Jovian and Saturnian systems has revealed that three moons—Europa, Titan and Enceladus-- have either liquid water oceans or, for Titan, seas of hydrocarbons that are potentially interesting abodes for life. I will describe what we know of these bodies, in what way we think they are habitable, and a strategy to determine if life is present on any or all of them. 

January 31 — "Exploring the Ocean Worlds of the Outer Solar System for Life"

• Presenter: Jonathan Lunine, Cornell University
• Host: Sascha Kempf
• Abstract: Planetary exploration of the Jovian and Saturnian systems has revealed that three moons—Europa, Titan and Enceladus-- have either liquid water oceans or, for Titan, seas of hydrocarbons that are potentially interesting abodes for life. I will describe what we know of these bodies, in what way we think they are habitable, and a strategy to determine if life is present on any or all of them. 

February 7 — "Dirac, Jordan, and von Neumann: Hilbert space and transformation theory"

• Presenter: Michel Janssen, University of Minnesota
• Host: Allan Franklin
• Abstract: In early 1927, Paul Dirac and Pascual Jordan, independently of one another, published their versions of a general formalism tying the various forms of the new quantum theory together and giving the theory's statistical interpretation in full generality. This formalism has come to be known as the Dirac-Jordan (statistical) transformation theory. A few months later, in response to these publications by Dirac and Jordan, John von Neumann published his Hilbert space formalism for quantum mechanics. The relation between the two formalisms can be captured in terms of a metaphor of arches and scaffolds that I have argued fits a number of instances of theory change in physics. What is unclear in this case is whether the story is best told with Hilbert space playing the role of the arch built on transformation theory as a scaffold to be dismantled once the arch could support itself, or with transformation theory playing the role of the arch and Hilbert space providing the scaffold built to prevent the arch from collapsing under the weight of its serious mathematical deficiencies. Either way, a narrative for this episode in the history of quantum mechanics based on the arches-and-scaffolds metaphor illustrates the promise of borrowing ideas from the approach to evolutionary biology known as evodevo for reconstructing genealogies of theories rather than species.

**Special Physics Faculty Candidate Search Colloquium** Thursday, February 8 — "Exploring the hydrodynamic limit of many-body quantum systems"

• Presenter: Andrew Lucas, Harvard University
• Location and time: JILA X317, 4:00 p.m
• Host: Paul Romatschke
• Abstract: The emergence of hydrodynamics in complicated microscopic many-body models has been a problem of interest in physics for more than a century. I will describe two ways that the classical theory of hydrodynamics is still teaching us interesting things about interacting many-body quantum systems. Firstly, I will derive the constraints relating classical hydrodynamic phenomena to the spreading of quantum information. These bounds sharpen and clarify many previous conjectures on whether quantum fluids have the smallest diffusion constants allowed in nature. I will briefly outline how an exotic class of quantum systems, which are holographically dual to a black hole in one higher dimension, serve as highly non-trivial checks on these diffusion bounds. Secondly, I will describe hydrodynamic theories of transport in metals. Even though measuring the thermal and electrical conductivity of a metal is very easy experimentally, conventional approaches to transport theory are not adequate for metals where the electronic interactions are strong. I will describe recent experimental evidence for hydrodynamic transport, including dramatic violations of the Wiedemann-Franz law, and present a new mechanism for electron-limited transport, possibly applicable to single-band materials including SrTiO3.

**Special Physics Faculty Candidate Search Colloquium** Monday, February 12 — "How low can the energy density go?"

• Presenter: Aaron Wall, University of Maryland
• Location: DUAN G125
• Host: Paul Romatschke
• Abstract: Quantum fields can sometimes have negative energy density.  In gravitational contexts, this threatens to permit both causality violations (such as traversable wormholes, warp drives, and time machines) and violations of the Second Law for black holes.  I will discuss the thermodynamic principles that rule out such pathological situations.  These principles have led us to an interesting lower bound on the energy flux, even for field theories in flat spacetime!
  This Quantum Null Energy Condition has recently been proven for general relativistic field theories.  I will give an intuitive argument explaining why such ``quantum energy conditions'' ought to hold.

February 14 — "Who ordered that: what lepton flavor may tell us about the Universe"

• Presenter: Yury G. Kolomensky, University of California, Berkeley
• Host: Eric Zimmerman
• Abstract: Muons, heavy cousins of electrons whose existence famously puzzled I.I. Rabi, may play an important role in understanding the behavior of particles and fields at the highest energies. Searches for transitions that violate the apparent lepton flavor conservation are sensitive to new interactions with the energy scale above those accessible in the modern colliders. Recent hints of deviations from the Standard Model expectations in the processes involving muons are intriguing, and motivate new measurements over the next decade. Among those, the Mu2e experiment at Fermilab will search for neutrinoless muon-to-electron conversion with unprecedented precision. I will discuss the history of the lepton flavor measurements, and describe the design and present status of Mu2e, which will improve the current constraints by four orders of magnitude. 

**Special Physics Faculty Candidate Search Colloquium** Thursday, February 15 — "Sources of student interest and engagement in Introductory Physics for Life Science"

• Presenter: Ben Geller
• Location: JILA Auditorium
• Time: 3:00 - 4:00 p.m.
• Host: Heather Lewandowski
• Abstract:  Effectively teaching an Introductory Physics for Life Science (IPLS) course means engaging life science students in a subject for which they may not have considerable preexisting interest. We have found that the inclusion of topical examples of relevance to life-science students can help to engage students whose initial interest in physics is less developed, but that different examples and models vary in their effectiveness. Examples that ground physical models in authentic biological and biochemical contexts with which the students are already familiar, and about which they may already have authentic driving questions, are especially effective. By analyzing data from (1) survey instruments assessing student attitudes and interest in particular life science examples, and (2) interviews conducted with students before and after instruction, we identify features of our IPLS course that appear to be particularly important for fostering student learning and engagement. We suggest that some of these features might also foster student interest in more traditional introductory physics courses. 

**Special Physics Faculty Candidate Search Colloquium** Thursday, February 15 — "Holographic phase diagrams"

• Presenter: Christiana Pantelidou, Imperial
• Location: JILA X317
• Host: Paul Romatschke
• Abstract: Over the last 20 years, the Gauge/Gravity correspondence has led to significant progress in the understanding of strongly coupled matter without quasiparticles, found both in Condensed Matter systems and Quantum Chromodynamics. In this talk, after an extended pedagogical introduction on the subject, I will discuss the various phases that have been discovered using this approach, how they compete with each other and what is currently known about their ground states.

**Special Physics Faculty Candidate Search Colloquium** Monday, February 19 — "The atoms of an expanding universe"

• Presenter: Dionysios Anninos, Harvard University
• Location: DUAN G125
• Host: Paul Romatschke
• Abstract: Observations over the past few decades have provided important evidence about the geometry of the large-scale universe.
  During the early inflationary era, as well as the current vacuum dominated era, the evidence points to a universe described by an exponentially expanding de Sitter spacetime. From the perspective of theoretical physics, the last two decades have seen remarkable progress in our understanding of spacetime as an emergent, collective phenomenon stemming from a microscopic holographic system with a large number of `atomic' constituents. This has been achieved with great success for negatively curved anti-de Sitter spacetimes. We will explore holography for a de Sitter universe. Our discussion will be guided by the construction and consideration of concrete mathematical models, providing an exact realization of the dS-CFT correspondence.
  We will also discuss similarities and distinctions between the cosmological horizon of a de Sitter universe and the horizon of an ordinary black hole from a modern, holographic perspective.

February 21 — "Superconducting Microresonators: From Astrophysics to Zero-Point Fluctuations"

• Presenter: Jonas Zmuidzinas, Caltech
• Host: Jason Glenn
• Abstract: The hallmark of superconductivity – zero electrical resistance – explains why superconducting resonators can have very low dissipation (high Q). Superconducting resonators at the macro scale have been developed for decades and enable energy-efficient high-energy accelerators such as the LHC. At the micro scale, superconducting resonators have attracted considerable attention in recent years and are being applied to a rapidly growing range of applications, from photon detectors for astrophysics to quantum circuits. This presentation will describe the advances in superconducting microresonators over the past two decades, focusing on their use as detectors and the relevant physical phenomena, but including a wide variety of other applications.

February 28 — "Command of Swimming Bacteria by Liquid Crystals"

• Presenter: Oleg Lavrentovich, Kent State University
• Host: Joe MacLennan
• Abstract: Self-propelled bacteria are marvels of nature. If we can control their dynamics, we could use it to power microsystems of the future. Unfortunately, bacteria swim mostly randomly in isotropic liquids such as water. It is difficult to control their dynamics by factors other than transient gradients of nutrients; visual, acoustic and tactile communication channels that humans use to control large animals are not effective. To establish communication, we replace water with a water-based lyotropic liquid crystal, which couples propulsion of bacteria to the orientational order of the medium. The long-range orientational order of the liquid crystal can be designed as uniform or be pre-patterned into various structures by a plasmonic photoalignment technique [1]. The preimposed patterns of liquid crystal orientation allow one to gain a significant control over the dynamics of bacteria, namely, their trajectories, polarity of swimming, spatial variation of concentration [2], and run-and-tumble behavior [3]. Topological defects of integer strength serve either as attractors or repellents of bacteria, while defect pairs and patterns with broken left-right symmetry pump the bacterial flows along a preselected polar axis. The study of bacteria-liquid crystal system might result in approaches to harness the energy of collective motion for micro-robotic, biomechanical, and sensing devices, as well as micro-mixing and transport of micro-cargo. The work is supported by NSF grants DMR-1507637 and DMS-1729509.

March 7 — "EPR and spatial-mode entanglement in spinor Bose-Einstein condensates"

• Presenter: Carsten Kempt, Institut für Quantenoptik, Leibniz Universität Hannover
• Host: Ana Maria Rey
• Abstract: Spin changing collisions in alkaline Bose-Einstein condensates can be employed to generate highly entangled atomic quantum states. Here, we will report on the generation of two classes of entangled states. Firstly, we demonstrate the generation of two-mode squeezed vacuum states and record their characteristic quadrature correlations by atomic homodyning. We prove that the correlations fulfill Reid’s criterion [1] for continuous-variable Einstein-Podolsky-Rosen entanglement. The homodyne measurements allow for a full tomographic reconstruction, yielding a two-mode squeezed state with a 78% fidelity. The created state can be directly applied to atom interferometry, as is exemplified by an atomic clock measurement beyond the Standard Quantum Limit.
  Secondly, we demonstrate entanglement between two spatially separated atomic modes. The entangled state is obtained by spatially splitting a Twin Fock state of indistinguishable atoms. The method opens a path to exploit the recent success in the creation of many-particle entanglement in ultracold atoms for the field of quantum information, where individually addressable subsystems are required. Finally, we will show how the measurement protocol can be extended to perform a Bell test of quantum nonlocality.

  [1] M. Reid, Phys. Rev. A 40, 913–923 (1989)

March 14 — "Throwing God's Dice"

• Presenter: Krister Shalm, NIST
• Host: John Price
• Abstract: In 1943 Einstein wrote to Max Born saying “As I have said so many times, God doesn't play dice with the world.” This discussion with Born was just one part of a much large debate on the consequences of quantum theory on the nature of reality. In 1935 Einstein, Podolsky, and Rosen famously published a paper with the aim of showing that the wave function in quantum mechanics does not provide a complete description of reality. The gedanken experiment showed that quantum theory, as interpreted by Niels Bohr, leads to situations where distant particles, each with their own “elements of reality”, could instantaneously affect one another. Such action at a distance seemingly conflicts with relativity. The hope was that a local theory of quantum mechanics could be developed where individual particles are governed by elements of reality, even if these elements are hidden from us. In such a theory, now known as local realism, these elements of reality or hidden variables could remove the randomness inherent in quantum mechanics.
  In 1964 John Bell in a startling result showed that the predictions of quantum mechanics are fundamentally incompatible with any local realistic theory. In other words, an experiment can be done that can rule out all theories based on local hidden variables. Carrying out this test has been technologically challenging. It wasn’t until 2015 when three independent groups were able to rule out local realism in experiments free of loopholes. In this talk I will discuss the loophole-free Bell test carried out at the National Institute of Standards and Technology. I will also discuss how we can use such a Bell test to build a random number generator that can be certified by quantum mechanics itself. Such a random number generator that can trace its roots back to the original Einstein thought experiments is the closest we can get to “throwing God’s dice.”

March 21 — "Exploring the 3D Nano and Atomic World: Coherent Diffractive Imaging and Atomic Electron Tomography"

• Presenter: John Miao, Deputy Director, NSF STROBE Science and Technology Center, Department of Physics and Astronomy and California NanoSystems Institute, University of California Los Angeles
• Host: Margaret Murnane
• Abstract: The discovery and analysis of X-ray diffraction from crystals by Max von Laue, William Henry Bragg and William Lawrence Bragg in 1912 marked the birth of crystallography. Over the last century, crystallography has been fundamental to the development of many fields of science. However, many samples in physics, chemistry, materials science, nanoscience, geology, and biology are non-crystalline, and thus their 3D structures are not accessible by traditional crystallography. Overcoming this major hurdle has required the development of new structure determination methods. In this talk, I will present two methods that can go beyond crystallography: coherent diffractive imaging (CDI) and atomic electron tomography (AET). In CDI, the diffraction pattern of a non-crystalline sample or a nanocrystal is first measured and then directly phased to obtain an image. The well-known phase problem is solved by combining the oversampling method with iterative algorithms. In the first part of the talk, I will illustrate several prominent CDI methods and highlight some important applications using high harmonic generation, 3rd generation synchrotron radiation and X-ray free electron lasers. In the second part of the talk, I will present a general tomographic method, termed AET, for 3D structure determination of crystal defects and disordered materials at atomic resolution. By combining advanced electron microscopes with novel data analysis and powerful computational algorithms, AET has been used to reveal the 3D atomic structure of crystal defects and chemical order/disorder, and to precisely localize the 3D coordinates of individual atoms in materials without assumption of crystallinity. The experimentally measured coordinates can then be used as direct input for quantum mechanical calculations of physical properties such as atomic spin and orbital magnetic moments and local magnetocrystalline anisotropy at the single-atom level. As large-scale and tabletop coherent X-ray sources and powerful electron microscopes are under rapid development worldwide, CDI and AET are expected to find broad applications in both the physical and biological sciences.
  1.    J. Miao, T. Ishikawa, I. K. Robinson and M. M. Murnane, “Beyond crystallography: Diffractive imaging using coherent x-ray light sources”, Science 348, 530-535 (2015). (Review)
  2.    J. Miao, P. Ercius and S. J. L. Billinge, "Atomic electron tomography: 3D structures without crystals", Science 353, aaf2157 (2016). (Review)
  3.    Y. Yang, C.-C. Chen, M. C. Scott, C. Ophus, R. Xu, A. Pryor Jr, L. Wu, F. Sun, W. Theis, J. Zhou, M. Eisenbach, P. R. C. Kent, R. F. Sabirianov, H. Zeng, P. Ercius and J. Miao, “Deciphering chemical order/disorder and material properties at the single-atom level”, Nature 542, 75-79 (2017).

March 28 — Spring Break; No Colloquium

April 4 — "Quantum-limited measurements: One physicist's crooked path from quantum optics to quantum information"

• Presenter: Carlton Caves, University of New Mexico
• Host: Scott Diddams
• Abstract: Quantum information science has changed our view of quantum mechanics. Originally viewed as a nag, whose uncertainty principles restrict what we can do, quantum mechanics is now seen as a liberator, allowing us to do things, such as secure key distribution and efficient computations, that could not be done in the realistic world of classical physics. Yet there is one area, that of quantum limits on high-precision measurements, where the two faces of quantum mechanics remain locked in battle. I will trace the history of quantum-limited measurements, from the use of nonclassical light to improve the phase sensitivity of an interferometer, to the modern perspective on the role of entanglement in improving measurement precision.

April 11 — "From the Einstein-Bohr debate to quantum information: the second quantum revolution"

• Presenter: Alain Aspect, Institut d’Optique Graduate School, Palaiseau
• Host: Jun Ye
• Abstract: In 1935, with co-authors Podolsky and Rosen, Einstein discovered a weird quantum situation, in which particles in a pair are so strongly correlated that Schrödinger called them “entangled”. By analyzing that situation, Einstein concluded that the quantum formalism is incomplete. Niels Bohr immediately opposed that conclusion, and the debate lasted until the death of these two giants of physics.
  In 1964, John Bell discovered that it is possible to settle the debate experimentally, by testing the now celebrated "Bell's inequalities", and to show directly that the revolutionary concept of entanglement is indeed a reality. A long series of experiments, started in 1972, have produced more and more precise results, in situations closer and closer to the ideal theoretical scheme.
  After explaining the debate, and describing some experiments, I will show how this conceptual discussion has prompted the emergence of the new field of quantum information, at the heart of the second quantum revolution.

**Special Physics Faculty Candidate Search Colloquium** Tuesday, April 17 — "Polarization in spectral lines: a window into the physics of the solar atmosphere"

• Presenter: Ivan Milić, Max Planck Institute for Solar System Research
• Location and time: JILA Auditorium, 4:00 p.m
• Host: Dmitri Uzdensky
• Abstract: The light that we receive from the Sun has passed through the solar atmosphere and undergone many scattering processes that leave their imprint on its spectral distribution and polarization state. These processes are influenced by the thermodynamic and magnetic properties of the plasma and thus the spectropolarimetric observations in spectral lines allow us to infer the temperature, velocity and magnetic field throughout the solar atmosphere.
  This talk will focus on the Zeeman effect, scattering polarization and the Hanle effect, and how they contribute to form line polarization patterns. More specifically, the emphasis will be on spectral lines formed in non-local thermodynamic equilibrium conditions. Such lines typically probe the physical conditions in the upper layers of solar atmosphere, regions that are particularly interesting for their highly dynamic and magnetic nature. The diagnostic techniques, however, can be extended to a multitude of astrophysical objects, such as atmospheres of other stars and different types of accretion or circumstellar disks.
  We will also discuss spectropolarimetric inversions, as the state-of-the-art tools used for the interpretation of the solar observations with high angular and spectral resolution. Finally, a brief outlook on the bright future of the field, especially in the light of Daniel K. Inouye Solar Telescope, will be presented.
  Image: Observed intensity and circular polarization (top two panels), and inferred temperature, line-of-sight velocity and line-of-sight magnetic field at two different depths in an observed patch of the solar atmosphere.

April 18 — "Optical atomic clock: a case study for the quantum technology revolution"

• Presenter: Jun Ye, JILA, University of Colorado Boulder
• Host: John Price
• Abstract: Precise quantum state engineering of individual atoms has led to the unprecedented measurement performance for time and frequency. The use of many atoms not only enhances the counting statistics, but is also emerging as a powerful tool to protect against systematic uncertainties. At the core of the new JILA three-dimensional optical lattice clock is a quantum gas of fermionic atoms that are spatially correlated to guard against motional and collisional effects.  The convolution of precision control of light and matter is helping bridge different disciplines in physics and fostering new capabilities to probe fundamental and emerging phenomena.

**Special Colloquium** Wednesday, April 25 — "Driven Phases of Many-Body Quantum Matter"

• Presenter: Vedika Khemani, Harvard University
• Location and time: JILA Auditorium, 12:00 p.m
• Host: Leo Radzihovsky
• Abstract: Recent years have witnessed a remarkable confluence of diverse areas of physics coming together to inform fundamental questions about many-body quantum matter. A unifying theme in this enterprise has been the study of many-body quantum dynamics in systems ranging from electrons in solids to cold atomic gases to black holes.  One of the foundational pillars in the study of many-body systems is the theory of equilibrium statistical mechanics characterized by two fundamental ideas: thermalization (that interacting systems generically approach thermal equilibrium at late times) and phase structure (that equilibrium states of matter can display various forms of order separated by sharp phase transitions). 

  Recent progress, particularly in the field of many-body localization, has led to generalizations of these fundamental ideas to the out-of-equilibrium setting. I will describe this progress, particularly as applied to periodically driven or Floquet systems. I will show that not only can non-equilibrium systems exhibit a sharp notion of phase structure, but that some of these phases are completely novel and unique to the out-of-equilibrium setting. For example, certain phases of matter that are forbidden in equilibrium, such as quantum time crystals, have found new life in the out-of-equilibrium setting.  I will describe some of the many fascinating properties of this phase, and comment on recent experiments that have detected signatures of time crystals in a variety of different physical settings. 

April 25 — "Hot tips for thermal nanophysics"

• Presenter: Fabian Menges, University of Colorado, Boulder
• Host: John Price
• Abstract: Temperature is one of the most central concepts of thermal physics with historical roots in the seventeenth century mechanics and nineteenth century thermodynamics. Building up on these early foundations, I will highlight how the understanding of temperature gets increasingly challenging when small systems are away from equilibrium, get scaled to ensemble sizes below classical limits, and when quantum effects become relevant. Today, the apparently simply question of ‘what is the temperature’ is again up for debate, fueled by the miniaturization of technology and the experimental progress to study physical properties and processes down to atomic length and ultrafast time scales. Aiming to clarify questions such as how to measure temperature on the length-scale of nanoscale transistors, how interfaces and contacts influence dissipation processes, and how fundamental heat and charge transport relations may change at the atomic scale, we have recently developed new thermal nanometrology techniques based on scanning probe methods [1]. By demonstrating the real-space quantification of local Joule and Peltier effects at metal-semiconductor contacts [2], and the first atomic scale validation of the Wiedemann-Franz law at room temperature [3], I will illustrate the application of these approaches. Finally, I’ll provide an outlook on our ongoing efforts to open the door for spatio-temporal characterization of thermal phenomena at the transition from the classical to the quantum regime.

  References:

  [1] F. Menges et al., Nanoscale thermometry by scanning thermal microscopy, Review of Scientific Instruments (87) 7, 074902, 2016. 

  [2] F. Menges et al., Temperature mapping of operating nanoscale devices by scanning probe thermometry, Nature Communications 7(10874), 2016.

  [3] N. Mosso et al., Heat transport through atomic contacts, Nature Nanotechnology 12, 430-433, 2017.

May 2 — "The Quest for a Quantum Spin Liquid*"

• Presenter: Collin Broholm, Johns Hopkins University
• Host: Dmitry Reznik
• Abstract: Magnetism has commanded human wonder through millennia and it is a central component of the technologies that shape our lives. For the past 50 years scientists have been in pursuit of a radically new form of magnetism that would constitute a new state of matter: The quantum spin liquid may exist within a crystalline solid composed of atoms that carry a magnetic dipole moment. However, quantum fluctuations of these dipole moments preclude the development of conventional magnetic order even at temperature far below the scale of inter-site interactions. The result is a quantum material with unique macroscopic properties driven by quantum coherence and entanglement. If we can realize the materials physics of the quantum spin liquid these properties can be fully explored and might lead to interesting applications much as other advances we have made in our understanding of magnetism.
  In the ongoing quest for a robust realization of a quantum spin liquid in the lab, a range of interesting magnetic materials and phenomena have been discovered. I shall review experiments probing interacting quantum spins on kagome, honeycomb, and triangular lattices in 2D and on the pyrochlore lattice in 3D. These frustrated quantum magnets feature an intermediate energy and temperature regime with spin-liquid-like properties but also unique low temperature phases driven by quenched disorder or lattice instabilities. Such inevitable deviations from ideal spin liquid models are interesting in their own right and their elucidation may contribute to understanding age old puzzles such as the phase diagram of V₂O₃ which I shall describe in greater detail in my seminar the following day.
  * Supported by U.S. DoE Basic Energy Sciences, DE-FG02-08ER46544 and by the Gordon and Betty Moore foundation GBMF 4532.

For more information about colloquia this semester, contact: John Price.

Colloquia are Wednesdays at 4:00 p.m. in DUAN G1B20, unless otherwise noted.

Coffee, tea and cookies will be available before regular colloquia beginning at 3:45 p.m. in DUAN G1B31.

August 30 — "What are Saturn’s rings made of?"

• Presenter: Sascha Kempf, University of Colorado, Boulder
• Host: 
• Abstract: When Galileo Galilei discovered Saturn’s rings 450 years ago he didn’t even know that he had observed rings. It was Christiaan Huygens who proposed that Saturn is surrounded by a thin solid ring - made of metal. Earth-based spectroscopic observations in 1970 revealed that the rings in fact consist of 95% clean water ice, but the nature of the other 5% of embedded material, which gives the rings their unique reddish tint, is still unknown. It is, however, this little amount of unknown material that will bring us closer to understanding the rings’ origin and provide valuable information about the formation of the Saturnian system as a whole.

  After almost 20 years in space, NASA's Cassini spacecraft has begun the final chapter of its remarkable story of exploration: its Grand Finale. Before the spacecraft plunges into Saturn’s atmosphere Cassini is undertaking a daring set of orbits that is, in many ways, a whole new mission. Starting in late April this year Cassini is performing weekly dives between the planet and the inner rim of Saturn’s rings. No other mission has ever explored this region that has been considered until a few years ago to be inaccessible for space probes. Those unique orbits allowed the Cosmic Dust Analyzer (CDA) on Cassini for the very first time to collect material originating from the main rings itself and to identify their composition.

  In my talk, I will report about the exciting findings by CDA during Cassini’s swan song. 

September 6 — "Generation and detection of tunable orbital angular momentum in polarization-maintaining optical fiber"

• Presenter: Juliet Gopinath, University of Colorado, Boulder
• Host: John Price
• Abstract: In the past few years, twisted light, or light with orbital angular momentum (OAM) has captured interest a diverse array of applications. It can be used to drive micromachines and in biophotonics, OAM is used in microscopy to achieve resolution orders of magnitude better than the diffraction limit. In astronomy, the OAM of light from distant stars carries information about the inhomogeneity of the interstellar medium and the shape of black holes. In quantum information science, OAM states can be entangled, which leads to surprising demonstrations of quantum mechanics and potentially new computational possibilities. OAM-enabled communications, of all the applications, has received the majority of the attention due to the potential increase in fiber optic bandwidth that would be realized in a move from binary to a classical or quantum mechanical-vast parameter space per photon provided by the integer OAM states.

  To date, specialty fiber has been used to carry OAM. In this talk, I will describe new methods for generating OAM light in commercially available polarization maintaining optical fiber. By adding up two higher order modes, generation of tunable OAM can be demonstrated. In addition, I will discuss a new quantitative detection method for light with OAM and extensions of the technique. Finally, I will present a method to control independently, both the OAM and the spatial beam profile.

  Biography:Juliet Gopinath is an Associate Professor of Electrical, Computer and Energy Engineering at the University of Colorado Boulder. She received her B.S. degree in Electrical Engineering from the University of Minnesota and her M.S. and Ph.D. degrees at MIT. She worked at MIT Lincoln Laboratory from 2005 to 2009 on topics including cryogenic Yb:YAG lasers, beam combining, and mode-locked diode lasers. Since 2009, she has run a group focused on optical devices and lasers at 91��ý. Her current research interests include ultrafast lasers, nonlinear optics, mid-infrared materials, spectroscopy, orbital angular momentum and adaptive optical devices. She is the recipient of an Air Force Young Investigator Award (2010), R & D 100 Award (2012), an NSF CAREER award (2016), the CU Provost Achievement Award (2016) and is an Associate Editor for IEEE Photonics Journal.

September 13 — "Enabling Technology Innovation through Plasma Modeling: Sustainability and Biotechnology as the Next Frontiers"

• Presenter: Mark Kushner, University of Michigan
• Host: John Cary
• Abstract: The field of low temperature plasmas (LTPs) has provided enabling sciences and technologies that are arguably responsible for huge swaths of our industrial and high technology infrastructure.  The information technology revolution has been singularly enabled by the ability to fabricate microchips using LTPs.  Virtually every human implant is fabricated or made biocompatible using LTPs.  All non-incandescent lighting sources rely on LTPs, either directly as the source of photons or indirectly through materials fabrication.  The current generation of jet engines are enabled by LTP processing, and interplanetary missions are made possible by LTP propulsion.  Now, LTPs have the potential to treat wounds and disease, and to convert green-house gases to high value chemicals.  Many of these advances are the end result of scientifically and experimentally inspired technology development, and in some cases incremental advances over many years, with little input from modeling.  The plasma chemistry and plasma surface interactions that are responsible for these successes have untold complexity that acutely challenge diagnostics and modeling, as well as the underlying AMO (atomic, molecular, optical) physics knowledge base.  A legitimate question is – has modeling and simulation been significantly influential in the development of LTP enabled technologies?  Are modeling and simulation LTPs capable of leading innovation?  In this talk, the role and potential of modeling and simulation in the LTP innovation chain will be reviewed with examples from materials processing and the next frontiers of biotechnology and sustainability.  Examples of where modeling has provided insights that stimulated, if not enabled, technology development will be discussed.

September 20 — "ColdQuanta Incorporated: The History and Future of a University Spinoff Company*"

• Presenter: Dana Anderson, JILA, University of Colorado, Boulder
• Host: John Price
• Abstract: ColdQuanta Inc. was founded in 2007 by three physicists and a businessman, myself, Theodor Hänsch, Jakob Reichel and Rainer Kunz.  Its mission is to develop and commercialize cold and ultracold matter technology that can enable the scientific and applications communities.  Much of ColdQuanta’s early technology emerged from a MURI grant from the Army Research Office and more so, a large DARPA grant to develop technology for guided Bose-Einstein Condensate (BEC) applications. Out of that effort came compact components for producing ultracold matter and in particular a sophisticated “atom chip” technology that made it possible to considerably reduce the size of BEC machines and similar systems.  A BEC machine that in the past took many months to build, occupied a large optical table and needed racks of electronic instrumentation can now be purchased from ColdQuanta, occupies a single rack, and can be up and running in six hours.  Since 2007 ColdQuanta has produced instruments for making cold atoms in undergraduate labs, and has enabled numerous applications and scientific experiments from optical lattices to quantum computing to the JPL/NASA mission to put cold atoms on the International Space Station.  This talk tells the story of how ColdQuanta was formed and highlights the role of students and spin-off companies in the University’s mission to provide an educated workforce, to promote scientific advancement, and to maintain this nation’s technological leadership.  It also covers some relevant “this is reality” topics such as starting a tech business, money, and conflicts of interest.  Regarding the latter: 

  *I hereby disclose that I have a financial interest in ColdQuanta, Inc. (!)

September 27 — "The Quest for the New Standard Model: Searching for BSM Physics with Rare-Isotope Beams"

• Presenter: Kyle Leach, Colorado School of Mines
• Host: Eric Zimmerman
• Abstract: The development of the Standard Model (SM) has been one of the crowning achievements in modern physics, and is the cornerstone of current subatomic studies. Despite its success, the SM is known to be incomplete, and providing limits on possible physics beyond the Standard Model (BSM) is crucial to our understanding of the natural universe.  Although they are generally complex, atomic nuclei can be exploited as a laboratory for these studies through the use of rare-isotope beams (RIBs).  The production of these short-lived, very exotic isotopes has opened new avenues of research in our search for BSM physics in the era of the LHC.  This work is at the precision and sensitivity frontiers, and helps to bridge the gap between atomic, nuclear, and particle physics using novel, state-of-the-art detection techniques. In this talk, I will use these topics to highlight the significant role of the atomic nucleus in our ongoing search for additional generations of quarks, new descriptions of the weak interaction, and light dark matter. These studies play a critical role in providing the groundwork for our quest to develop the "New Standard Model".

October 4 — "Broadening the Searchlight: New Ideas in Dark Matter Detection"

• Presenter: Kathryn Zurek, LBNL
• Host: Ethan Neil
• Abstract: Searches for massive dark matter have largely focused on a mass window near the weak scale, the so-called "WIMP window". This window is, however, becoming increasingly closed by both the LHC and the unprecedented sensitivity of direct detection experiments. At the same time, theoretical work in recent years has shown lighter dark matter candidates in a hidden sector are theoretically well-motivated, natural and arise generically in many theories beyond the standard model. New ideas are needed to search for dark matter with mass below a GeV and as light as the warm dark matter limit of a keV. We propose new ideas to search for such light dark matter with superconductors, semi-conductors, graphene, Dirac materials, and superfluid helium. We show that these same experiments, through inelastic processes, may also be sensitive to dark matter with masses in the meV to keV mass window, broadening the mass reach to light dark matter by many orders of magnitude.

October 11 — "Spin Orbit Coupled Quantum Magnetism"

• Presenter: Kate Ross, Colorado State University
• Host: Chuck Rogers
• Abstract: Strong quantum fluctuations and spin entanglement can lead to exotic emergent many body properties of materials, such as fractionalized excitations predicted for quantum spin liquids, or the field-tuned Bose Einstein Condensation observed in quantum dimer crystals.  Traditionally, the materials space in which these types of phases are sought has been limited to materials with “pure" spin 1/2, as obtainable from Cu2+ for instance, and most theories have therefore focused on the isotropic exchange limit.  However, attention has recently shifted towards quantum materials in which an interplay of strong spin orbit coupling and crystal electric field effects lead to a "pseudo-spin" 1/2.  The magnetic interactions in these materials can be described by anisotropic effective exchange models, which can lead to new predicted quantum many body phenomena such as the Majorana fermion excitations of Kitaev quantum spin liquid, or emergent electrodynamics in quantum spin ice.  I will discuss some recent material examples that exemplify this new paradigm.

October 18 — "The Remarkable Ways in Which Gases Dissolve and React in Water"

• Presenter: Gilbert Nathanson, Department of Chemistry, University of Wisconsin, Madison
• Host: David Nesbitt
• Abstract: Interfacial reactions between atmospheric gases and sea spray play a vital role in our air quality and climate.  These reactions also display fascinating dynamics at the atomic scale.  From this microscopic perspective, the interfacial encounter begins when gas molecules strike the surface of an aqueous solution that might contain ions and biological molecules.  We rely on understanding the physics of these gas-liquid collisions to construct a “blow-by-blow” picture of the solvation and reaction of acids, bases, and oxidizers in such complex solutions.  I will describe experiments using microjets and coated wheels that enable us to explore sea-spray mimics inside a vacuum chamber and help reveal how aerosol-mediated reactions takes place.

October 25 — "Generation and Application of Attosecond Laser Pulses"

• Presenter: Andreas Becker, JILA, University of Colorado, Boulder
• Host: John Price
• Abstract: High harmonic generation provides a unique technique to generate coherent light up to keV photon energies, which is emitted in pulses a few tens of attoseconds in duration. Such pulses can now be used to probe dynamics in matter on the time scale of electronic motion. I will present our theoretical efforts to perform numerical calculations capturing the highly nonlinear process of attosecond pulse generation on both the microscopic level of a single atom and the macroscopic level of the generating gas medium consisting of billions of atoms. In the second part I will then discuss examples of the application of attosecond pulses to resolve electron dynamics in atoms and molecules.

November 1 — "Transient Crosslinkers Tune the Patterns of Microtubule Filaments"

• Presenter: Jennifer Ross, University of Massachusetts, Amherst 
• Host: Meredith Betterton
• Abstract: The cell is a complex autonomous machine taking in information, performing computations, and responding to the environment. To enable agile read/write capabilities, much of the molecular biochemistry that performs these computations must be transient and weak, allowing signals to be carried as a function of the concentration of numerous and coupled interactions. Traditionally, biochemical experiments can only measure strongly interacting systems that can last for long times in dilute concentrations. We have developed microscopy measurements to enable to visualization of weak, transient interactions and the resulting emergent behaviors of coupled systems. I will present excerpts from stories where many weak, transient interactions can have strong repercussions on the overall activity and can, in fact, overpower strongly interacting systems. These studies involve the microtubule cytoskeleton and the transport motor, kinesin-1.  Our results reveal a fundamentally important aspect of cellular self-organization: weak, transient interacting species can tune their interaction strength directly by tuning the local concentration to act like a rheostat. The tunability of weak, transient interactions is a fundamental activity of biological systems, and our insights will ultimately enable us to learn how to engineer these systems to create biological or biomimetic devices.

  Biography: Ross is the director of the new Massachusetts Center for Autonomous Materials (MassCAM) and an award-winning biophysicist studying the organization of the microtubule cytoskeleton and microtubule-based enzymes using high-resolution single molecule imaging techniques. She has a degree in Physics and has studied the microtubule cytoskeleton for over a decade. As a Cottrell Scholar, Ross has pioneered innovative teaching techniques that are being adopted around the world. Specifically, she has taught at several international short courses on microscopy including Analytical and Quantitative Microscopy (AQLM) at the Marine Biology Laboratory and the Bangalore Microscopy Course at the National Centre for Biological Science in Bangalore, India. She has also served as the President of NESM in the past. She is also an advocate for women and under-represented groups and has a blog to help others make it in academics.

November 8 — "Tests of quantum mechanics and gravitation with atom interferometry"

• Presenter: Mark Kasevich, Stanford University
• Host: Scott Diddams
• Abstract: Recent de Broglie wave interference experiments with atoms have achieved wavepacket separations as large as 54 cm over time intervals of 2 sec. These experiments, and their impact on gravitational and quantum physics, will be discussed.

November 15 — No colloquium

November 22 — Fall Break; No Colloquium

November 29 — "What's Next in Higgs Physics?"

• Presenter: Sridhara Dasu, University of Wisconsin 
• Host: Bill Ford
• Abstract: The discovery of the Higgs boson in 2012 by the LHC experiments, ATLAS and CMS, was celebrated enthusiastically by the world. The new datasets from 2015-16 have solidified that discovery, further establishing the H(125 GeV) to be Standard Model (SM)-like. Nevertheless, explorations of the Higgs sector beyond the SM are important, as they often involve additional scalar and pseudo-scalar particles. We are making significant headway in mapping out the details of the Higgs sector already, searching for heavier Higgs bosons and rare decays of the H(125). This scalar Higgs sector could also be a portal to the "dark" sector, which we know little about from particle physics point of view. This talk will discuss the latest status of the CMS Higgs sector exploration.
  Further exploration of Higgs sector is promising, but is experimentally challenging, due to low energies of the Higgs decay products.
  Implications for future improvements needed for the experimental facilities will also be briefly discussed.

December 6 — "From simple to complex atoms for atomic qubits and scalable quantum computing"

• Presenter: Mark Saffman, University of Wisconsin 
• Host: Dana Anderson
• Abstract: Quantum computing is a few decades old and is currently an area where there is great excitement and rapid developments. A handful of distinct approaches have shown the capability of on-demand generation of entanglement and execution of basic quantum algorithms.
  One of the daunting challenges in developing a quantum computer is the need for a very large number of qubits. Neutral atoms are one of the most promising approaches for meeting this challenge. I will give a snapshot of the current status of atomic quantum computing, describe the physics underlying neutral atom qubits and quantum gates, and show how one of the most complicated atoms in the periodic table may lead to some simple solutions to hard problems.   
• Bio: Mark Saffman is an experimental physicist working in the areas of atomic physics, quantum and nonlinear optics, and quantum information processing. He has made significant contributions to the physics of optical solitons, pattern formation, sources of entangled light, and quantum computing. His current research effort is devoted to the development of neutral atom based quantum computing devices. His research team was the first to demonstrate a quantum CNOT gate between two trapped neutral atoms, and the deterministic entanglement of a pair of neutral atoms. This was done using dipole mediated interactions between highly excited Rydberg atoms. He is currently developing scalable neutral atom platforms using arrays of trapped atoms.  
  He is a Professor of Physics at the University of Wisconsin-Madison, and a fellow of the American Physical Society and the Optical Society of America. He has been recognized with the Alfred P. Sloan Fellowship and a University of Wisconsin Vilas Associate Award. He also serves as an Associate Editor for Physical Review A.

December 13 — "First Results from CUORE: Majorana Neutrinos and the Search for Neutrinoless Double-Beta Decay"

• Presenter: Lindley Winslow, MIT
• Host: Alysia Marino
• Abstract: The neutrino is unique among the Standard Model particles. It is the only fundamental fermion that could be its own antiparticle, a Majorana particle. A Majorana neutrino would acquire mass in a fundamentally different way than the other particles and this would have profound consequences to particle physics and cosmology. The only feasible experiments to determine the Majorana nature of the neutrino are searches for the rare nuclear process neutrinoless double-beta decay.  CUORE uses tellurium dioxide crystals cooled to 10 mK to search for this rare process.  In this talk, I will present the first results from this detector and highlight my group’s R&D efforts and our other efforts including axions and nanoparticle-based liquid scintillators.

For more information about colloquia this semester, contact: John Price.

Colloquia are Wednesdays at 4:00 p.m. in DUAN G1B20, unless otherwise noted.

Coffee, tea and cookies will be available before regular colloquia beginning at 3:45 p.m. in DUAN G1B31.

January 18 — "Gravitational Waves, Colliding Black Holes, And Tornadoes In Space-Time: The Dawn Of A New Astronomy"

• Presenter: David Reitze, LIGO Laboratory and University of Florida, Gainesville
• Host: Henry Kapteyn
• Abstract: The first direct detections of gravitational waves in late 2015 were made possible by a forty year experimental campaign to design, build, and operate LIGO, the Laser Interferometer Gravitational-wave Observatory. In this colloquium, I’ll cover gravitational waves and what makes them so difficult to detect and at the same time such powerful and unique probes of the universe. I’ll also give a flavor of the somewhat complicated history of how LIGO was conceived and built. Most of the presentation will focus on the interferometers, the LIGO detections and their astrophysical implications. Time permitting, I’ll give a preview of where LIGO intends to go in the next decade and beyond.  

January 25 — "15 Years of PhET Interactive Simulations: Cycles of Innovation, Learning, and Impact"

• Presenter: Kathy Perkins, University of Colorado, Boulder
• Host: John Price
• Abstract: With a collection of 134 interactive simulations for teaching science and math, and over 100 million simulation uses per year worldwide, the PhET Interactive Simulations project has come a long way since its beginning in 2002. Founded by our colleague Carl Wieman as the “Physics Education Technology” project, his vision was to make physics engaging and accessible for all learners by tapping into a natural curiosity about real world phenomena. The journey from 2002 to today has included many cycles of innovation and learning as PhET expanded from physics to chemistry to math, from college to middle school, and from a local resource to an international mainstay. We will reflect on these past cycles – the challenges and the solutions – and how physics and the physicist’s perspective has shaped and advanced our work throughout. We will also look ahead to what’s on the horizon for PhET – bringing science inquiry to students with disabilities, advancing assessment of science practices, transforming middle school math, and building a sustainable business model.

**Special Colloquium** Monday, January 30 — "Unlocking neutrino mysteries with the NOvA experiment"

• Presenter: Christopher Backhouse, California Institute of Technology
• Host: Kevin Stenson
• **NOTE SPECIAL LOCATION: DUAN G130
• Abstract: The 2015 Nobel Prize in Physics was awarded for the discovery of the phenomenon of neutrino oscillations, which implies that neutrinos are not massless as we had previously believed. This raises a wealth of new and intriguing questions. What is the ordering of the neutrino mass states? Might they violate matter/antimatter symmetry? What structure, if any, does the neutrino mixing matrix have? The NOvA experiment directly addresses these questions by measuring changes undergone by a powerful neutrino beam over an 810 km baseline, from its source at Fermilab, Illinois to a huge 14 kton detector in Ash River, Minnesota. I will give a brief overview of neutrino oscillations, then present our latest results, their implications, and prospects for the future.

February 1 — "The 1,358,954,496 Matrix Elements to Get From SUSY Diff Eq's to Pictures, Codes, Card Games, Music, Computers, and Back Again"

• Presenter: S. James Gates, University of Maryland
• Host: Clifford Bridges
• Abstract: In this presentation a discussion is given of a recent derivation of a supersymmetrical QM representation spectrum that took some surprising twists and turns along the way.

February 8 — "Quarks of Many Colors"

• Presenter: Tom DeGrand, University of Colorado Boulder
• Host: John Price
• Abstract: Numerical simulation of quantum chromodynamics  (QCD) is a successful way to compute the properties of strongly interacting particles from first principles. However, it doesn't give much insight into why the numbers are what they are. Long ago Hooft argued that QCD could be understood most simply when the “three'” of the three colors was taken to be a large number. But nobody could compute anything like a reduced matrix element with this idea. Recently, people have begun to marry these two stories.  I'll tell you a bit about them both, put them together, and  see what we find. Remarkable regularities emerge.

Special Colloquium — Monday, February 13 "Physics at the LHC: Why and How to go Beyond the Higgs Discovery"

• Presenter: John Alison, University of Chicago
• NOTE SPECIAL LOCATION: DUAN G130
• Host: Kevin Stenson
• Abstract: The Standard Model (SM) of particle physics is a spectacularly successful theory that is known to be fundamentally incomplete.  The recent discovery of the Higgs boson at the Large Hadron Collider is, on one hand, the final missing piece of the SM and, on the other, a window into what lies beyond.  I will discuss the motivations and experimental challenges of searching for physics beyond the SM at the LHC.  Emphasis will be placed on using the Higgs boson as a probe of new physics in processes involving pairs of Higgs bosons.

February 15 — "Looking for Fossils of the Big Bang in Molecular Spectra"

• Presenter: Eric Cornell, JILA and University of Colorado, Boulder
• Host: John Price
• Abstract: How can you learn about the early moments of the universe? How can you discover evidence for new sub-atomic particles? We usually think of ever-more exotic telescopes, or of ever-larger particle accelerators. I will talk about the third leg of the stool: precision measurement. We will see that the humble two-atom molecule should be thought of as an ultrahigh electric-field laboratory.

Special Colloquium — Monday, February 20 "Strategies for searches of physics beyond the Standard Model in the XXI century"

• Presenter: Manuel Franco Sevilla, University of California, Santa Barbara
• NOTE SPECIAL LOCATION: DUAN G130
• Host: Kevin Stenson
• Abstract: At the end of the XIX century, Lord Kelvin summarized a widespread feeling among physicists by saying that "physics is essentially complete, save for two little clouds". The "clouds" he was (apocryphally) referring to were the puzzling results from two measurements, the Michelson-Morley experiment and the Black-body spectrum, whose explanations ushered in an unprecedented era of discoveries that stretched throughout most of the XX century. After the culmination of the Standard Model in the 70's, the field of particle physics has found itself in a similar situation. Today, the "clouds" guiding the searches for physics beyond the Standard Model are issues like dark matter or the hierarchy problem. Using SUSY searches at CMS and the measurement of B->D(*)TauNu decays at BaBar as models, I will give an overview of some of the main strategies that are being followed in the quest to find new physics in the XXI century.

February 22 — "The Difficult Search for CP Violation in Neutrinos"

• Presenter: Michael Wilking, Stony Brook University
• Host: Kevin Stenson
• Abstract: To observe CP violation, experiments must make precise, differential measurements of the appearance of electron neutrinos and anti-electron neutrinos. This requires unprecedented control of systematic uncertainties, and, in particular, an understanding of neutrino-nucleus interactions that is beyond the capabilities of existing theoretical models. The resulting "neutrino energy measurement problem" that will be confronted by current and future long-baseline neutrino oscillation experiments, as well as potential experimental solutions, will be discussed.

March 1 — "Searching for Supersymmetry at the LHC"

• Presenter: Keith Ulmer, Texas A&M
• Host: Kevin Stenson
• Abstract: The Large Hadron Collider (LHC) at CERN currently provides the highest energy particle collisions ever produced in a laboratory. These collisions were reconstructed and analyzed by the CMS and ATLAS experiments to claim the discovery of the Higgs Boson in 2012, thus completing the standard model of particle physics. This talk explores what's next for the LHC, including the implications of the Higgs discovery on the search for new physics beyond the standard model. In particular, such open questions as the nature of dark matter and the gauge hierarchy problem may find eloquent solutions in supersymmetry, a proposed new symmetry of nature relating fermions and bosons. I will discuss the current state of experimental searches for supersymmetry at CMS, including the near term prospects for discovery, and will conclude with an example of the innovative new technological solutions being explored to continue the hunt for new physics into the High Luminosity LHC era set to begin in the coming decade.

March 8 — "Searching for axion and hidden photon dark matter with lumped element electromagnetic resonators and SQUIDs"

• Presenter: Kent Irwin, Stanford University
• Host: Nils Halverson
• Abstract: About 85% of the matter in the universe is made up of an unknown "dark" component that is mostly non-baryonic. For several decades, searches for this mysterious substance has principally focused on one candidate particle: the WIMP. Superconducting Quantum Interference Devices (SQUIDs) have played an important role in these searches, which have successfully ruled out significant phase space for WIMP dark matter. Recently, there has been a surge in theoretical interest in ultra-light-field dark matter, including QCD axions (spin 0 bosons) and hidden photons (spin 1 bosons). The Dark Matter Radio (DM Radio) is a tunable superconducting high-Q lumped-element resonator also using SQUIDs for detection. I will discuss the motivation, status and prospects for the DM Radio experiment in searching for both axions and hidden photons, and the remarkable phase space that DM Radio will search over the next several years.

March 15 — "Tunable Materials and Metasurfaces – from Quantum to Perfect"

• Presenter: Harry Atwater, California Institute of Technology
• Host: Scott Diddams
• Abstract: Tuning the Fermi level and complex dielectric function of low-dimensional nanophotonic structures including layered materials and nanoantenna arrays enables scientific exploration of quantum materials such graphene, phosphorene and topological insulators and, as well applications including electronic phase and amplitude modulators for the near infrared (conducting oxides) and mid infrared (graphene). We discuss light-matter interactions in materials and report dynamically tunable metasurfaces exhibiting >π phase modulation and ‘perfect’ absorption approaching 100%.

March 22 — "High-Capacity Optical Communications using Multiplexing of Multiple Orbital-Angular-Momentum Beams"

• Presenter: Alan Willner, University of Southern California
• Host: Scott Diddams
• Abstract: Optical communications has historically experienced tremendous capacity growth by multiplexing many channels and transmitting them simultaneously. Recently, the community has turned to research the possibility of space-division-multiplexing (SDM) as the next domain to exploit, and multiple spatially overlapping orthogonal modes can achieve a subset called mode-division-multiplexing (MDM). Indeed, the ability to multiplex multiple data-carrying modes over the same physical medium represents the potential for increasing system capacity and spectral efficiency.
  Generating different amounts of orbital-angular-momentum (OAM) on different optical beams has emerged as a technique for such mode multiplexing. A beam can carry OAM if its phase front twists in a helical fashion as it propagates, and the amount of OAM corresponds to the number of 2*pi phase shifts that occur in the azimuthal direction. Each OAM beam is orthogonal to other beams, and such beams can be efficiently multiplexed, transmitted, and demultiplexed with little inherent crosstalk. This presentation will explore the achievements of and challenges to OAM-based optical and millimeter-wave communication systems, including transmission, turbulence compensation, and link design.

March 29 — Spring Break; No Colloquium

April 5 — "Thermoelectrics and Thermoelectric Materials"

• Presenter: David Singh, University of Missouri
• Host: Gang Cao
• Abstract: Thermoelectric devices are used for the conversion of thermal and electrical energy. They offer a number of advantages over competing technologies including scalability to small sizes and temperature differences, simple reliable designs and often low cost. However, these devices have not seen wide application in energy applications due to their limited conversion efficiency. This is a consequence of the limited performance of current thermoelectric materials, which can be characterized by a dimensionless figure of merit, ZT=σS2T/κ. There is no known fundamental limit on ZT. However, the combination of transport parameters entering ZT is a combination that does not occur in ordinary materials. This talk presents an overview of ZT and discusses strategies for optimizing ZT as well as recent results that point to ways of identifying new high ZT compositions. An important finding is that electronic structure plays a remarkably subtle role in thermoelectric performance that can however be simply visualized in terms of iso-energy surfaces. Finally, a connection is drawn between topological insulators and high ZT thermoelectrics, explaining the overlap between these two interesting materials classes. Characteristics that can be used to identify new thermoelectric compositions are presented and discussed.

April 12 — "Structure and Dynamics with Ultrafast Electron Microscopes… or how to make atomic-level movies of molecules and materials"

• Presenter: Brad Siwick, McGill University
• Host: John Price
• Abstract: In this talk I will describe how combining ultrafast lasers and electron microscopes in novel ways makes it possible to directly ‘watch’ the time-evolving structure of condensed matter on the fastest timescales open to atomic motion.  By combining such measurements with complementary (and more conventional) spectroscopic probes one can develop structure-property relationships for materials under even very far from equilibrium conditions.

  I will give several examples of the remarkable new kinds of information that can be gleaned from such studies and describe how these opportunities emerge from the unique capabilities of the current generation of ultrafast electron microscopy instruments.  For example, in diffraction mode it is possible to identify and separate lattice structural changes from valence charge density redistribution in materials on the ultrafast timescale and to identify novel photoinduced phases that have no equilibrium analogs.   It is also possible to directly probe the strength of the coupling between electrons and phonons in materials across the entire Brillouin zone and to probe nonequilibrium phonon dynamics (or relaxation) in exquisite detail.  In imaging mode, real space pictures of nano- to microstructural evolution in materials at unprecedented spatio-temporal resolution can be obtained. 

  I will assume no familiarity with ultrafast lasers or electron microscopes.

  References

  1. G. Sciani and  R. J. D. Miller, Femtosecond electron diffraction:  Heralding the era of atomically resolved dynamics, Rep. Prog. Phys. 71 (2011) 096101
  2. R. P. Chatelain, V. Morrison, C. Godbout, and B.  J. Siwick, Ultrafast electron diffraction with radio-frequency compressed electron pulses, Appl. Phys. Lett. 101 (2012) 081901.
  3. V. Morrison, R. P. Chatelain, K. Tiwari, A. Hendaoui, M. Chakker and B.  J.  Siwick, A photoinduced metal-like phase of monoclinic vanadium dioxide revealed by ultrafast electron diffraction, Science 346 (2014) 445 – 448.
  4. R. P. Chatelain, V. Morrison, Bart L. M. Klarenaar and B.  J.  Siwick, Coherent and incoherent electron-phonon coupling in graphite observed with radio-frequency compressed ultrafast electron diffraction, Phys. Rev. Lett. 113 (2014) 235502.
  5. L. Nikolova, M. Stern, T. LaGrange, B. Reed, N. Browning, G. H. Campbell, J.-C. Kieffer, F. Rosei and B. J. Siwick, Complex crystallization dynamics in amorphous germanium studied with dynamic TEM. Phys. Rev. B 87 (2013) 064105. 

April 19 — No Colloquium

April 26 — "Building a Proportional Cell: Lessons from Physics"

• Presenter: Jané Kondev, Brandeis University
• Host: Loren Hough
• Abstract: The inside of a living cell is spatially organized into different functional structures called organelles. For example, the nucleus is a membrane bound compartment that contains the cell's DNA, while the cytoskeleton is made up of dynamic protein fibers which allow cells to move and change shape.  

  Gulliver noticed 140 years ago that the size of the cell's nucleus is proportional to the size of the cell. Similar observations have been made about other micron-sale structures within the cell. These experiments suggest that cells measure and control the size of their organelles, and they raise a simple question: How does the cell establish a micron-scale ruler with nothing more at its disposal than nanometer-sized molecules that diffuse around the cell and on occasion bump into each other?  In this talk I will describe quantitative experiments and related theory that are beginning to reveal general principles of how cells control the size of their organelles. 

May 3 — "Imaging the Surface States of a Strongly Correlated Topological Insulator"

• Presenter: Jennifer Hoffman, Harvard University
• Host: Dan Dessau and Minhyea Lee
• Abstract: The prediction and subsequent discovery of robust spin-polarized surface states on topological band insulators has launched a new subfield of physics over the last decade. In the last few years it has been recognized that when topology is combined with strong electron-electron correlations, even more interesting and potentially useful states of matter can arise, such as new topological classifications, fractionalized states, and many-body localization that preserves the topology of the insulating state against thermal destruction. Here we show the first direct proof of a strongly correlated topological insulator. Using scanning tunneling microscopy to probe real and momentum space structure, our measurements on the heavy fermion material SmB₆ reveal the evolution of the insulating gap arising from strong interactions. Within the narrow gap, we directly image a dispersing surface state that converges to a Dirac point close to the chemical potential. Our observations present the first opportunity to explore a strongly correlated topological state of matter.

For more information about colloquia this semester, contact: John Price.

Colloquia are Wednesdays at 4:00 p.m. in DUAN G1B20, unless otherwise noted.

Coffee, tea and cookies will be available before regular colloquia beginning at 3:45 p.m. in DUAN G1B31.

August 24 — "NASA’s Juno Mission to Jupiter: What’s Inside the Giant Planet?"

• Presenter: Fran Bagenal, University of Colorado Boulder
• Host: Mihaly Horanyi
• Abstract: Jupiter is a planet of superlatives: the most massive planet in the solar system, rotates the fastest, has the strongest magnetic field, and has the most extensive satellite system of any planet. NASA’s Juno mission was launched in August 2011 and went into orbit over Jupiter’s poles on July 4th this summer.  Juno’s principal goal is to understand the origin and evolution of Jupiter. Underneath its dense cloud cover, Jupiter safeguards secrets to the fundamental processes and conditions that governed our solar system during its formation. As our primary example of a giant planet, Jupiter can also provide critical knowledge for understanding the planetary systems being discovered around other stars. With its suite of science instruments, Juno will investigate the existence of a solid planetary core, map Jupiter's intense magnetic field, measure the amount of water and ammonia in the deep atmosphere, and observe the planet's auroras. Juno will let us take a giant step forward in our understanding of how giant planets form and the role these titans played in putting together the rest of the solar system. In Greek and Roman mythology, Jupiter drew a veil of clouds around himself to hide his mischief. It was Jupiter’s wife, the goddess Juno, who was able to peer through the clouds and reveal Jupiter’s true nature.  Juno is also the first spacecraft to fly over Jupiter’s aurora and will measure both the energetic particles raining down on the planet and the bright “northern & southern lights” they excite.

August 31 — "Cosmic Dust Research at the Colorado Dust Accelerator"

• Presenter: Tobin Munsat, University of Colorado Boulder
• Host: John Price
• Abstract: Cosmic dust grains play a central role in astrophysics and planetary science; the interaction of dust grains with planetary surfaces and atmospheres can drive weathering processes, surface chemistry (for airless bodies), and even atmospheric chemistry on Earth. Dust-detecting instruments can provide insight into the environment where the particles were formed, whether that is the interstellar medium, comets, asteroids, or even ejected from the surfaces of planets and moons. From a basic physics point of view, hypervelocity impact experiments can access extremely high energy density levels. To address many facets of cosmic dust research in the lab, we have recently developed a dust accelerator on the east campus of CU. This is a 3 MV electrostatic linear accelerator which launches particles in the size range of 0.1 to a few micrometers and velocities up to 100 km/s onto a variety of targets. This talk will describe the basic details of the accelerator and provide an overview of several lines of research being carried out by our group.  Specifically, I will discuss hypervelocity impact cratering experiments in thin films, impact experiments into cryogenic (ice) targets, making "shooting stars" in the lab, and the local development of dust detector instruments that have flown to the moon and will soon fly to Jupiter's moon Europa.

September 7 — "The End of Inflation"

• Presenter: Tom Giblin, Kenyon College
• Host: Ethan Neil
• Abstract: The evidence for a hot, dense, early stage of the Universe is overwhelming.  Setting up a Universe in this hot-dense stage is more of a challenge.  While Cosmic Inflation as a paradigm has been successful at negotiating  the issues of the Hot Big Bang, its conclusion is marked by a cold, empty state that needs to quickly and efficiently reheat.  Violent processes, turbulent phase transitions and non-linear processes characterize this period and might help us shed light on the particle physics of the very young, very energetic early Universe.  I will describe how we have been using large numerical simulations to calculate precision observables that will help to constrain the growing number of early universe scenarios.

September 14 — "Apples vs. Oranges: Comparison of Student Performance in a MOOC vs. a Brick-and-Mortar Course"

• Presenter: Michael Dubson, University of Colorado Boulder
• Host: John Price
• Abstract: In the fall of 2013, my colleagues and I taught the calculus-based introductory physics course to 800 tuition-paying students at the University of Colorado at Boulder. At the same time we taught a free massive open online course (MOOC ) version of the same course, through Coursera.com. The initial enrollment in the MOOC was 10,000 students, of whom 255 completed the course. Students in both courses received identical lectures with identical embedded clicker questions, identical homework assignments, and identical timed exams. We present data on participation rates and exam performance for the two groups. We find that the MOOC is like a drug targeted at a very specific population. When it works, it works well, but it works for very few students. This MOOC worked well for older, well-educated students, who already had a good understanding of Newtonian mechanics.

September 21 — "Tragic tale of the mathematical genius Ramanujan"

• Presenter: Ramamurti Shankar, Yale University
• Host: Rahul Nandkishore
• Abstract: In this talk I will describe the remarkable life and mathematics of Srinivasa Ramanujan (1887-1920), an unknown self-educated clerk from South India who stunned the world of western mathematics with his results on number theory, modular functions etc., that continue to challenge us till this day. I will describe his early life in India, his discovery by Cambridge mathematician G H Hardy, his few years in Cambridge and his untimely death at age 32.

September 28 — "Shared vibrations:  How photosynthetic light harvesting approaches 100% efficiency”

• Presenter: David Jonas, Department of Chemistry and Biochemistry, University of Colorado Boulder
• Host: John Price
• Abstract: Almost all life on earth depends on photosynthesis, and almost all the power our civilization uses has been stored as fuel by photosynthesis.  In photosynthesis, light is harvested by antenna proteins that can transfer the photon’s energy to a reaction center with near unit quantum efficiency.  The remarkable efficiency of these energy transfer processes has been a mystery for over 60 years and we cannot replicate it artificially.  Recent femtosecond two-dimensional (2D) spectroscopy experiments on antenna proteins found signatures initially attributed to electronic coherence that have sparked discussion about the role of quantum mechanics in biology; it is puzzling that these signatures persist for longer than the apparent energy transfer timescale.   We have recently shown that intramolecular vibrations shared across pigments can drive electronic energy transfer outside the Born-Oppenheimer picture of fast electrons and slow vibrations.  Such shared vibrations on the ground electronic state of the antenna generate all of the observed 2D signatures and their properties reveal the design principle for the antenna.  There are indications that this new mechanism may be operative in a variety of antenna proteins using 5 different photosynthetic pigments that are responsible for over half of the light harvesting on our planet.

**Special Colloquium/Nuclear Particle Physics Seminar** Monday, October 3 — "The Physics of Non-Hydrodynamic Modes"

• Presenter: Paul Romatschke, University of Colorado Boulder
• NOTE SPECIAL LOCATION: DUAN G125
• Abstract: Examples for hydrodynamic collective modes are sound waves, shear and diffusive modes. But what are non-hydrodynamic collective modes? Most physicists likely have never ever heard about non-hydrodynamic modes in their entire career. Indeed, there does not seem to be a single textbook on this topic. This colloquium will give an introduction to the physics of non-hydrodynamic modes, featuring gravitational waves, string theory predictions for experiment, the coolest and hottest stuff on earth and high-temperature superconductors.

October 5 — “A synthetic quantum magnet made of hundreds of trapped ions”

• Presenter: John Bollinger, NIST Boulder
• Host: Scott Diddams
• Abstract: Entanglement between individual quantum objects exponentially increases the complexity of quantum many-body systems, so systems with more than 30-40 quantum bits cannot be fully studied using conventional techniques and computers. To make progress at this frontier of physics, Feynman’s pioneering ideas of quantum computation and quantum simulation are now being pursued in a wide variety of well-controlled quantum platforms.  Trapped-ions are naturally suited for simulating quantum magnetism, and exhibit desirable properties such as high-fidelity state preparation and readout, and long trapping and coherence times.  I will discuss how variable range, quantum magnetic interactions can be engineered with trapped ions, focusing on our work with 2-dimensional arrays of several hundred ions crystallized in a Penning ion trap.  In particular, I will highlight our recent experiments that benchmark quantum dynamics and entanglement, and utilize our ability to time-reverse the dynamics to measure out-of-time-order correlation functions that quantify the spread of quantum information throughout the system.

October 12 — "On string theory, particle physics and cosmology"

• Presenter: Fernando Quevedo, Director - ICTP, Trieste and Cambridge University
• Host: Shanta de Alwis
• Abstract: An overview will be given for the recent developments in string theoretical scenarios to address particle physics and cosmology questions.

October 19 — "From BEC to CEO: the Entrepreneurial Experience"

• Presenter: Chris Myatt, CEO - MBio Diagnostics
• Host: Scott Diddams
• Abstract: Can academic training in physics – at CU, JILA, and at NIST – prepare you to start a company?  While the subject of your research may not be directly applicable to industrial problems—in my case, Bose-Einstein condensation and quantum computing—the skill sets and tools you develop are of great value in preparing you to start a company, or to find a job in industry.  I will discuss briefly the two companies I have founded, provide an overview of the technology of each, what it takes to get going, and the lessons learned in doing so.

October 26 — "Exploiting Disorder for Global and Local Response"

• Presenter: Sidney Nagel, University of Chicago
• Host: Noel Clark
• Abstract: We are customarily taught to understand ordinary solids by considering perturbations about a periodic structure.  This approach becomes increasingly untenable as the amount of disorder in the solid increases.  In a crystal with only one atom per unit cell, all atoms play the same role in producing the solid's global response to external perturbations. Disordered materials are not similarly constrained and a new principle emerges: independence of bond-level response. This allows one to drive the system to different regimes of behavior by successively removing individual bonds. We can thus exploit disorder to achieve unique, varied, textured and tunable global response. We can use similar pruning techniques to achieve long-range interactions inspired by allosteric behavior in proteins. This allows a local input strain to control the local strain at a distant site in the network.

November 2 — "A Bridge Too Far: The Demise of the Superconducting Super Collider"

• Presenter: Michael Riordan, Author
• Host: Allan Franklin
• Abstract: In October 1993 the US Congress terminated the Superconducting Super Collider — at over $10 billion the largest and costliest basic-science project ever attempted.  It was a disastrous loss for the nation’s high-energy physics community. With the 2012 discovery of the Higgs boson at CERN’s Large Hadron Collider, Europe has assumed world leadership in this field.
  A combination of fiscal austerity, continuing SSC cost overruns, intense Congressional scrutiny, lack of major foreign contributions, waning Presidential support, and the widespread public perception of mismanagement led to the project’s demise nearly five years after it had begun. Its termination occurred against the political backdrop of changing scientific needs as US science policy shifted to a post-Cold War footing during the early 1990s. And the growing cost of the SSC inevitably exerted undue pressure upon other worthy research, thus weakening its support in Congress and the broader scientific community.
  As underscored by the Higgs boson discovery, at a mass substantially below that of the top quark, the SSC did not need to collide protons at 40 TeV in order to attain its premier physics goal. The selection of this design energy was governed more by politics than by physics, given that Europeans could build the LHC by eventually installing superconducting magnets in the LEP tunnel under construction in the mid-1980s. In hindsight, there were good alternative projects the US high-energy physics community could have pursued that did not involve building a gargantuan, multibillion-dollar machine at a green-field site in Texas.

November 9 — "The physics of cell division"

• Presenter: Meredith Betterton, University of Colorado Boulder
• Host: John Price
• Abstract: Cells are the basic unit of life. All life on earth depends on cells’ ability to duplicate themselves. In order to divide successfully, cells must solve fascinating physics problems, which this talk will introduce assuming no biology background. A key step in cell division is ensuring that each of the daughter cells inherits a single copy of the genetic material. In eukaryotes, a self-organized machine called the mitotic spindle exerts forces that physically move the chromosomes. This cellular machine is composed of microtubules, molecular motors, and associated molecules. We are using theory, simulation, and experiment to address fundamental physics questions related to mitosis, including how the mitotic spindle structure self assembles and achieves the correct size, how the spindle organizes and moves chromosomes, and how these same components outside of cells can create nonequilibrium materials that exhibit new physics.

November 16 — "What’s next after Moore’s law: quantum computing"

• Presenter: John Martinis, Google and UC Santa Barbara
• Host: John Price
• Abstract: As microelectronics technology nears the end of exponential growth over time, known as Moore’s law, there is a renewed interest in new computing paradigms such as quantum computing. After many years of fundamental research on superconducting quantum devices, I recently moved my research program to Google with the goal of building a useful quantum computer. Following Feynman’s vision, I will highlight a proof-of-principle experiment to simulate a chemical reaction that finds an interaction cross section. I will also outline a “quantum supremacy” experiment that will demonstrate the exponential power of a quantum processor by checking its output with a classical computer, which is intractable for even the world's most advanced classical supercomputer beyond 45-50 qubits. We are working to perform this experiment in the next year.
  John Martinis currently heads the quantum-hardware team at Google. John started the field of research on quantum devices as a graduate student at UC Berkeley in 1985, and has continued this research at NIST Boulder, UC Santa Barbara, and now Google. In 2010 he was awarded the AAAS science breakthrough of the year, and in 2014 was awarded the London Prize for low-temperature physics research.

November 23 — Fall Break; No Colloquium

November 30 — "Transport and Localization in many-body nuclear spin systems"

• Presenter: Paola Cappellaro, Massachusetts Institute of Technology
• Host: Ana Maria Rey
• Abstract: A large-scale quantum computer could solve problems that would take classical computers longer than the age of the universe to crack, with profound implications for cryptography, chemistry, material science, and many areas of physics. However, to reach this goal we need to control large quantum systems, where the many-body dynamics becomes often fragile and very complex.
  Among the many questions and challenges that arise when working toward this goal, I will address two questions in my talk: How can we transfer quantum information from one quantum register to another?
  How can we preserve quantum information in the presence of strong interactions?
  Using a nuclear spin chain as an exemplary experimental system, and the tools of Hamiltonian engineering, I will show how spin chains can act as quantum wires in a distributed quantum computing architecture, transporting information and entanglement. I will then show how disorder can quench the transport of information, a phenomenon known as localization. This phenomenon might actually be a feature in some situations, as it allows preserving local quantum information for later retrieval and prevents thermalization. Is localization however possible even in the presence of long-range interaction? I will show experimental signatures that a logarithmic growth of long-range correlation is still present in interacting systems, a sign of many-body localization.

**Special Colloquium: Monday, December 5 at 4:00 p.m.** — "Symmetry, Topology and Bacteria"

• NOTE SPECIAL LOCATION: DUAN G125
• Presenter: Ivan Smalyukh, University of Colorado Boulder
• Abstract: Einstein introduced the “colloidal atom” paradigm, but only high-symmetry colloidal crystals could be realized within a century or so. Born developed a theory of polar ordered fluids, but only nonpolar ones could be observed. Heisenberg proposed 3D solitons as models of particles in continuous fields, but Derrick put forward a theorem that they cannot be stable. I will discuss how, addressing these long-standing challenges, we realize the lowest symmetry triclinic colloidal crystals [1] and biaxial ferromagnetic fluids [2], as well as the static 3D topological solitons within them [3]. With production aided by bacteria, such unusual materials promise a solution to the window inefficiency problem and other technological uses.
  1. H. Mundoor, B. Senyuk & I.I. Smalyukh. Science352, 69 (2016).
  2. Q. Liu, P.J. Ackerman, T.C. Lubensky & I.I. Smalyukh. PNAS 113, 10479 (2016).
  3. P.J. Ackerman & I.I. Smalyukh. Nature Mater. DOI:10.1038/NMAT4826 (2016).

For more information about colloquia this semester, contact: John Price.

Coffee, tea and cookies will be available before regular colloquia beginning at 3:45 p.m. in DUAN G1B31.

January 13 — "Looking for Dark Matter with Atomic Clocks and Gravitational Wave Detectors"

• Presenter: Asimina Arvanitaki, Perimeter Institute
• Host: Shanta de Alwis
• Abstract: When we think about Particle physics the first thing that comes to mind is colliders and high energies. In this talk, I will describe how low energy experiments can probe new phenomena, new particles and new forces of nature. I will demonstrate this through two seemingly different experiments that test the same aspects of Dark Matter.  In String Theory, fundamental constants, such as the electron mass or charge, are determined by fields known as moduli. When these fields are the Dark Matter of our Universe, they will cause the fundamental constants to oscillate locally with a frequency set by the Dark Matter mass. For frequencies smaller than 1 Hz atomic clocks can easily pick up these oscillations with their immense sensitivity. For higher frequencies above 1 kHz, Dark Matter can excite acoustic modes in resonant mass detectors originally designed to detect gravitational radiation from astrophysical sources. Both techniques can extend searches for this type of Dark Matter by several order of magnitude in the near future.

January 20 — "From atoms to brains: Microfabricated atomic sensors for non-invasive magnetic brain imaging?"

• Presenter: Svenja Knappe, NIST Boulder
• Host: Markus Raschke
• Abstract: Laser spectroscopy of atoms can be used as a sensitive tool to measure many physical properties such as time, length, acceleration, and temperature. It has also been used to measure magnetic fields as precisely as the best superconducting magnetometers currently available. Our group has been trying to miniaturize these atom-based magnetic field sensors by use of microfabrication technologies. We have developed a non-invasive multichannel magnetic imaging system sensitive enough to detect the tiny magnetic fields emitted by the brain. These uncooled small sensors could open new and exciting possibilities for brain imaging. They could improve the spatial resolution of the images, open the door for imaging during real-world scenarios, long-term telemetry, and maybe even enable wearable systems in the future.

January 27 — "Simulating QCD at non-zero matter density"

• Presenter: Philippe de Forcrand, ETH Zurich and CERN
• Host: Paul Romatschke
• Abstract: The study of the properties of Quantum Chromodynamics by Monte Carlo simulations, called Lattice QCD, has been highly successful in the regime where matter and anti-matter densities are equal. When they are not, the notorious "sign problem" occurs, and standard Monte Carlo sampling cannot be applied.
  I review the nature of the "sign problem", the importance of addressing it, and offer some views on current efforts to circumvent it.

RESCHEDULED Thursday, January 28 — SPECIAL Physics Colloquium "Studying the Dark Side of the Nucleus:  From Neutron Skins to Neutron Stars"

• Presenter: Seamus Riordan, Stony Brook University
• Host: Jamie Nagle
• NOTE SPECIAL LOCATION: DUAN G130
• Abstract: The neutron densities in atomic nuclei are notoriously difficult to observe with high precision: the standard tool of electromagnetic interactions which has been used to map out the nuclear charge distributions simply doesn't see them. In fact, it has only recently been experimentally confirmed that the neutron-rich lead nucleus even has a neutron skin, and is only a fraction of a neutron radius thick. Encoded in these distributions is a wealth of important information about how the strong nuclear force builds systems where the number of protons and neutrons are unequal. This information has bearing not only for our understanding of asymmetric nuclei, but also in the construction of extreme systems like neutron stars. Fortunately, nature gives us a novel way to image this side of the nucleus: through fundamental weak force interactions, which interact primarily to neutrons rather than protons. In this colloquium I will discuss why these neutron distributions play an important part in our understanding of nuclear physics and astrophysics, how one images such tiny systems with electron beams, and the recent and upcoming experimental efforts for such measurements.

Monday, February 1 — SPECIAL Physics Colloquium "High-Energy-Density Physics: Laser-driven Fusion to Fundamental Plasma Science"

• Presenter: Mario Manuel, Smithsonian Astrophysical Society
• Host: Tobin Munsat
• NOTE SPECIAL LOCATION: DUAN G130
• Abstract: Inertial confinement fusion (ICF) is a major research effort underway in the US and abroad to achieve net energy production through nuclear fusion of hydrogen isotopes. Rather than using externally applied magnetic fields, as in magnetic confinement fusion (e.g. as in a tokamak), ICF utilizes spherical capsules of deuterium-tritium fuel. These capsules are compressed through the use of high-power lasers to produce fuel temperatures and densities higher than those in the center of the sun. The fuel is arranged in such a way as to create a runaway thermal instability for the short period that the fuel is held together by its own inertia. In this talk, I will briefly describe the general ICF concept and the facilities where this research takes place.
  Laser facilities used to study ICF also provide unique opportunities to study fundamental plasma science in the high-energy-density regime, i.e. systems with pressures >1 Mbar. One of the great challenges to the ICF program is understanding how the Rayleigh-Taylor (RT) instability behaves in the HED system produced by the interaction of intense lasers with matter. This classical hydrodynamic instability also occurs in laser-driven plasma systems where the acceleration direction opposes the density gradient. One complexity involved with RT in these systems is the generation of magnetic fields due the the Biermann battery mechanism. Creation of RT-induced magnetic fields in HED plasmas will be thoroughly discussed and I will describe our experimental results that first demonstrated their existence 30+ years after the theoretical predictions.

February 3 — "The Water Surface: Complex, Dynamic and Wet"

• Presenter: Mischa Bonn, Max-Planck-Institute for Polymer Sciences, Mainz
• Host: Markus Raschke
• Abstract: Water surfaces and interfaces are ubiquitous, not just in nature (e.g. at the boundaries of cells, in rain drops, oceans, rivers and lakes) but also in many technological applications (such as electrochemistry and photocatalytic water splitting). Water is a rather unique liquid, owing to its strong intermolecular interactions: strong hydrogen bonds hold water molecules together. At the surface of water, the water hydrogen-bonded network is abruptly interrupted, conferring properties on interfacial water different from bulk water [1].
  We elucidate the structure and structural dynamics of interfacial water using surface-specific vibrational spectroscopy of interfacial water molecules, with femtosecond time resolution. Specifically, we find that the interface is structurally more heterogeneous [2,3] and substantially more dynamical [4] than bulk water. We reveal the nature of the heterogeneity, and quantify the reorientational dynamics of specifically interfacial water molecules. Finally, we show that the evaporation of water – i.e. the release of individual water molecules from the bulk into the gas phase – is not a purely stochastic event. Rather, the evaporation follows one specific pathway, involving a delicately timed, concerted motion of several water molecules to ‘launch’ a single molecule from the surface [5].
  References
  [1] Bonn, M.; Nagata, Y.; Backus, E.H.G.; Angew. Chem.-Intern. Ed. 2015, 54, 5560.
  [2] Hsieh, C.S.; Okuno, M.; Backus, E. H. G.; Hunger, J.; Nagata, Y.; Bonn, M.. Angew. Chem.-Intern. Ed. 2014, 31, 8146 (VIP paper).
  [3] van der Post, S. T.; Hsieh, C.-S.; Okuno, M.; Nagata, Y.; Bakker, H. J.; Bonn, M.; Hunger, J. Nat. Commun. 2015, 6, 8384.
  [4] Hsieh, C. S.; Campen, R. K.; M. Okuno; E. H. G. Backus; Y. Nagata; and M. Bonn, Proc. Nat. Acad. Sci. USA 2013, 110, 18780.
  [5] Nagata, Y.; Usui, K.; Bonn, M., Phys. Rev. Lett. 2015, 115, 236102.

February 10 — "Atomic physics meets nanophotonics: creating complex quantum states of matter and light"

• Presenter: Darrick Chang, Institute of Photonic Sciences
• Host: Markus Raschke
• Abstract: Significant efforts have been made to interface cold atoms with micro- and nano-photonic systems in recent years. Originally, it was envisioned that the migration to these systems from free-space atomic ensemble or macroscopic cavity QED experiments could dramatically improve figures of merit and facilitate scalability for applications such as quantum information processing. However, a more interesting scenario would be if nanophotonic systems could yield new paradigms for controlling quantum light-matter interactions, which have no obvious counterpart in macroscopic settings.
  Here, we describe one paradigm for novel physics, based upon the coupling of atoms to photonic crystal structures. In particular, we show that atoms can become dressed by localized photonic "clouds" of tunable size. This cloud behaves much like an external cavity, but which is attached to the position of the atom. This dynamically induced cavity can then mediate long-range spin interactions or forces between atoms, yielding an exotic quantum material where spins, phonons, and photons are strongly coupled.

Thursday, February 11 — SPECIAL Physics Colloquium "Imaging the Physics of Magnetically Confined Fusion"

• Presenter: Dr. Benjamin Tobias, Princeton Plasma Physics Laboratory
• Host: Tobin Munsat
• NOTE SPECIAL TIME AND LOCATION: 2:00 p.m. DUAN F1117 (Commons Room)
• Abstract: Electron Cyclotron Emission Imaging (ECEI) and Microwave Imaging Reflectometry (MIR) are powerful visualization tools that are purpose built for tokamak research, and they have become integral to experiments at the cutting edge of fusion science. They provide unique data that was simply not available to previous generations of scientists and bring us closer than ever to understanding and controlling instabilities. 2D and 3D images of the core plasma help to validate modeling of Alfvénic oscillations that can limit plasma self-heating in a burning reactor by diagnosing their interaction with energetic ions. Images of the edge plasma help to explain how and why transients occur, and what techniques can excite benign modes to regulate the stability of this region. To address the most dangerous eventuality, a rapid and violent disruption of plasma confinement, we apply these tools in the study of helical modes that tear the internal magnetic surfaces. The vast majority of tokamak disruptions are attributed to these tearing modes, and using microwave imaging data we are able to validate magnetohydrodynamic (MHD) modeling, quantify the forces omitted by single fluid resistive theory, and accurately prescribe inputs to more sophisticated analysis that can help us understand the successes and failures of disruption avoidance and mitigation techniques.

Monday, February 15 — SPECIAL Physics Colloquium "Probing the quark–gluon plasma with light-flavor hadrons with the ALICE detector at the LHC"

• Presenter: Michele Floris, CERN
• Host: Jamie Nagle
• NOTE SPECIAL LOCATION: DUAN G125
• Abstract: Quantum ChromoDynamics (QCD) describes the interaction between the elementary constituents of hadronic matter, the quarks and gluons. Quarks and gluons are not observed as free particles, but are confined inside color-singlet hadrons. QCD, however, predicts the existence of a phase of matter at high temperature (T > 10^12 K) where quarks and gluons are no longer confined, called the Quark–Gluon Plasma (QGP). The QGP can be created and studied in the laboratory colliding heavy nuclei at ultra-relativistic energies. The ALICE experiment at the CERN LHC is pursuing this program at the energy frontier (\sqrt{s_NN} > 2.76 TeV).

  After a brief introduction on the physics of confinement and heavy-ion experiments, I will discuss recent results from the ALICE experiment on the collective properties and hadronization mechanism of the system created in the collision. Measurements of light-flavor hadrons obtained in lead–lead, proton–proton and proton–lead collisions will be compared and contrasted. These studies revealed unexpected features, which are currently not clarified and could lead to a better understanding of high-energy hadronic interactions.

February 17— "Investigative Science Learning Environment: Turning our students into collaborative participants in the practice of physics"  

• Presenter: Eugenia Etkina, Rutgers University
• Host: Noah Finkelstein
• Abstract: Scientists and especially physicists have their own, unique ways of developing new knowledge, solving new problems, and communicating about what they do. These form a set of cultural norms and practices that we call “physics.” Can students become enculturated into physics in a one year introductory course, or does “doing physics” remain the exclusive purview of professionals who have acquired their skills through years of training? Development of the Next Generation Science Standards, revisions to AP courses, and a new MCAT suggest that these aspects of physics (and other sciences) are as valuable as the final product of scientific labor—concepts and mathematical representations—that traditionally have been the sole focus of science courses. Science practices are the central elements of all these innovations. In my talk I will describe Investigative Science Learning environment - an instructional philosophy that allows us to make these practices a centerpiece of learning physics without losing conceptual and mathematical focus. I will also discuss ways to assess these complex practices.

Thursday, February 18 — SPECIAL Physics Colloquium "Nature's Keys: Two of the Fundamental Plasma Processes that Unlock our World"

• Presenter: Seth Dorfman, UCLA
• Host: Tobin Munsat
• NOTE SPECIAL LOCATION: DUAN F1117 - Commons Room
• NOTE SPECIAL TIME: 2:00 p.m.
• Abstract: Plasma makes up 99.9% of the visible matter in the universe and is crucial to building a working fusion power plant, but many fundamental processes that occur in space and laboratory plasmas remain poorly understood. Interesting unsolved problems abound: Why is the outer layer of the sun (the solar corona) much hotter than the photosphere below, when from adiabatic considerations we would expect the reverse? How can we reduce losses to improve the performance of fusion plasmas? What determines the evolution of space weather that can damage our sensitive infrastructure? I will discuss fundamental plasma physics as the key to solving these and other problems, focusing on two examples: magnetic reconnection and non-linear physics of Alfvén waves.

  Magnetic reconnection is an important energy release mechanism to consider in contexts such as eruptions on the sun and loss of plasma in experimental fusion devices. In both cases, a slow build up phase is followed by a quick release of magnetic energy, but this impulsive behavior is not fully understood. I will show results from a dedicated laboratory experiment at Princeton University in which 3-D physics is necessary to explain the observed impulsive events. The experiments have key features in common with space observations that will inform future comparative studies.

  Alfvén waves are fundamental modes of a plasma with a magnetic field. The non-linear behavior of these waves may be key in contexts such as space weather and the scattering and loss of energetic particles in fusion devices. One important non-linear process with a long history of theoretical and simulation studies is a class of parametric instabilities in which a large amplitude Alfvén wave produces various daughter modes. I will show results from recent experiments at the Large Plasma Device at UCLA which represent the first observation of this type of instability in the laboratory.

Monday, February 22 — SPECIAL Physics Colloquium "Jet and photon probes of hot, dense nuclear matter at the Large Hadron Collider"

• Presenter: Dennis Perepelitsa, Brookhaven National Laboratory
• Host: Jamie Nagle
• NOTE SPECIAL LOCATION: DUAN G125
• Abstract: When large nuclei are accelerated to relativistic energies and brought into collision at the CERN Large Hadron Collider (LHC), the enormous temperature and energy density in the nuclear system trigger a phase transition into an exotic form of matter. The resulting quark-gluon plasma (QGP) evolves in an expanding, flowing fireball, and investigating its remarkable properties allows us to study how the theory of the strong nuclear force (quantum chromodynamics, or QCD) manifests itself in a novel high-density, high-temperature regime. Since the first nucleus-nucleus collisions in late 2010, a rich experimental program with the large, state-of-the-art detectors at the LHC has flourished. In this talk, I will focus on two valuable experimental tools developed to explore the QGP: reconstructed jets which arise from cascades of strongly-interacting quarks and gluons passing through the plasma; and high-energy photons which escape the interaction zone without further interaction.

February 24 — "Chaos and Thermalization in Many-Body Quantum Systems"

• Presenter: Mark Srednicki, University of California, Santa Barbara
• Host: Paul Beale
• Abstract: The question of how isolated systems come to thermal equilibrium has been debated for more than a century. I will present a simple picture of how thermalization of many-body systems can occur, with universal properties of chaotic quantum dynamics as the underlying mechanism. A large array of analytic, numerical, and experimental evidence now supports this picture in many cases.

Thursday, February 25 — SPECIAL Physics Colloquium, "Plasma Wakefield Acceleration: Surfing on a Wave of Plasma"

• Presenter: Michael Litos, SLAC National Accelerator Laboratory
• Host: Tobin Munsat
• NOTE SPECIAL LOCATION: DUAN G126
• NOTE SPECIAL TIME: 2:00 p.m.
• Abstract: The passage of an intense laser pulse or particle beam through a cold plasma can set up an oscillation in the plasma electrons with properties that are attractive for the acceleration of particle beams in a scheme known as plasma wakefield acceleration. These relativistic plasma waves can sustain incredibly high longitudinal electric fields traveling with a phase velocity near the speed of light. By pushing the plasma harder with a more intense driver pulse, non-linear responses may be evoked with yet more interesting properties. One of the greatest challenges in the field right now is the ability to form, control, and diagnose the plasma source with a sufficiently high resolution to better understand and manipulate the beam-plasma interaction. This line of research offers great promise for the creation of compact, inexpensive particle accelerators that may be used in applications ranging from high energy colliders to X-ray free electron lasers that can fit in a university lab. It also serves as an interesting example of how the proper combination of plasmas, lasers, and particle beams can lead to a sum that is greater than its parts.

Monday, February 29 — SPECIAL Physics Colloquium, "Jet Quenching in Heavy-Ion Collisions : Probing the Perfect Liquid"

• Presenter: Aaron Angerami, Columbia University
• Host: Jamie Nagle
• NOTE SPECIAL LOCATION: DUAN G125
• Abstract: Relativistic heavy-ion collisions produce matter at the highest temperatures accessible in the laboratory. Results from the RHIC experiments show that the system produced in these collisions behaves like a near-perfect fluid. These observations indicate that strong-coupling dynamics dominate the long wavelength behavior of the system and are in surprising contrast to initial expectations that system would form a weakly-coupled plasma. This exciting discovery has led to a number of questions about the nature of strongly coupled quantum systems and how such behavior can emerge from a theory possessing asymptotic freedom. Another discovery of the RHIC program was the observation of jet quenching, where energetic partons produced in the early stages of the collisions lose energy though their interactions with the medium. Jets probe the medium at a variety of length scales and are thus sensitive to both its microscopic structure and the onset of the strong coupling. In this colloquium I will present the latest LHC jet measurements performed with the ATLAS experiment, in which precision techniques developed in high energy physics have been adapted to the heavy-ion environment. I will discuss the rapidly improving theoretical picture of jet quenching, the implications of new measurement capabilities during the next lead ion runs at the LHC, and the role of a future detector to measure jets at RHIC. 

March 2 — "The Search for 100 Earths"

• Presenter: Debra Fischer, Yale University
• Host: Scott Diddams
• Abstract: Exoplanets are an important subfield of astronomy and there has been an impressive rate of discovery over the past two decades. Hundreds of exoplanets have been detected using the Doppler technique and the NASA Kepler mission discovered thousands of transiting planets, providing important statistical information about the size and ubiquity of other worlds. However, the vast majority of planets around nearby stars have escaped detection; a radial velocity precision of about 10 cm/s is required to discover potentially habitable worlds and to unravel complex multi-planet architectures. Such high precision would also dramatically improve the efficiency of space-based transit missions by providing the masses needed to estimate bulk densities and model the internal structure of exoplanets. The future of exoplanet science has very different trajectories depending on the precision that can ultimately be achieved with Doppler measurements.
  I will describe the work my team is carrying out to reach 10 cm/s precision with the radial velocity technique. We are commissioning a new instrument, the EXPRES spectrograph, at the Discovery Channel Telescope to carry out a search for 100 Earths in 2017.

Monday, March 7 — SPECIAL Physics Colloquium, "Probing Hot Nuclear Matter with Heavy Quarks at the Relativistic Heavy Ion Collider"

• Presenter: Darren McGlinchey, University of Colorado
• Host: Jamie Nagle
• NOTE SPECIAL LOCATION: DUAN G125
• Abstract: High energy heavy ion collisions, like those performed at the Relativistic Heavy Ion Collider (RHIC), allow us to test our understanding of nuclear physics at the highest temperatures accessible in the laboratory. At these high temperatures, a new phase of nuclear matter called the Quark Gluon Plasma (QGP), where the fundamental quarks and gluons are no longer confined within hadrons, is created. The RHIC experiments discovered that the QGP is strongly coupled and behaves as a nearly perfect fluid, contrary to initial expectations. Understanding the microscopic structure of the QGP, and its strongly coupled nature, requires calibrated probes at varying length scales. Heavy quarks, namely charm and bottom, are produced in initial hard scatterings, and therefore provide an excellent tool for these studies as they experience the full evolution of the produced medium. In this colloquium, I will discuss the use of heavy quarks as a probe of the medium produced in heavy ion collisions including recent experimental advances made possible by detector upgrades of the PHENIX experiment at RHIC.

March 9 — "Quantum simulations: from condensed matter to high energy models"

• Presenter: Ignacio Cirac, Max Planck Institute of Quantum Optics
• Host: Murray Holland
• Abstract: Many-body quantum systems are very hard to describe and simulate in general, since the dimension of the state space grows exponentially with the number of particles, volume, etc. Cold atoms in optical lattices as well as trapped ions may help us in that task, as one can engineer the interactions among the atoms in order to emulate many-body quantum problems. In this talk I will briefly summarize proposals to simulate condensed matter as well as high energy physics models with those systems. I will also show how photonic crystal structures can be used to design subwavelength optical lattices in two dimensions for ultracold atoms, potentially achieving a better performance than current experimental set-ups.

Monday, March 14 — SPECIAL Physics Colloquium, "Testing the fundamental nature of neutrinos with double beta decay"

• Presenter: Dr. David Moore, Stanford University
• Host: Jamie Nagle
• NOTE SPECIAL LOCATION: DUAN G125
• Abstract: The discovery of neutrino oscillations has demonstrated that neutrinos have small, but non-zero masses. These masses are at least 10^6 times smaller than those of the other fundamental particles, suggesting that neutrinos could be getting their masses through a different physical mechanism. Understanding the fundamental nature of neutrinos and the mechanism by which they acquire mass are key goals of nuclear and particle physics in the coming years.
  In this talk I will describe current and future searches for neutrinoless double beta decay with the Enriched Xenon Observatory (EXO). This extremely rare nuclear decay can occur if neutrinos are Majorana particles, i.e. if neutrinos and anti-neutrinos are identical. Searches for this lepton number violating decay will probe extensions to the Standard Model that attempt to account for the mechanism by which neutrinos acquire mass and could constrain the absolute neutrino mass scale. I will discuss the current status of the EXO experimental program and plans for next-generation detectors, which have substantial possibility to observe this beyond the Standard Model process.

March 16 — "Lessons of the warm Pliocene – Melding knowledge from geology and physics"

• Presenter: Bette Otto-Bliesner, NCAR
• Host: Scott Diddams
• Abstract: Studying past geologic warm periods can give valuable insight into important processes that may operate in the climate system during a warm regime. It also allows testing of our climate models ‘out-of-sample’. That is, climate models are most often developed and tested for the period of instrumental measurements, before being used to project the future. But the likely future long-term consequences of anthropogenic greenhouse gas changes is outside the range of this testing. 

  The mid-Pliocene warm period (~ 3 million years ago) has been considered a possible analog for the future. Atmospheric carbon dioxide concentrations for this period have been estimated to be approximately 350 to 450 ppmv. We have now nearly surpassed globally the mid-point of this range and could surpass 450 ppmv by shortly after year 2030 in the most dire pathway of future emissions. In this talk, I will review the geological reconstructions for the mid-Pliocene that indicate strong warming at polar latitudes and much higher sea level than today, and the various hypotheses that have been proposed to explain this warmth. These mechanisms and feedbacks are tested with climate models that simulate the responses of the atmosphere, ocean, sea ice, and land to the greenhouse gas forcing. I will conclude with a discussion of the possible reasons for the mismatches between the data and models.

March 23 - Spring Break; No Colloquium

March 30 — "New Findings in Materials in Extreme Environments"

• Presenter: Russel Hemley, Geophysical Laboratory, Carnegie Institution of Washington
• Host: Markus Raschke and Mike Ritzwoller
• Abstract: Extreme environments, and in particular high pressures and temperatures, produce profound effects on condensed matter and materials -- from structure, bonding, and chemical reactivity to bulk transport properties. It is now possibly to generate multimegabar pressures (e.g., >300 GPa) and temperatures from millikelvins to thousands of degrees in the laboratory under static conditions. A broad range of novel phenomena and materials are being observed, including unusual transitions between insulating and metallic phases, new superconductors and ferroelectrics, and novel structural and superhard materials.  Of particular interest have been pressure-induced transitions in simple elemental and molecular systems such as hydrogen. Altogether, these studies have implications for problems in physics and chemistry, planetary science, geoscience, astrophysics, and even soft matter and biology, and the new materials being discovered may find potential applications in energy and other technologies. In this effort, large experimental facilities such as accelerator-based x-ray, neutron scattering, and large laser sources are allowing new types of measurements to be made as well as still more extreme environments to be reached in the laboratory. 

April 6 — "Twisting the quantum"

• Presenter: Charles Clark, National Institute of Standards and Technology and the University of Maryland
• Host: Judah Levine
• Abstract: Wave motions in nature were known to the ancients, and their mathematical expression in physics today is essentially the same as that first provided by d'Alembert and Euler in the mid-18th century. Yet it was only in 1992 that physicists managed to control a basic property of light waves: their capability of swirling around their own axis of propagation. During the past decade such techniques of control have also been developed for quantum particles: atoms, electrons and neutrons.

  I will present a simple description of these phenomena, emphasizing the most basic aspects of wave and quantum particle motion, and showing how these are used in our recent work on twisting neutron wavefunctions [1].

  1. “Controlling Neutron Orbital Angular Momentum,” C. W. Clark, R. Barankov, M. G. Huber, M. Arif, D. G. Cory and D. A. Pushin, Nature 525, 504 (2015)

April 13 — "How old are Saturn’s rings, what is going on within Saturn’s moon Enceladus, and how dust measurements help to answer these questions"

• Presenter: Sascha Kempf, University of Colorado Boulder, LASP
• Host: Anna Hasenfratz
• Abstract: Even 450 years after Galileo Galilei’s discovery of Saturn’s rings, their origin and evolution is still not known. The rings are the brightest of the four ring systems in the solar system and have at least the mass of the moon Mimas. Interactions with Saturn’s moons and viscous spreading of the ring material seem to imply a ring age of about a tenth of the age of the Saturnian system. A young ring age is problematic because the disruption of a Mimas-sized body or a comet in the Roche zone of Saturn would result in a ring with a much larger rock content than observed today. The unique ring color resulting mainly from the pollution of the ring material with interplanetary meteoroids provides a key for constraining the ring age. Direct measurements of the meteoroid flux into the Saturnian system by Cassini’s Cosmic Dust Analyzer (CDA) allowed us for the first time to constrain the ring age.

  The discovery that Saturn’s small ice moon Enceladus maintains an active ice volcano is surely one of the most exiting findings by the Cassini mission and data obtained by the Cosmic Dust Analyzer contributed significantly to our understanding of this phenomenon. In fact, knowledge of the dynamics and composition of the ice particles forming the plume allow us to obtain information about the moon’s interior and in particular provide evidence for a subsurface ocean. In October 2015 Cassini performed its last close flyby at Enceladus. Data obtained during this flyby together with numerical simulations allow us to constrain the Enceladus dust source rate and to draw conclusions about the emission of plume particles along the fractures in the south polar terrain.

April 20 - "Dipolar quantum gases of strongly magnetic Erbium atoms"

• Presenter: Francesca Ferlaino, University of Innsbruck 
• Host: Ana Maria Rey and Debbie Jin
• Abstract: Given their strong magnetic moment and exotic electronic configuration, lanthanide atomic species disclose a plethora of intriguing phenomena in ultracold quantum physics. The large magnetic moment of these atoms reflects on a strong interparticle dipole-dipole interaction, which has both a long-range nature and an isotropic character. Here, we report on our latest results on quantum many-body and few-body physics based on a strongly-magnetic lanthanide, erbium (Er). Particular emphasis will be given to the dipolar Fermi gas and to the scattering properties of bosonic and fermionic Er, in which unconventional and fascinating phenomena appear as a result of both the magnetic and the orbital anisotropy of the underlying the native interactions between atoms.

Wednesday, April 27 — SPECIAL Physics Colloquium, "Ultrafast laser-driven processes in magnetic materials: A theoretical perspective"

• Presenter: Peter Oppeneer, Dept. of Physics and Astronomy, Uppsala University, Uppsala, Sweden
• Host: Margaret Murnane
• NOTE SPECIAL LOCATION: JILA X317
• NOTE SPECIAL TIME: 11:00 a.m.
• Abstract: Through excitation of materials with ultrashort laser pulses it might be possible to reach novel, transient states of matter that cannot be reached under equilibrium conditions. One prominent example is that of ultrafast laser-induced demagnetization which was discovered 20 years ago. Femtosecond laser excitation of a ferromagnetic transition metal was found to cause an ultrafast magnetization quenching within about 300 fs, much faster than the equilibrium spin-lattice relaxation time. The origin of the unexpectedly fast demagnetization is not understood and is currently a topic of intense debates. In this talk I will give a survey of recent theories and discuss in more detail the electron-phonon spin-flip scattering and the superdiffusive transport models, in connection to insightful measurements that help to bring the problem’s solution closer. I will provide an outlook on future ultrafast magnetic processes, including all-optical magnetization reversal and laser-imparted magnetization.

April 27 — "Trillion Degree Matter"

• Presenter: Gunther Roland, MIT 
• Host: Paul Romatschke
• Abstract: Experiments at large particle colliders like RHIC at Brookhaven Lab and LHC at CERN have used collisions of heavy nuclei to study a simple question:
  What are the properties of matter at extremely high temperature, in excess of several trillion Kelvin? At such temperatures, matter exists in the form of a plasma of quarks and gluons resembling the universe shortly after the Big Bang. We have found that the produced state of matter exhibits fascinating and somewhat surprising properties: Although its density exceeds that of water by 16 orders of magnitude, the quark-gluon plasma behaves like a near-perfect liquid. I will review the most striking experimental observations and describe new experiments aimed at elucidating the emergence of the plasma's unusual properties.

For more information about colloquia this semester, contact: Markus Raschke.

Colloquia are Wednesdays at 4:00 p.m. in DUAN G1B20, unless otherwise noted.

Coffee, tea and cookies will be available before regular colloquia beginning at 3:45 p.m. in DUAN G1B31.

August 26 — "The New Horizons Mission to Pluto: First Results"

• Presenter: Mihaly Horanyi, LASP, University of Colorado Boulder
• Host: Markus Raschke
• Abstract: New Horizons, the fastest spacecraft ever, reached Pluto on July 14, 2015, following a close to a decade-long journey.  It gathered high-resolution images, spectral data, plasma and dust measurements. The small fraction of the data that has been downloaded to date already prompted the need to revise our understanding about the outer most regions of our solar system. Contrary to our expectations, Pluto turned out to be geologically active, and shows an exceptionally wide variety of terrain, including possibly Rocky Mountain sized ice glaciers. The talk will discuss the scientific motivation for the mission, its instruments, and some of the preliminary results. One of the seven instruments onboard New Horizons is the Student Dust Counter that was designed, built, and operated by generations of students at the University of Colorado, many of them from our Department of Physics.

September 2 — "Advocating Science"

• Presenter: Rush Holt, C.E.O., American Association for the Advancement of Science (AAAS)
• Host: Tobin Munsat
• Abstract: Among the various ways of thinking and knowing about the universe and ourselves, science is special. Asking questions that can be answered empirically and engaging in open communication so that others can collectively review and verify possible answers lead to the most reliable knowledge—a knowledge that is powerfully applicable in daily life. Science is, as physician and essayist Lewis Thomas wrote, the “shrewdest maneuver” for discovering the world. This grand and clever enterprise, while surely not removing all worldly woes, brings beauty, wonderfully fulfilling intellectual pleasure, and cultural enrichment. It can lead to improved human interaction, more constructive commerce, and a better quality of life. Science helps bring what is a deep human need—a sense of progress. That progress is not assured, however. To thrive, science needs the support of the society it serves, and that support must be cultivated.

September 9 — "Water Storage in Planetary Interiors: Hydrogen Incorporation in High Pressure Minerals"

• Presenter: Joe Smyth, University of Colorado Boulder
• Host: Michael Ritzwoller
• Abstract: Although Earth is a water planet, and water dominates all of the surface processes, the oceans are only 0.023 % of the planet’s mass (230 ppm). Hydrogen is soluble, to greater or lesser extent, in all of the oxygen minerals of the interior. The Earth’s mantle is solid rock composed of oxygen minerals, mostly silicates, and composes two thirds of the mass of the planet.  Small amounts of H incorporated in these minerals are thus a much larger potential reservoir of water than is the ocean. Hydrogen has a unique geochemistry and usually occurs as a hydroxyl (OH) group in minerals. Its substitution in nominally anhydrous silicates is typically charge-balanced by vacancy at a divalent metal site, which reduces the density of the crystal and increases the compressibility. Hydrogen is relatively mobile in these crystals and can facilitate deformation and increase electrical conductivity. By controlling deformation, H controls viscosity and thus convection. H substitution in these minerals can decrease P-wave and S-wave seismic velocities, as well as density, and lower the temperature onset of melting by hundreds of degrees C.  ‘Water’, incorporated in minerals, may thus control many of the processes of the interior that drive plate tectonics as well the surface processes.

September 16 — "Changing his Mind at the Speed of Thought: Einstein's Failed Attempts to Undiscover Gravitational Waves"

• Presenter: Daniel Kennefick, University of Arkansas
• Host: Markus Raschke and Allan Franklin
• Abstract:  100 years ago, in November 1915, Einstein published his theory of General Relativity. This talk examines Einstein's life over the next several years, a period of war, poor health and personal turmoil, which ended with him becoming world famous in 1919 as a result of the famous British eclipse expedition which vindicated his theory. We also examine one important physics prediction which he deduced from the theory, that of gravitational waves. Here too, the path was far from smooth, and Einstein remained, for many years, very skeptical of their existence. This culminated in 1936 when he angrily withdraw a paper from the Physical Review after being criticized for trying to prove that his theory did not, after all, predict their existence.
• Daniel Kennefick is a native of Ireland who earned his Ph.D. at Caltech working under a leading General Relativist, Kip Thorne. His fields of interest are gravitational waves, black holes and galactic structure. he is also a historian of gravitational physics and astronomy. He is a fellow of the American Physical Society. He has written a book on the history of gravitational waves and helps to edit the Collected Papers of Albert Einstein. He is a co-author of "An Einstein Encyclopaedia" to appear next month from Princeton University Press.

**Special Seminar** Thursday, September 17 — "Evidence in favor of the density wave wave theory of galactic spiral arms"

• Presenter: Daniel Kennefick, University of Arkansas
• Location: JILA Auditorium
• Abstract: Since the work of Lin and Shu in the 1960s the density wave theory, in several different forms, has been the leading candidate for a theory explaining the ubiquitous spiral arm patters seen in disk galaxies. Other theories exist, and rival versions of the density wave theory exist, but there has been little success in using observations to decide which theories are more successful. We present three such observational tests which we have been working on recently. Firstly we present a relationship between spiral arm pitch angle (which measures how tightly wound a spiral arm is), central bulge mass and gas density in the disk of a galaxy. Such a relationship is predicted by the original density wave theory of Lin and Shu. Secondly we show that spiral arm pitch angle varies between measurements made in optical versus infrared images in a way which is predicted by the density wave theory. Thirdly we examine the extent to which the number of arms in a spiral pattern varies with disk density, a prediction made by the swing amplification version of the density wave theory.

September 23 — "Interferometry in a strong light"

• Presenter: Cindy Regal, University of Colorado Boulder
• Host: Paul Beale
• Abstract: The best rulers are made from light!  Optical interferometry is at the heart of many precise measurements from gravitational wave searches to modern surface diagnostics and microscopy.  Generally one improves interferometer precision by increasing the light intensity, as well as by calming the many technical sources of noise that can perturb the mirrors or optical path.  However, at extreme levels of light strength where radiation forces are significant, a new and interesting disturbance should appear – the quantum shaking associated with random arrival of individual photons at a mirror of the interferometer.  This quantum backaction of light has been long foreseen in gravitational wave searches and played a formative role in quantum optics theory.  In this talk I will discuss an experiment in which we used a particularly compliant micro-scale, vibrating drum to finally observe this effect in an interferometer.  In this strong-light limit, interferometer mirrors can also be used as a nonlinear medium to manipulate light – for example to make squeezed light.  We are pursuing ways to translate more complex quantum states into light by coupling superconducting elements to an interferometer mirror.   

September 30 — "Quantum thermalization and many-body Anderson localization"

• Presenter: David Huse, Princeton
• Host: Leo Radzihovsky
• Abstract: Progress in physics and quantum information science motivates much recent study of the behavior of extensively-entangled many-body quantum systems fully isolated from their environment, and thus undergoing unitary time evolution.  What does it mean for such a system to go to thermal equilibrium?  I will explain the Eigenstate Thermalization Hypothesis (ETH), which posits that each individual exact eigenstate of the system's Hamiltonian is at thermal equilibrium, and which appears to be true for many (but not all) quantum many-body systems.  Prominent among the systems that do not obey this hypothesis are quantum systems that are many-body Anderson localized and thus do not constitute a reservoir that can thermalize itself.  When the ETH is true, one can do standard statistical mechanics using the `single-eigenstate ensembles', which are the limit of the microcanonical ensemble where the `energy window' contains only a single many-body eigenstate.  These eigenstate ensembles are more powerful than the traditional statistical mechanical ensembles, in that they can also "see" the novel quantum phase transition in to the many-body localized phase, as well as a rich new world of many-body localized phases with symmetry-breaking and/or topological order.

October 7 — "The Space Economy"

• Presenter: George Sowers, United Launch Alliance
• Host: Markus Raschke
• Abstract: Different questions about the space economy will be addressed.  Why is space so expensive?  What can be done to lower the cost of space activity? And most importantly, how can we create a self sustaining space economy?  A self sustaining space economy is the key to affordable space and to bring the vast resources of space into the economic purview of humankind.
  
  Dr. George F. Sowers is vice president of Advanced Concepts and Technologies for United Launch Alliance (ULA) headquartered in Denver, Colorado. In this role, Sowers is responsible for assessing product offerings, technology roadmaps, operations concepts, and business models that meet various potential customerfuture space architecture needs and buying approaches. The overall goal of Sowers’ team is to enable the design of the products, processes and infrastructure to meet our customers’ future requirements. Prior to this position, Sowers was the vice president of Human Launch Services.
  Before joining ULA, Sowers was director of Business Development for Lockheed Martin Space Systems Company.
  Sowers previously served as director of Mission Integration for the Atlas program and Chief Systems Engineer and director of the Systems Engineering and Integration Team (SEIT) for Atlas V development.
  Sowers began his career with Martin Marietta in 1981 as a flight design engineer on the Titan program.
  Sowers received his Bachelor of Science degree in physics from Georgia Tech in 1980 and his PhD in physics from the
  University of Colorado in 1988.
   

October 14 — "The Shining High-Energy Universe as a Plasma Astrophysicist's Playground"

• Presenter: Dmitri Uzdensky, University of Colorado Boulder
• Host: Paul Beale
• Abstract: Most of the visible, shining Universe (the regular, non-dark matter) is made of plasma, a hot ionized gas that can carry electric currents and thus generate and interact with electromagnetic fields.  Cosmic plasmas are complex, turbulent, almost always immersed in magnetic fields and often collisionless, with nonthermal particle distributions. Many of the most spectacular phenomena in our magnetized plasma Universe involve rapid and bright high-energy bursts and flares, often exhibiting nonthermal spectra. These violent events are believed to be powered by just a few basic kinetic plasma processes: turbulence, shocks, and magnetic reconnection (rapid rearrangement of magnetic field topology, leading to a violent release of magnetic energy).  Studying them is the realm of Plasma Astrophysics.  Often, the intense radiation produced by these energy dissipation processes exerts an strong backreaction on them, controlling their energetics and dynamics and hence their observational signals.  However, radiative effects (e.g., radiative cooling, radiation pressure, photon-drag resistivity, pair creation) have so far been largely ignored in traditional theoretical studies.  My research program aims at building a coherent theoretical framework for the new frontier — Radiative Plasma Astrophysics — and at exploring its astrophysical applications.  Most of my research so far has focused on radiative effects on magnetic reconnection. I will give an overview of my group’s research in this area, including the development of new computational tools capable of investigating radiative kinetic plasma processes, and I will illustrate our work with several important astrophysical applications.

October 21 — "Pentaquarks: Quark Model Revisited"

• Presenter: Tomasz Swarnicki, Syracuse University
• Host: John Cumalat
• Abstract: The LHCb experiment has recently reported the observation of pentaquark candidates: bound states of four quarks and an antiquark. Such objects have been predicted for over 50 years, but until recently believed not to exist. I will describe the bumpy road which led to this recent observation, starting from the birth of the Quark Model, through its spectacular success in describing known mesons and baryons and searches for quark structures made out of more than the minimal quark content. I will describe the LHCb pentaquark measurement and conclude with implications of the recent observations of tetraquark and pentaquark candidates on our understanding of the fabric of matter made out of quarks.

October 28 — "The new SI"

• Presenter: David Newell, National Institute of Standards and Technology, Gaithersburg, Maryland
• Host: Scott DIddams
• Abstract: The International System of Units (SI from the French Le Système International d’Unités) is the universally accepted method of expressing physical measurements for world commerce, industry, and science. Though officially established in 1960, the origins of the present SI can be traced back to the creation of the decimal Metric System during the French Revolution. The SI has proven to be a living, evolving system, changing as new knowledge and measurement needs arise, and once again international consensus is building to advance the SI to reflect contemporary understanding of the physical world.  The new framework of the future SI will no longer be based on definitions of units such as the meter, kilogram, and second.  Instead it will adopt exact values for seven fundamental constants of nature upon which all SI units will be realized. Gone are the base units and their definitions.

November 4 — "New condensation and pairing mechanisms for high-Tc cuprate and unconventional superconductors inferred from muon and neutron experiments"

• Presenter: Tomo Uemura, Columbia University
• Host: Minhyea Lee
• Abstract:Superconductivity of simple metals (Al, Sn, etc.) was explained by Bardeen, Cooper and Schrieffer who obtained Nobel Prize in 1972 for this contribution.  Soon after discovery of high-Tc superconductors in 1986 (which led to Nobel Prize to Bednorz and Mueller in 1987), we started muon spin relaxation (MuSR) measurements of the magnetic field penetration depth, and demonstrated nearly linear correlations between Tc and the superfluid density, which are manifest not only in cuprates but also in many other unconventional superconductors including Alkali doped C60, FeAs and some of the heavy-fermion systems.  The strong dependence of Tc on carrier density is not expected in the BCS condensation, but is a main characteristic of Bose Einstein Condensation.  Another important scaling of Tc is seen with the energy of the magnetic resonance mode measured by neutron scattering, which can be viewed as an excitation analogous to rotons in superfluid He. Rotons are short-range and short-time solid-like correlations, called soft mode, in superfluid state for which Landau was given Nobel Prize in 1962. This roton-like scaling is expected when condensed bosons are excited with phase fluctuations to another bosonic state, without breaking of a boson into two fermion pairs.  This again is different from Cooper pair-breaking excitations across the energy gap in BCS superconductors. We discuss these features and adopting BEC-BCS crossover picture for the cuprates, which I proposed in 1994, several years earlier than the burst of experiments by cold atoms.  Further consideration lead us to a possible pairing mechanism based on comparable spin and charge energy scales in cuparate and many other unconventional superconductors.

   

November 11 — "Polymer Drag Reduction"

• Presenter: James Brasseur, Aerospace Engineering Sciences, University of Colorado (Formerly Penn State University)
• Host: Mike Ritzwoller
• Abstract: Fluid turbulence refers to a chaotic state of fluid motion that is dominated by strong nonlinearity among “eddies” spread over a broad range of length and time scales. It has been known since the 40’s that polymers injected at very low concentrations from the submerged surfaces of ships can dramatically reduce the drag on the ship hull by 40% or more, but that “polymer drag reduction” manifests only when the flow state is highly turbulent! The mechanical processes underlying the effect are so complicated that they are still under debate. Indeed, Pierre-Gilles DeGennes, who won the 1991 Nobel Prize in physics “for discovering that methods … can be generalized to more complex forms of matter, in particular to liquid crystals and polymers” wrote a paper on the topic that is still hotly debated by those who have researched the mechanisms. Why does the flow have to be turbulent and how do molecules orders of magnitude smaller than the smallest turbulent flow eddies cause such dramatic reductions in surface shear stress (i.e., drag)? I will address these questions using high-fidelity computer simulations of turbulent flow to argue that the essential mechanism is the suppression of momentum transport to the surface by strong interaction between polymer, turbulence and shear adjacent to the surface.

November 18 — "The Department of Physics and the German V-2 Legacy"

• Presenter: R.C. "Merc" Mercure, Jr.
• Host: John Cumalat
• Abstract: The speaker will discuss the Department of Physics during the period of 1948 to 1956, the time when he was student in the Department. The preeminence in space science and space physics of the University, the Department of Physics, LASP as well as Ball Aerospace and Technology, Inc., can be directly traced back to 1947 when the Department of Physics won a contract to use captured German V-2 rockets to lift instruments above the earth’s atmosphere to study the sun. The early history of Ball Aerospace and of LASP is discussed.
  Dr. Ruel C. Mercure earned his PhD in physics in 1957, under William Rense. He is the 21st student at CU to earn a PhD in Physics.

November 25 Fall Break - No Colloquium

December 2 — "The Physics of Phagocytosis: How Polymerizing Polymers Drive Cell Engulfment of Particles"

• Presenter: Jennifer Curtis, Georgia Institute of Technology
• Host: Meredith Betterton
• Abstract: Cell processes have evolved to be extremely robust. According to engineering principles, building a machine with robust function requires modular design and compartmentalization.  This is consistent with the observation that seemingly disparate cell processes like cell motility, phagocytosis and cytokinesis possess common sets of molecules that work together to achieve similar activities within a larger cellular event. For example, all of these processes require actin-driven membrane deformation, the exertion of contractile forces, and exquisite control of membrane tension. 
  With this broader perspective in mind, we investigate phagocytosis, the dramatic mechanical process by which individual cells engulf foreign bodies or debris.  It is critical to the immune response, being the hallmark behavior of white blood cells.   Considerable effort has been devoted to elucidating the biochemical pathways that direct phagocytosis.  However, mechanics of phagocytosis has received less attention.  We present a systematic study of mechanical signature of phagocytosis, including the forces and dynamics that occur during cell engulfment of a substrate, and the reproducibility from one phagocytotic event to another.  Our studies show that the majority of cells behave in a similar manner, having distinct phases of initial-contact, rapid spreading, and late-stage contraction.  Further the engulfment dynamics appear to collapse onto a single ‘master curve’ similar to that found for cell spreading in other unrelated scenarios. Our studies provide new territory for gathering details about the ‘modular’ actomyosin machinery that regulates phagocytosis and other cell mechanical processes.  In addition to addressing a long-standing puzzle about actin architecture in phagocytosis, we expect that our observations will provide insights into systems dependent on the actomyosin molecular machinery and chemo-mechanical coupling.

December 9 — "Programming colloidal self-assembly: Colloids with directional and DNA-mediated interactions"

• Presenter: David Pine, NYU
• Host: Ivan Smalyukh
• Abstract: The self-assembly of colloids has been largely limited to spheres and rods with non-specific interactions and no directionality other than that imparted by a rod.  Recently a variety of new colloidal particles have been introduced, including particles with sticky patches, dimples, cavities, and other complex shapes, that enable assembly into spectrum of new structures.  Here we focus on the development of DNA-coated particles with highly specific, programmable, and thermally reversible interactions provides unprecedented control over particle assembly.  Of particular interest is the ability to assemble colloidal particles made from different materials, including polymers, inorganics, and metals, into complex hybrid structures with programmed particle placement.  A key recent development is the fabrication of DNA coatings that allow particles to bind and roll so that they can avoid kinetic traps and anneal into structures that minimize the free energy.

For more information about colloquia this semester, contact: Markus Raschke.

Colloquia are Wednesdays at 4:00 p.m. in DUAN G1B20, unless otherwise noted.

Coffee, tea and cookies will be available before regular colloquia beginning at 3:45 p.m. in DUAN G1B31.

January 14 — "Autonomous motility in soft active matter"

• Presenter: Zvonomir Dogic, Brandeis University
• Host: Meredith Betterton
• Abstract: The laws of equilibrium statistical mechanics impose severe constraints on the properties of conventional materials assembled from inanimate building blocks. Consequently, such materials cannot exhibit spontaneous motion or perform macroscopic work; i.e., a fluid in a beaker remains quiescent unless driven by external forces. Inspired by biological phenomena such Drosophila cytoplasmic streaming, our goal is to develop a new category of soft active materials assembled from animate, energy-consuming building blocks. Released from the constraints of the equilibrium, such internally driven materials are able to change-shape, crawl, flow, swim, and exert forces on their boundaries to produce macroscopic work. Active liquid crystals can serve as a platform for developing novel material applications, testing fundamental theoretical models of far-from-equilibrium active matter and potentially even shedding light on self-organization of living cells.

January 21 — "Quantum matter without quasiparticles"

• Presenter: Subir Sachdev, Harvard University
• Host: Leo Radzihovsky
• Abstract: The quasiparticle concept is the foundation of our understanding of the dynamics of quantum many-body systems. It originated in the theory of metals, which have electron-like quasiparticles; but it is also useful in more exotic states like those found in fractional quantum Hall systems. However, modern materials abound in systems to which the quasiparticle picture does not apply, and developing their theoretical description remains one of the most important challenges in condensed matter physics. I will describe recent progress in understanding the dynamics of two systems without quasiparticles: the superfluid-insulator transition of ultracold atoms, and the `strange metal’ found in the high temperature superconductors. Some of this progress relies on holographic methods which map non-quasiparticle quantum systems to the dynamics of black hole horizons.

January 28 — "Optical Multi-Dimensional Coherent Spectroscopy"

• Presenter: Steven T. Cundiff, Physics, University of Michigan, JILA, National Institute of Standards and Technology & University of Colorado, Boulder
• Host: Markus Raschke
• Abstract: The concept of multidimensional Fourier transform spectroscopy originated in NMR where it enabled the determination of molecular structure.  In either NMR or optics, a sample is excited by a series of pulses. The key concept is to correlate what happens during multiple time periods between pulses by taking a multidimensional Fourier transform. The presence of a correlation, which is manifest as an off-diagonal peak in the resulting multidimensional spectrum, indicates that the corresponding resonances are coupled. Migrating multidimensional Fourier transform spectroscopy to the infrared and visible regimes is difficult because of the need to obtain full phase information about the emitted signal and for the phase difference between the excitation pulses to be stable and precisely incremented. I will give an introduction to optical two-dimensional coherent spectroscopy, using an atomic vapor as simple test system, but also show unexpected results due to atomic interactions. I will then present our use of it to study optical resonances in semiconductor nanostructures. In quantum wells, our results show that many-body effects dominate the light-matter interaction for excitons in semiconductors and provide a rigorous and quantitative test of the theory.  In quantum dots, there is inhomogeneous broadening due to size dispersion, however two-dimensional coherent spectroscopy can make size-resolved measurements without the need to isolate individual quantum dots.
   

February 4 — "Plasma-Based Particle Accelerators: There's Plenty of Room at the Bottom"

• Presenter: Mike Downer, University of Texas, Austin
• Host: John Cary
• Abstract: Over the past few years, compact plasma-based particle accelerators have advanced sufficiently that it is no longer a pipe dream* to imagine a tabletop x-ray free-electron laser in every major university in the world, or proton cancer therapy on a scale that many hospitals could afford.  I will survey recent experimental highlights in the field that make these hopes more realistic than even a few years ago.  These include a milestone achieved recently using the Texas Petawatt Laser:  nearly mono-energetic acceleration of plasma electrons to 2 GeV with unprecedented sub-milliradian beam divergence.  I will discuss near-term prospects for improving plasma-based accelerators further, and for obtaining tunable x-ray radation from them.   Finally I will describe new holographic techniques that enable experimenters to visualize the electron density waves that lie at the heart of plasma-based accelerators.  Such 4D visualization, previously available only from intensive computer simulations, helps physicists understand how plasma-based particle accelerators work, and how to make them work better.

February 11 — "Spin and pseudospins in 2-dimensional semiconductors and heterostructures"

• Presenter: Xiaodong Xu, University of Washington
• Host: Markus Raschke
• Abstract: Conventional semiconductors (e.g. Silicon and Gallium Arsenide) and their heterostructures are the foundation of condensed matter physics and present solid-state technologies, such as light emitting diodes, solar cells, and high speed transistors. Recently, new classes of quantum materials at 2-dimensional limit have been discovered, including atomically thin monolayers of graphite, boron nitride, and transition metal dichalcogenides (TMDs). These monolayer materials not only have remarkable physical properties on their own, but also they can be stacked together to form van der Waals heterostructures for new functionalities, offering unprecedented opportunities to explore new realms in materials science, device physics, and engineering. In this talk, I will focus on 2-dimensional semiconductors and their heterostructures. Fundamentally new properties in these structures include the relationship between electron spin, the "valley index" specifying which of the two inequivalent momentum states the electron occupies in the 2D lattice, and the "layer index" specifying which layer the electron is in when two monolayers form a bilayer. I will discuss the experimental progress towards understanding and optoelectronic control of spins and pseudospins in both monolayers and atomically-thin semiconducting heterostructures.

CANCELLED - February 18 — "What have we learned about the QGP from run I at the LHC?"

• Presenter: Constantin Loizides, Lawrence Berkeley National Laboratory
• Host: Jamie Nagle
• Abstract: Nucleus-nucleus collisions at the Large Hadron Collider (LHC) occur at energies that enable the creation of a new state of matter, called the quark-gluon plasma (QGP), reaching energy densities similar to those achieved in the early universe shortly after the Big Bang. The system created in such collisions exhibits collective behavior characteristic of a strongly coupled, nearly inviscid fluid, which undergoes rapid three-dimensional expansion. Tomographic probes, e.g. involving the measurements of energetic partons or heavy flavor quarks, exhibit properties of the system consistent with expectations of hot partonic matter. I will discuss the most striking results from run I at the LHC, and will compare them
  with those from the Relativistic Heavy Ion Collider and to models, leading to an exploration of the QGP state over more than two orders of magnitude in collision energy.

February 25 — "Coherent Diffraction Imaging and Atomic Resolution Electron Tomography"

• Presenter: Jianwei (John) Miao, Department of Physics & Astronomy and California NanoSystems Institute, University of California, Los Angeles
• Host: Meredith Betterton and Markus Raschke
• Abstract: For centuries, lens-based microscopy, such as light, phase-contrast, fluorescence, confocal and electron microscopy, has played an important role in the evolution of modern science and technology. In 1999, a novel form of microscopy, which is known as coherent diffractive imaging (CDI) or lensless imaging, was developed and transformed our traditional view of microscopy, in which the diffraction pattern of a non-crystalline object or a nanocrystal was first measured and then directly phased to obtain an image. The well-known phase problem was solved by combining the oversampling method with iterative algorithms. In the first part of the talk, I will present the principle of CDI and illustrate some applications using synchrotron radiation, X-ray free electron lasers and high harmonic generation.
  
  In the second part of the talk, I will present a general tomographic method for determining the 3D local structure of materials at atomic resolution. By combining scanning transmission electron microscopy with a novel data acquisition and image reconstruction method known as equally sloped tomography, we achieve electron tomography at 2.4 Å resolution and observe nearly all the atoms in a multiply-twinned Pt nanoparticle. We find the existence of atomic steps at 3D twin boundaries of the Pt nanoparticle and, for the first time, image the 3D core structure of edge and screw dislocations in materials at atomic resolution. We expect this atomic resolution electron tomography method to find broad applications in solid state physics, materials sciences, nanoscience, chemistry and biology.
   

March 4 — "Of Soot and Sunflowers"

• Presenter: Chris Sorensen, Kansas State University
• Host: John Cumalat
• Abstract: Soot, the by-product of combustion, that smoky, black crap from chimneys, power plants and the inside of the tail pipe of my roadster, what scientist would ever bother to study soot? As a “particle physicist”, that’s what I do, and I find that soot has mysteries and beauties that can entertain any curiosity. In this talk I will describe some of my researches into soot and other aggregate structures; an unlikely journey of discovery to find fractal structures with non-Euclidian dimensionality, gel networks of graphene that tenuously span space and common Fibonacci themes for non-equilibrium phenomena such as sunflowers and soot.
  Christopher M. Sorensen is the Cortelyou-Rust University Distinguished Professor and a University Distinguished Teaching Scholar in the Departments of Physics and Chemistry (adjunct). He has won numerous teaching awards. In 2007 he was named the CASE/Carnegie Foundation United States Professor of the year for doctoral universities.
  He is also an active scientist with over 280 publications, six patents and three pending. In 2003 he won the Sinclair Award of the American Association for Aerosol Research, and he is a past president of that organization. He is a Fellow of the AAAR, the APS and the AAAS.
  Chris graduated from the University of Nebraska in 1969 where he was Phi Beta Kappa and a Woodrow Wilson Fellow. He was drafted and served in Vietnam. He earned his PhD in physics from the University of Colorado in 1977. In 2008 he was named a Norlin Distinguished Graduate of that university.

March 11 — "The early evolution of the Moon: Looking beneath the surface with GRAIL gravity data"

• Presenter: Jeff Andrews-Hanna, Colorado School of Mines
• Host: Shijie Zhong
• Abstract: The geological record of the earliest history of the Moon is poorly preserved as a result of the heavy impact bombardment of the surface prior to 3.7 Ga.  However, the geophysical signatures of early lunar evolution are preserved in the subsurface.  Recent data from NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission is providing a view of the lunar subsurface at unprecedented resolution. Linear gravity anomalies reveal a population of ancient igneous intrusions that likely formed during an early period of thermal expansion of the Moon, providing an important constraint on lunar formation.  Later intrusive activity was dominated by the formation of circular or arcuate dikes within the ring structures surrounding the major impact basins.  In the absence of ring dikes, the gravitational signatures of tectonic offsets across the rings reveal the nature of the basin ring tectonics.  The largest magmatic-tectonic structure revealed by GRAIL is a quasi-rectangular set of linear density anomalies ~2500 km in diameter, encompassing the Procellarum region on the lunar nearside.  The gravitational signatures of the Procellarum border structures are consistent with volcanically flooded rift valleys, formed by extension driven by the gradual cooling and contraction of the Procellarum KREEP terrain. These and other observations from GRAIL are shedding new light on the early history of Earth's nearest neighbor.

March 18 — "Time-reversal-symmetry-breaking in unconventional superconductors"

• Presenter: Aharon Kapitulnik, Stanford University
• Host: Leo Radzihovsky
• Abstract: BCS theory of conventional superconductivity describes pairing of spin-singlet time-reversed states, and is characterized by an order parameter which breaks U(1)-gauge symmetry leading to basic superconducting properties, such as the Meissner effect, persistent current and flux quantization. By contrast, unconventional superconductors exhibit additional broken symmetries, which often lead to distinct superconducting phases with unique properties. In this talk I will survey current searches for time reversal symmetry breaking (TRSB) in unconventional superconductors. After a brief discussion on the origin of TRSB in condensed matter systems I will introduce various ways to detect such effects, and in particular the use of magneto-optic like measurements that utilize the Sagnac interferometer that was invented in our group. I will show results on a variety of unconventional superconductors and discuss their implications to the determination of the possible pairing symmetry in each system.

Spring Break - No Colloquium March 25

April 1 — "Magnetic fields from the ocean floor to outer space"

• Presenter: Margaret Kivelson, UCLA
• Host: Mihaly Horanyi
• Abstract:  Magnetic fields are ubiquitous in the universe. They control behavior of charged particles that fill most of space. They account for some of the structure observed in the universe. We earthlings live in a bubble in space, shielded by Earth’s magnetic field from direct interaction with most of the energetic particulate matter, some potentially destructive, that fills the solar system. This presentation will start with some historical background and will briefly discuss the generation of planetary magnetic fields, a subject still not fully understood.  Using data from spacecraft that have explored all the planetary bodies of the solar system, the talk will describe magnetic properties of some solar system bodies and how these properties affect their interaction with the plasma that surrounds them.

Physics Colloquium: Thursday, April 2 — "Quantum Tapestries" Sponsored by Annals of Physics

• Presenter: Matthew Fisher, UC Santa Barbara
• NOTE SPECIAL LOCATION: 11th Floor Commons Room
• Host: Victor Gurarie
• Abstract: Within each of nature’s crystals is an exotic quantum world of electrons weaving to and fro. Each crystal has its own unique tapestry, as varied as the crystals themselves. In some crystals the electrons weave an orderly quilt. Within others, the electrons are seemingly entwined in a entangled web of quantum motion. In this talk I will describe the ongoing eff orts to disentangle even nature’s most intricate quantum embroidery. Cutting-edge quantum many-body simulations together with recent ideas from quantum information theory, such as entanglement entropy, are enabling a coherent picture to emerge.

April 8 Joint Physics/APS Colloquium — "Detection of B-mode polarization at 150GHz and degree angular scales by BICEP2
and Keck Array"

• Presenter: Clem Pryke, University of Minnesota
• Host: Nils Halverson
• Abstract: The theory of Cosmic Inflation postulates that our entire observable universe was spawned from a quantum fluctuation in an incredibly brief burst of hyper expansion. Inflation makes several predictions which appear to match features of the actual Universe in which we find ourselves, and, in addition, predicts that a background of gravitational waves will exist which may produce a specific observable feature in the polarization pattern of the Cosmic Microwave Background - the long sought B-mode polarization. Using data from a specialized radio telescope called BICEP2 operating from the South Pole in Antarctica our collaboration reported last year a highly significant detection of B-modes at 150GHz and few degree angular scales. However in a recently published joint analysis with the Planck space mission we find that, once an unexpectedly high level of polarized emission from dust grains in our own galaxy is taken into account, there is not currently significant evidence for a gravitational wave signal. I will describe the background, the results, and the continuing hunt for inflationary gravitational waves with the BICEP/Keck-Array experimental program.

April 15 — "A half-century of particle physics:  from CP violation to the Higgs boson"

• Presenter: Bill Ford, University of Colorado, Boulder
• Host: Paul Beale
• Abstract: When as a grad student in 1964 I took my first particle physics course, the violation of CP symmetry had just been discovered, and the concept of "quarks" had newly been proposed.  In this talk I offer some personal perspectives on the evolution of the Standard Model of particle physics and some of the field's still-open questions.

April 22 — "Muon g-2 and the quest for TeV scale physics"

• Presenter: Chris Polly, Fermilab
• Host: Eric Zimmerman
• Abstract: One of the most imperative questions in particle physics today is whether or not new physics will emerge at the few TeV scale.  Observational hints for new physics have arisen from several sectors with exciting theoretical implications that can potentially be explained by supersymmetry, dark matter, or other exotic models.  One of the most persistent hints comes from the Brookhaven muon g-2 experiment, where an ultra-precise measurement of the muon anomalous magnetic moment differs by >3 sigma from the theoretical expectation.  The anomalous magnetic moment of the muon provides a unique window into the TeV scale, and a new effort is underway at Fermilab to improve the experimental precision.  A review of the physics, the principles behind the experiment, and the incredible journey to bring the experiment to the point it is at today will be discussed.

April 29 — "Physics Textbooks Don't Always Tell the Truth

• Presenter: Allan Franklin, University of Colorado, Boulder
• Host: 
• Abstract: Anyone who studies the history of physics quickly realizes that the history of physics presented in physics textbooks is often inaccurate. This is not necessarily a bad thing. The purpose of textbooks is to help students learn physics. An inaccurate history may serve a pedagogical purpose. It may help to explain concepts more clearly than the actual history. I believe, however, that it is important for those of us who teach physics to know the accurate history. In this talk I will discuss three episodes from the history of modern physics: 1) Millikan’s experiments on the photoelectric effect; 2) the Michelson-Morley experiment; and 3) the Ellis-Wooster experiment on the energy spectrum in β decay. Everyone knows that Millikan’s work established the photon theory of light and that the Michelson-Morley experiment was crucial in the genesis of Einstein’s special theory of relativity. The problem is that what everyone knows is wrong. The Ellis-Wooster experiment, on the other hand, is rarely discussed in physics texts, but it should be. In this talk I will present a more accurate history of these three experiments.

Special Colloquia

**SPECIAL Colloquium: Monday, January 12, Note date and location — "Topological soft matter: from linkages to kinks"

• Presenter: Bryan Chen, Instituut-Lorentz for Theoretical Physics, Leiden University
• Location: Duane G125
• Host: Leo Radzihovsky
• Abstract: Networks of rigid bars connected by joints, termed linkages, provide a minimal framework to design robotic arms and mechanical metamaterials built out of folding components.  These linkages may admit motions that perform useful functions.  Can these motions be made to be topologically robust?  I will explain this question and illustrate our answer with a chain-like linkage that, according to linear elasticity, behaves like a topological mechanical insulator whose zero-energy modes are localized at the edge. Simple experiments we performed using prototypes of the chain vividly illustrate how this edge mode can in fact propagate unobstructed all the way to the opposite end. Indeed, the chain is a mechanical conductor, whose carriers are nonlinear solitary waves, not captured within linear elasticity. This chain can be regarded as the simplest example of a topological mechanical metamaterial whose protected excitations are solitons, moving domain walls between distinct topological mechanical phases.  Live demonstrations on real toys will be performed.

**SPECIAL Colloquium: Monday, January 26 — "Broken-symmetry states in topological insulators"

• Presenter: Yihua Wang, Stanford University
• Location: DUAN G125
• Host: Dan Dessau/Leo Radzihovsky
• Abstract: Breaking the time-reversal symmetry (TRS) on the surface of a three-dimensional topological insulator (TI) transforms its metallic surface into a Chern insulator. The TRS-broken surface states are essential for many exotic emergent particles in condensed matter. In this talk, I will show broken TRS surface states of TI induced by magnetism and by light imaged with scanning microscopy and photoemission spectroscopy respectively. Our capability to manipulate mesoscopic magnetic structures as well as to shape ultrafast light pulses makes broken-symmetry states in TI promising platforms to simulate elusive fundamental particles such as magnetic monopoles and Majorana fermions.

**SPECIAL Colloquium: Thursday, January 29 — "Quantum Dynamics and Control of Chemical Reactions: From Cold Molecules to Quantum Biology"

• Presenter: Timur Tscherbul, University of Toronto
• Location: DUAN G130
• Host: Andreas Becker
• Abstract: Ultracold molecular gases hold the promise of revolutionizing atomic, molecular, and chemical physics by enabling high-resolution quantum control of molecular interactions with external electromagnetic fields. In the first part of this talk, I will present an efficient theoretical method for calculating low-temperature chemical reaction rates in the presence of electromagnetic fields. Using this method, I will show that electric fields with magnitudes <150 kV/cm can be used to control the total reaction cross sections, product state distributions, and the branching ratios for reactive vs inelastic scattering via electric-field-induced Feshbach resonances. In the second part of my talk, I will show how long-lived quantum coherences can be generated in multilevel atomic and molecular systems weakly illuminated by solar light. These noise-induced coherences arise due to Fano interference between incoherent pump transitions, and I will discuss the conditions under which such interference is likely to occur in real atomic and molecular systems. Finally, I will show that noise-induced coherences can play an important role in the initial stages of cis-trans photoisomerization of retinal in rhodopsin, the primary photoreaction in vision.

**SPECIAL Colloquium: Monday, February 2 — "Mechanics and Self-Organization of Biological Active Matter"

• Presenter: Daniel Chen, Brandeis University
• Location: DUAN G130
• Host: Dan Dessau
• Abstract: A remarkable feature of living matter is that mechanical motion at large length scales is routinely accomplished with exquisite precision by the cooperative action of millions of nanometer-sized molecular motors and fibers. Examples range from the way cells in our body divide to the way cilia lining our airways collectively beat to transport fluid. From a technological standpoint, it would be highly desirable to create artificial materials that could inherit, at least in part, this unique capability of living materials. Toward this goal, I will describe our creation of a versatile model system composed of microtubules, kinesin molecular motors, and polymer depletant that self-organizes into highly dynamic network of active bundles. Driven far from equilibrium by chemical energy from ATP hydrolysis, this active gel exhibits continuous structural remodeling, akin to the cell cytoskeleton, but has far fewer components and thus more amenable to experimental measurement and theoretical modeling. I will show that these active gels exhibit unique mechanical properties with no analogues in conventional equilibrium materials, including the ability to ‘cloak’ mechanical information at large length scales and to spontaneously flow under confinement. These properties exemplify new challenges and opportunities at the intersection of materials science, biology, and active matter physics.

**SPECIAL Colloquium: Tuesday, February 3 — "Beyond Independence: Emergent Neural Function and the Natural World"

• Presenter: Ann Hermundstad, University of Pennsylvania
• Location: JILA Auditorium
• Host: Meredith Betterton
• Abstract: The brain is composed of billions of neurons that exchange diverse patterns of electrical activity.  Collectively, these activity patterns encode information about the external environment (such as patterns of light intensity, chemical signals, and acoustic pressure waves), and they ultimately support a vast set of behavioral functions (such as the abilities to see in the dark, to learn to ride a bicycle, and to plan a career path).  I am interested in understanding how the anatomical and dynamical organization of neuronal populations facilitates these diverse functionalities, and more generally, why a particular organization is advantageous or disadvantageous.  Efficient coding—the notion that information processing systems are efficiently tuned to the statistical regularities of their natural inputs—provides a powerful organizing framework for approaching these questions.  I will highlight two applications of efficient coding in the visual system, where it is possible to both characterize the statistical structure of visual signals, and measure how these signals are encoded in populations of neurons.  In one application, we use behavioral experiments to measure human visual sensitivity to correlated textures (visual patterns with controlled statistical properties), and we show that this sensitivity is matched to the distribution of multipoint correlations present in natural scenes.  In another application, we use multi-electrode arrays to record the activity of visual neurons responding to natural and artificial movies, and we show evidence that the distribution of population activity patterns is matched to the statistical structure of natural movies.  Together, these studies provide evidence that the visual system is efficiently tuned to signals in the natural visual environment, and they raise a broad set of questions about the generality of these results across different neuronal and biological systems.

**SPECIAL Colloquium: Thursday, February 5 — "Dirac fermions and broken symmetries in topological crystalline insulators"

• Presenter: Ilijia Zeljkovic, Madhavan Lab, Boston College
• Location: DUAN G130
• Host: Dan Dessau
• Abstract: Topological crystalline insulators (TCIs) are a recently discovered class of topological materials which harbor massless Dirac surface states (SS). Theory predicts that these SS are protected by crystalline symmetries, and that SS electrons can acquire a mass if these symmetries are broken. In this talk, I will present our recent scanning tunneling microscopy (STM) investigations of the TCI Pb1-xSnxSe utilizing two different spectroscopic methods: quasiparticle interference imaging and Landau level spectroscopy. First, based on the interference patterns of scattered surface electrons, we uncover a distinct orbital texture of the Dirac bands in TCIs. Moreover, simultaneous imaging of the atomic and electronic structures reveals that a fraction of the Dirac electrons acquire mass due to a broken mirror symmetry. Our experiments suggest a novel pathway for manipulation of electrons in two-dimensional systems via lattice distortions.

**SPECIAL Colloquium: Monday, February 9 — "Many-Body Physics in the Attosecond World"

• Presenter: Stefan Pabst, ITAMP, Harvard University
• Location: DUAN G125
• Host: Andreas Becker
• Abstract: Ultrafast science is a young, vibrant, and highly interdisciplinary research field combining areas of biology, chemistry, and physics. Attosecond and strong-field physics focus on the electron motion in atoms, molecules, and solid state systems. In this talk, I discuss in more detail the importance of many-body processes in electronic motion as they appear in noble gas atoms when exposed to ultrashort and strong-field pulses. Furthermore, I explain the challenges and the perspectives of many-body theories in the strong-field regime. I show, in particular, how a time-dependent configuration interaction (TDCIS) approach can be applied to uniquely identify collective many-body effects in high-harmonic generation, and to discover an ultrafast decoherence mechanism that entangles the photoelectron with its parent ion within a few hundreds of attoseconds.

**SPECIAL Colloquium: Thursday, February 12 — "Emergent properties hidden in plain view: Strong electronic correlations at oxide interfaces"

• Presenter: Jak Chakhalian, Center for Artificial Quantum Materials, University of Arkansas
• Location: DUAN G130
• Host: Dan Dessau
• Abstract: Complex oxides are a class of materials characterized by a variety of competing interactions that create a subtle balance to define the lowest energy state and lead to a wide diversity of intriguing properties ranging from high Tc superconductivity to exotic magnetism and orbital phenomena. By utilizing bulk properties of these materials as a starting point, interfaces between different classes of complex oxides offer a unique opportunity to break the fundamental symmetries present in the bulk and alter the local environment. Harnessing our recent advances in complex oxide growth, we can combine materials with distinct or even antagonistic order parameters to create novel materials in the form of heterostructures with atomic layer precision. The broken lattice symmetry, strain, and altered chemical and electronic environments at the interfaces then provide a unique laboratory to manipulate this subtle balance and enable novel quantum manybody states not attainable in bulk. Understanding of these phases however requires detailed microscopic studies of the heterostructure properties. In this talk I will review our recent results on unit-cell thin nickelates, titanates and cuprate-manganite heterostructures to illustrate recentlyuncovered principles of rational materials design and control of collective quantum phenomena by the interface.
   

**SPECIAL Colloquium: Monday, February 16 — "Atomic giants in a new light: Quantum many-body physics with Rydberg atoms and photons"

• Presenter: Thomas Pohl, Max Planck Institute for the Physics of Complex Systems, Dresden, Germany
• Location: DUAN G130
• Host: Andreas Becker
• Abstract: The study of highly excited atoms has a long history in AMO physics, from the early days of quantum mechanics to the more recent development of cavity-QED. Lately, the advent of ultracold gases has ushered in a renaissance of Rydberg atom physics, allowing to create, probe, manipulate and utilize extreme atomic states with unprecedented precision. In particular, the combination of ultra-low temperatures, high densities and strong atomic interactions leads to rich physical behavior with promising applications in optical and information science.
  In this talk, I will outline different ideas to turn laser-driven Rydberg gases into a versatile platform for exploring exotic many-body phenomena in synthetic quantum systems, resembling quantum magnets, quantum fluids or quantum solids. Emerging correlations in such settings can have a substantial back-action onto the light field that excites the atoms. The resultant intricate interplay of light-matter and atom-atom interactions gives rise to unrivaled optical nonlinearities that open the door to manipulating light at the level of single quanta. I will present our progress towards understanding photon propagation in this new regime of quantum optics, from classical nonlinearities and few-photon applications to the quantum many-body physics of light.

**SPECIAL Colloquium: Thursday, February 19 — "Age of Oxides: New Physics and Foundations of Modern Technology"

• Presenter: Gang Cao, University of Kentucky
• Location: DUAN G130
• Host: Dan Dessau
• Abstract: Modern condensed matter physics research has produced novel materials with fundamental properties that underpin a remarkable number of cutting-edge technologies. Moreover, whoever discovers novel materials generally controls the science and technology that flows from them. It is now generally accepted that novel materials are necessary for critical advances in technologies. Transition metal oxides have recently attracted enormous interest within both the basic and applied science communities.  However, for many decades, the overwhelming balance of effort was focused on the 3d-elements and their compounds; the heavier 4d- and 5d-elements (which constitute two thirds of the d-transition elements listed in the Periodic Table) and their compounds have been largely ignored until recently. We review the unusual interplay between the competing interactions present in the 4d- and 5d-transition metal oxides, and how they offer wide-ranging opportunities for the discovery of new physics and, ultimately, new device paradigms.   

**SPECIAL Colloquium: Monday, February 23— "Many body localization: a new frontier for quantum statistical physics"

• Presenter: Dr. Rahul Nandkishore, Princeton University
• Location: DUAN G130
• Host: Leo Radzihovsky
• Abstract: The existing theory of quantum statistical mechanics describes open systems in contact with large reservoirs. However, experimental advances in the construction and control of isolated quantum systems have highlighted the need for an analogous theory of isolated systems. It has been realized that isolated quantum systems can support behavior which has no analog in open quantum systems. A prominent example is the phenomenon of many body localization.
  Many body localization occurs in isolated quantum systems, usually with strong disorder, and is marked by absence of dissipation, absence of thermal equilibration, a strictly zero DC conductivity (even at energy densities corresponding to high temperatures), and a memory of the initial conditions that survives in local observables for arbitrarily long times. Recently, my co-workers and I have demonstrated that many body localization also opens the door to new states of matter which cannot exist in thermal equilibrium, such as topological order at finite energy density, or broken symmetry states below the equilibrium lower critical dimension. We have also uncovered a host of unexpected properties, such as a set of universal spectral features and a non-local charge response, that have striking implications for fields as diverse as quantum Hall based quantum computation and quantum control. In this talk, I review the essential features of the many body localization phenomenon, and present some of the recent progress that I have made in this field. I also discuss the implications of these results for both theory and experiment.

**SPECIAL Colloquium: Thursday, February 26 — "Age of Oxides: New Physics and Foundations of Modern Technology"

• Presenter: Michael Kolodrubetz, Boston University
• Location: DUAN G130
• Host: Andreas Becker
• Abstract: Michael Fisher once wrote that “the task of theory is to ... understand the universal aspects of the natural world.” This goal has motivated physicists in understanding phenomena from phase transitions to topological physics. In this talk, I will discuss how the quest for universality has progressed in an emerging field of research: the dynamics of coherent quantum systems. Dynamics plays a crucial role in ultracold AMO experiments and has recently become important in solid state systems such as superconducting circuits, where coherence is necessary for protecting and manipulating quantum information. I discuss canonical examples of universality in these systems, such as eigenstate thermalization, many-body localization, and the dynamics near phase transitions. Then I detail recent work where, in collaboration with two experimental groups, we demonstrated that universal topological properties can be measured from the non-equilibrium response of superconducting qubits. With these experiments as a guidepost, I discuss how these robust probes can be extended to larger many-body systems, where they may be a useful tool in probing novel quantum states and performing metrology.

**SPECIAL Colloquium: Monday, March 2 — "From Quantum Communications to Small Quantum Computers"

• Presenter: Graeme Smith, IBM TJ Watson Research Center
• Location: DUAN G130
• Host: Andreas Becker
• Abstract: Physical information carriers obey quantum laws. Taking proper account of this fact has led over the past few decades to profound generalizations of both communication and computation theory. First, I’ll discuss the central question in the theory of quantum communication: What are the capabilities of a noisy quantum communication link? Addressing this question leads us to concepts like entanglement, a fundamentally quantum form of correlations which turns out to be a remarkably useful resource, new capabilities such as unconditionally secure cryptographic key agreement, and classically impossible kinds of synergy between independent communication links. Second, I’ll discuss two key questions in the race to build a quantum computer: what can we do with a small quantum computer, and how can we know that we’ve done it? These will be central questions in the coming decade as the size of quantum computing experiments begins to outstrip our capacity to do effective modeling on classical machines.

**SPECIAL Colloquium: Monday, March 9 — "Spatiotemporal control of the active forces that shape living tissues"

• Presenter: Karen Kasza, Zallen lab, Memorial Sloan Kettering Cancer Center
• Location: DUAN G125
• Host: Matt Glaser
• Abstract: The ability of multicellular tissues to physically change shape, move, and grow is a key feature of life. These processes are often accomplished by local movements or rearrangements of cells within the tissues. Many cell movements are actively driven by contractile forces generated in cells by the motor protein myosin II. During embryonic development, forces are patterned to orient cell movements, resulting in changes in tissue shape that build functional tissues and organs. To uncover how force-generation by myosin drives cell movement and determines the physical behavior of tissues, I use the fruit fly embryo as a model system, where polarized patterns of myosin activity orient cell movements and rapidly elongate the embryo. I will discuss how studying embryos made with engineered myosin variants allows us to dissect mechanisms underlying tissue behavior. In particular, I will describe how myosin variants with enhanced activity accelerate cell movement but, surprisingly, also alter the spatial pattern of forces and result in reduced tissue elongation. These experiments reveal that the levels and patterns of forces are controlled by the same biological cue and suggest a trade-off between the speed and orientation of cell movements within tissues. These studies of how forces shape the fruit fly embryo shed light not only on physical principles at work in active, living materials but also on how defects in cell movements contribute to human birth defects and tumor metastasis.

**SPECIAL Colloquium: Thursday, March 12 — "Revealing hidden symmetry breaking in strongly correlated matter"

• Presenter: Darius Torchinsky, Caltech
• Location: DUAN G130
• Host: Daniel Dessau
• Abstract: Essential to a microscopic understanding of strongly correlated materials is a clear picture of the relationship between their myriad quantum ground states. However, in phenomena ranging from unconventional magnetism to high temperature superconductivity, this picture is often obscured by the presence of broken symmetries hidden from view of existing experimental techniques. This may include hidden structural symmetries or tensor order parameters representing complex spatial arrangements of multipolar electric and magnetic moments. It may even include electronic forms oforder which come in and out of existence on ultrashort timescales, invisible to static probes. I will demonstrate how ultrafast time resolved and nonlinear optical methods can reveal hidden symmetry breaking in some of the most intensely researched strongly correlated materials of the past decade, including high-temperature superconductors, spin-orbit coupled transition metal oxides and heavy fermion materials, and I will discuss how the newly uncovered symmetries play a fundamental role in their physics.

**SPECIAL Colloquium: Thursday, March 19 — "Magnetic Resonance with Single Nuclear-Spin Sensitivity"

• Presenter: Alex Sushkov, Harvard University
• Location: DUAN G130
• Host: Konrad Lehnert
• Abstract: Our method of nanoscale magnetic sensing and imaging makes use of nitrogen-vacancy (NV) color centers a few nanometers below the surface of a diamond crystal. Using individual NV centers, we perform NMR experiments on single protein molecules, labeled with carbon-13 and deuterium isotopes. In order to achieve single nuclear-spin sensitivity, we use isolated electronic-spin quantum bits (qubits), that are present on the diamond surface, as magnetic resonance "reporters". Their quantum state is coherently manipulated and measured optically via a proximal NV center. This system is used for sensing, coherent coupling, and imaging of individual proton spins on the diamond surface with angstrom resolution, under ambient conditions, at room temperature. Our approach may enable magnetic structural imaging of individual complex molecules, and realizes a new platform for probing novel materials, and manipulation of interacting spin systems.

For more information about colloquia this semester, contact: Markus Raschke.

Colloquia are Wednesdays at 4:00 p.m. in DUAN G1B20, unless otherwise noted.

Coffee, tea and cookies will be available before regular colloquia beginning at 3:45 p.m. in DUAN G1B31.

August 27 — "Looking inside quantum materials with the Spallation Neutron Source"

• Presenter: Dmitry Reznik, University of Colorado, Boulder
• Host: Paul Beale
• Abstract: High temperature superconductivity discovered in 1987 precipitated a revolution in solid state physics. All of a sudden physicists were forced to study complicated dirty materials with large unit cells and many interacting degrees of freedom. On a philosophical level, it became apparent that sometimes simple, striking, and very cool phenomena derive from complexity: Emergent phenomena in complex quantum materials became a new frontier in physics. Understanding these phenomena is often beyond traditional theory, although great progress is being made. It requires detailed direct knowledge of the inner workings of materials that can only be obtained through measurements. This necessity stimulated huge investments into new tools to study materials, many of which can rival high energy physics facilities. In fact solid state physicists took advantage of advances made by high energy physicists in accelerator science and built accelerator-based neutron and x-ray sources that can provide an extremely detailed picture of atomic and electronic structure, as well as dynamics of materials. I will describe some theoretical challenges of studying materials with complex interactions and tell about one of the newest large facilities for meeting the challenge from experimental side: the Spallation Neutron Source. It combines the most powerful pulsed neutron beams in the world with as a set of giant "eyes" that look into a wide range of materials by analyzing how neutrons bounce from investigated samples. I will give an example of a specific measurement from my research to illustrate its capabilities.

September 3 — "Light from Dark"

• Presenter: Harry Nelson, University of California, Santa Barbara
• Host: Eric Zimmerman
• Abstract: We know from astrophysical evidence that the vast majority of matter in our Universe neither absorbs nor emits light. Does this `Dark Matter' have any relation to the strong, electromagnetic, or weak interactions that comprise our `Standard Model' of particle physics? I'll discuss the research program that seeks an answer to that question, by the attempt to detect direct interactions of the dark matter in laboratory experiments. I'll focus on the recent results obtained by the LUX experiment that operates a mile underground in an old gold mine in South Dakota. At the heart of the LUX experiment is the potential detection of light induced by the impinging dark matter particles I'll also discuss the recent decisions that have been made concerning future experiments, particularly the LZ experiment, that will follow LUX.

September 10 — "The Birth, Care, and Feeding of Cat States in Circuit QED: Quantum Jumps of Photon Parity"

• Presenter: Rob Schoelkopf, Yale University 
• Host: Jun Ye
• Abstract: Dramatic progress has been made in the last decade and a half towards realizing solid-state systems for quantum information processing with superconducting quantum circuits. Artificial atoms (or qubits) based on Josephson junctions have improved their coherence times more than a million-fold, have been entangled, and used to perform simple quantum algorithms. The next challenge for the field is demonstrating quantum error correction that actually improves the lifetimes, a necessary step for building more complex systems. I will describe recent experiments with superconducting circuits, where we store quantum information in the form of Schrodinger cat states of a microwave cavity, containing up to 100 photons. Using an ancilla qubit, we then monitor the gradual death of these cats, photon by photon, by observing the first jumps of photon number parity. This represents the first continuous observation of a quantum error syndrome, and may enable new approaches to quantum information based on photonic qubits.

September 17 — "High Temperature Superconductivity – Insights from Einstein’s Electrons"

• Presenter: Zhi-Xun Shen, Stanford
• Host: Dan Dessau
• Abstract: It is now over 100 years since superconductivity was discovered and it took 45 years before a theory was formulated by Bardeen-Cooper-Schrieffer (BCS). Once understood, the impact has been felt far behind superconductivity itself. High-temperature superconductivity in cupper oxides, with critical temperature well above what was anticipated by the BCS, was discovered 25 years ago and remains a major unsolved physics problem today.
  
  The challenge of this problem is symbolized by a complex phase diagram consists of intertwined states with extreme properties in addition to unconventional superconductivity. None of them can be described by conventional theory, thus compounding the difficulty to understand high-temperature superconductivity itself as these states are different manifestations of the same underlying physical system, making an integrated understanding a necessity.
  
  Angle-resolved photoemission spectroscopy (ARPES), derived from Einstein’s formulation of photoelectric effect, has emerged as a leading experimental tool to push the frontier of this important field of modern physics. Over the last two decades, the improved resolution and carefully matched experiments have been the keys to turn this technique into a sophisticated many-body physics tool. As a result, ARPES played a critical role in setting the intellectual agenda by testing new ideas and discovering surprises. This talk focuses on the insights we have gained on the rich phase diagram of the copper oxide superconductors – a pre-requisite for a complete understanding of high temperature superconductivity.
  
  References: 
  ZX Shen et al., Phys. Rev. Lett. 70 1553 (1993)
  D.S. Marshall et al., Phys. Rev. Lett. 76, 4841 (1996)
  A.G. Loeser et al., Science, 273, 325 (1996)
  K. Tanaka, Science 314, 1910 (2006)
  W.S. Lee et al., Nature 450, 81 (2007)
  M. Hashimoto et al., Nature Physics 6, 414-418 (2010)
  R. He et al., Science, 331, 1579 (2011)
  I. Vishik., PNAS 109, 18332 (2012)
  M. Hashimoto et al., Nature Physics 10, 483 (2014)
  M. Hashimoto et al., arXiv:1405.5199, to be published in Nature Materials, 2014

September 24 — "The Origin of the Mass of the Visible Universe"

• Presenter: Zoltán Fodor, Eötvös Loránd University
• Host: Anna Hasenfratz
• Abstract: More than 99% of the mass of the visible universe is made up of protons and neutrons. Both particles are much heavier than their quark and gluon constituents, and the Standard Model of particle physics should explain this difference. A full ab initio calculation of these masses is carried out. To that end one uses the basic equations of the strong interactions on the lattice. Even the tiny mass difference between the neutrons and protons can be determined. The results completely agree with experimental observations. The generation of this hadronic mass is related to a transition in the early universe, which is also discussed.

October 1 — "Superconductors: Old and New"

• Presenter: Robert Birgeneau, University of California, Berkeley
• Host: Dmitry Reznik
• Abstract: Solid State Physics is a field which continuously renews itself through the discovery of new materials and new phenomena. This has been particularly true for the subfield of superconductivity. We will review the progress in this field from Kammelingh Onnes's discovery of superconductivity in mercury in 1911 to the Bednorz-Mueller ground breaking discovery of high temperature superconductivity in the lamellar copper oxides in 1986 to recent work on the Fe arsenides and selenides. Research on superconductivity has produced theoretical insights which have implications not only for superconductivity itself but for systems as varied as liquid crystal gels to the fundamental constituents of the universe.

October 8 — "Chasing Away the Noise in Short-Pulse Lasers"

• Presenter: Thomas Schibli, University of Colorado, Boulder
• Host: Paul Beale
• Abstract: Mode-locked lasers combined with phase-locking techniques have revolutionized precision measurements and spectroscopy applications alike. The technique that has driven these magnificent advances over a large range of scientific and engineering disciplines is indeed active feedback control, which rids a laser’s output of most amplitude and phase/timing fluctuations. Ideally, the ultimate noise level one could achieve in such systems would be solely determined by the coherence of an external reference, which is usually an ultra-narrow linewidth continuous-wave laser. In practice, the effectiveness of active stabilization relies on the available in-loop gain and bandwidth, both of which depend on the complex dynamics of pulse evolution, gain-photon coupling, and on the available actuators. While existing mode-locked lasers typically don’t limit the performance of an optical clock, they are typically the limiting link in applications that require good short-term stability. Such applications are currently enthusiastically pursued by many groups in form of timing dissemination at free electron lasers or for generating ultra-low noise microwaves.
  Based on our ongoing efforts in Boulder, I will describe our approach of making a low-noise frequency comb with the goal of reaching quantum limited noise performance from sub-Hz to the Nyquist frequency. I will describe a few basics of the laser dynamics to explain some of the technical and quantum mechanical noise sources in a laser and how they couple to phase noise in a frequency comb. This will lead us to design constraints of the laser and the development of novel actuators that enable very large feedback gains and bandwidths without the typical crosstalk of existing electro-optic modulators.

October 15 — "Unlocking Neutrino Properties with Long-Baseline Beams"

• Presenter: Alysia Marino, University of Colorado, Boulder
• Host: Paul Beale
• Abstract: Over the past 15 years compelling evidence has emerged that neutrinos have non-zero masses and that neutrinos change from one flavor to another. Intense neutrino beams generated by particle accelerators are now being used in order to more precisely probe the spectrum of neutrino masses and mixing.
  This talk will briefly review the experimental evidence and the framework that describes neutrino oscillations. As an example of a man-made neutrino beam, it will focus on the Tokai-to-Kamioka (T2K) experiment, which creates a beam of muon neutrinos at J-PARC on the east coast of Japan. With two neutrino detectors, one located near the origin of the beam, and another located 295 km away, T2K has seen the disappearance of muon neutrinos and the appearance of electron neutrinos. The talk will conclude with a brief discussion of future long-baseline neutrino experiments, especially the efforts in the US to send a high-intensity beam of neutrinos from Fermilab to a former gold mine in South Dakota.

October 22 — "Curing Picophobia: LISA Pathfinder and Gravitational Wave Detection in Space"

• Presenter: James Ira Thorpe, Goddard Space Flight Center, NASA
• Host: Peter Bender
• Abstract: The milliHertz band of the gravitational-wave spectrum offers an opportunity to study a rich variety of astrophysical systems and provide insight on galaxy formation, black hole growth, stellar evolution, gravitation, cosmology, and other areas. Observing in this band will require a large space-borne interferometer such as the Laser Interferometer Space Antenna (LISA). The LISA mission concept employs measurement techniques and technologies that are familiar to the precision measurement physics community but new to space flight. The LISA Pathfinder mission will provide an on-orbit demonstration of several of the key technologies for LISA-like missions. Led by the European Space Agency, with contributions from several European member states and from NASA, LISA Pathfinder expects to launch in 2015. A successful Pathfinder mission will open opportunities not only for gravitational wave detection, but also for other science applications of precision measurement in space. I will provide an overview of the Pathfinder mission, it's current status, and it's role in technology development for space-based gravitational wave observatories.

October 29 — "Seeing With the Nano-Eye: Accessing Structure, Function, and Dynamics of Matter on its Natural Length and Time Scales"

• Presenter: Markus Raschke, University of Colorado, Boulder
• Host: Paul Beale
• Abstract: To understand and ultimately control the properties of functional materials, from molecular soft-matter to quantum materials, requires access to the structure, coupling, and dynamics on the elementary time and length scales that define the microscopic interactions in these materials. To gain the desired nanometer spatial resolution with simultaneous spectroscopic specificity we combine scanning probe microscopy with different optical, including coherent, nonlinear, and ultrafast spectroscopies. The underlying near-field interaction mediated by the atomic-force or scanning tunneling microscope tip provides the desired deep-sub wavelength nano-focusing enabling few-nm spatial resolution. I will introduce our generalization of the approach in terms of near-field impedance matching to a quantum system based on special optical antenna-tip designs. The resulting enhanced and qualitatively new forms of light-matter interaction enable measurements of quantum dynamics in an interacting environment or to image the electromagnetic local density of states of thermal radiation. Other applications include the inter-molecular coupling and dynamics in soft-matter hetero-structures, surface plasmon interferometry as a probe of electronic structure and dynamics in graphene, and quantum phase transitions in correlated electron materials. These examples highlight the general applicability of the new near-field microscopy approach, complementing emergent X-ray and electron imaging towards the ultimate goal of probing matter on its most elementary spatio-temporal level.

November 5 — "Student Understanding at the Upper Division: Thermal Physics and the Related Mathematics"

• Presenter: John Thompson, University of Maine
• Host: Steve Pollock
• Abstract: Recent frontiers in physics education research include systematic investigations in the upper division.  I have been involved in collaborative efforts to conduct research on student learning in thermal and statistical physics.  The focus in thermodynamics has been on student ideas about the First and Second Laws and the associated concepts (e.g., work, heat, entropy); several studies yield additional insights about broader ideas, such as state functions.  Research in statistical physics has focused on the concepts underlying multiplicity and related ideas in probability.  Our research interests have included aspects of more advanced physics thinking, including connections between physics and relevant mathematics concepts in many of these areas, in order to explore the interaction of the mathematics and the physics in student understanding; examples include student interpretation of canonical representations, such as pressure-volume (P-V) diagrams and Taylor series expansions.  We have recently extended our work to investigate student understanding in analogous mechanical and chemical engineering courses.  Results from research are guiding the development of curricular materials designed for the upper division.

**SPECIAL COLLOQUIUM (Note Date and Location) Monday, November 10, Duane G125 — "Non-Dissipative Viscosity, Quantum Hall Effect and Topological Invariants"

• Presenter: Victor Gurarie, University of Colorado, Boulder
• Host: Paul Beale
• Abstract: Topological states of matter are a rapidly developing area of research in condensed matter physics. Beginning from quantum Hall effect in the 80s-90s, and continuing with topological insulators a decade later, this field of study is now full of theoretical models with anticipated exotic behavior which await their experimental discovery and observation. After a brief introduction into this field, I will describe my work to study topological states of matter with a certain class of topological invariants first proposed in the context of topological insulators and now generalized to include potentially all topological states. I will also discuss the proposed relationship between these invariants and the non-dissipative viscosity, a quantity originally known from the study of plasmas in a magnetic field which more recently received attention in the context of quantum Hall effect. 

November 12 — “Mapping and Time-Stamping the Universe with the Pan-STARRS Sky Survey”

• Presenter: Ken Chambers, University of Hawaii
• Host: David Bartlett
• Abstract: The Pan-STARRS Sky Survey is the first substantial synoptic digital sky survey, ranging from a few to thousands of measurements of some 6 billion astronomical objects. I will review the Pan-STARRS System and highlights of the scientific results to date. These include discoveries of Near Earth Objects, Comets, Kuiper Belt Objects, ultracold brown dwarfs; the 3-dimensional distribution of dust in the Milky Way, new features in the stellar and dynamical structure of the Milky Way, new galaxies in the local group, eclispsing binaries in M31 (critical for the distance ladder); ultraluminous supernovae, SnIa supernova and their constraints on dark matter, dark energy and quintessence; black hole tidal disruption events, high redshift quasars, and large scale structure. I will conclude with a brief description of the current Pan-STARRS NEO Survey, its expected data products, and the potential for identifing the source of gravitational wave events detected by LIGO.
  
  The Pan-STARRS1 Surveys have been made possible through contributions of the Institute for Astronomy of the University of Hawaii; the Pan-STARRS Project Office; the Max-Planck Society and its participating institutes: the Max Planck I nstitute for Astronomy, Heidelberg and the Max Planck Institute for Extraterrestrial Physics, Garching; The Johns Hopkins University; Durham University; the University of Edinburgh; Queen's University Belfast; the Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observatory Global Telescope Network Incorporated; the National Central University of Taiwan; the Space Telescope Science Institute; the National Aeronautics and Space Administration under Grant No. NNX08AR22G issued through the Planetary Science Division of the NASA Science Mission Directorate; the National Science Foundation under Grant No. AST-1238877; the University of Maryland; the Eotvos Lorand University; and the Los Alamos National Laboratory.

November 19 — "Restoration of Early Sound Recordings using Optical Metrology and Image Analysis"

• Presenter: Carl Haber, Lawrence Berkeley National Laboratory
• Host: Eric Zimmerman
• Abstract: Sound was first recorded and reproduced by Thomas Edison in 1877. Until about 1950, when magnetic tape use became common, most recordings were made on mechanical media such as wax, foil, shellac, lacquer, and plastic. Some of these older recordings contain material of great historical interest, may be in obsolete formats, and are damaged, decaying, or are now considered too delicate to play.
  Unlike print and latent image scanning, the playback of mechanical sound carriers has been an inherently invasive process. Recently, a series of techniques, based upon non-contact optical metrology and image processing, have been applied to create and analyze high resolution digital surface profiles of these materials. Numerical methods may be used to emulate the stylus motion through such a profile in order to reconstruct the recorded sound. This approach, and current results, including studies of some of the earliest known sound recordings, are the focus of this talk and will be illustrated with sounds and images.
  Additional information can be found at 
• Speaker Bio: Carl Haber is an experimental physicist.  He received his Ph.D. in Physics from Columbia University and is a Senior Scientist in the Physics Division of Lawrence Berkeley National Laboratory at the University of California. His career has focused on the development of instrumentation and methods for detecting and measuring particles created at high energy colliders, including Fermilab in the United States and at CERN near Geneva, Switzerland.  Since 2002 he, and his colleagues, have also been involved in aspects of preservation science, applying methods of precision optical metrology and data analysis to early recorded sound restoration.  He is a 2013 MacArthur Fellow and a Fellow of the American Physical Society and the John Simon Guggenheim Memorial Foundation.

November 26 Fall Break - No Colloquium 

December 3 — "The long and short of optical time keeping"

• Presenter: Scott Diddams, University of Colorado, Boulder
• Host: Markus Raschke
• Abstract: In the past decade, clocks “ticking” at optical frequencies have surpassed in performance their much slower microwave counterparts.  The implication is that time intervals can now be sub-divided to below the femtosecond (10-15 s) that represents a typical optical cycle.   In this new generation of clocks, a laser plays the role of the pendulum, which has its frequency guided on long timescales by an atomic reference transition.  A laser frequency comb functions as the clockwork that accumulates optical cycles in order to ultimately realize a second.
  
  In this talk, I will provide an overview of the science and technology behind optical clocks, with a focus on practical and fundamental limitations to timing precision on long and short timescales.  In particular, I will highlight recent developments that provide a means to synthesize signals from the radio frequencies to millimeter waves with attosecond (10-18 s) timing precision.  Finally, I will attempt to offer some perspective on the challenges and opportunities for optical timekeeping that might lie ahead.  Along these lines, I will describe recent experiments with a new class of low-noise lasers and frequency combs based on monolithic microresonators.  Among their benefits, these chip-scale devices have the potential to significantly reduce the bulk, cost, and complexity of key components needed to move optical timing beyond the research lab.

December 10 — "Swellable Colloidal Particles are Swell"

• Presenter: Arjun Yodh, University of Pennsylvania
• Host: Ivan Smalyukh
• Abstract: I will discuss soft matter experiments from my lab that explore fundamental issues in condensed matter physics. Briefly, soft materials deform easily when pushed, and their behavior is often dominated in surprising ways by entropy. Examples of soft matter include colloidal suspensions, emulsions, oil-water interfaces, polymer & surfactant solutions, liquid crystals, and mixtures thereof. These soft materials find applications in the paint, food science, & cosmetics industries, in practical control of complex fluid rheology & microfluidics, in cell biology, in high-tech problems such as photonics, printing & lithography, biochemical sensing, and in design of composites. Our recent work develops and takes advantage of a novel class of colloidal suspension composed of temperature-sensitive hydrogel particles; the temperature “knob” permits easy control of particle packing and simultaneous viewing by video microscopy [1]. These features, in turn, enable us to use colloidal crystals to answer fundamental questions about “how and where” crystal melting begins and about the mechanisms by which one crystal transforms to another during a solid-solid phase transition [2]. These features have also enabled us to critically explore open questions about disordered solids, e.g., how glasses rearrange internally when responding to mechanical stress [3]. After a broad introduction about soft matter and colloidal suspensions, I will describe these new experiments.

  REFERENCES
  [1]   Yunker, P.J., Chen, K., Gratale, M.D., Lohr, M.A., Still, T., Yodh, A.G., Reports on Progress in Physics 77, 056601 (Epub 2014).
  [2]   A.M. Alsayed, M.F. Islam, J. Zhang, P.J. Collings, A.G. Yodh, Science 309, 1207-1210 (2005); Y. Han, Y. Shokef, A. Alsayed, P. Yunker, T. C. Lubensky, A. G. Yodh,  Nature 456, 898-903 (2008); Peng, Y., Wang, F., Wang, Z., Alsayed, A.M., Zhang, Z., Yodh, A.G., and Han, Y., Nature Materials (Epub 2014).
  [3]   Chen, K., Ellenbroek, W.G., Zhang, Z.X., Chen, D.T.N., Yunker, P.J., Henkes, S., Brito, C., Dauchot, O., van Saarloos, W., Liu, A.J., and Yodh, A.G., Phys. Rev. Lett. 105, 025501 (2010); Chen, K., Manning, M.L., Yunker, P.J., Ellenbroek, W.G., Zhang, Z., Liu, A.J., and Yodh, A.G., Phys. Rev. Lett. 107, 108301 (2011); Still, T., Goodrich. C.P., Chen, K., Yunker, P.J., Schoenholz, S., Liu, A.J., and Yodh, A.G., Phys. Rev. E 89, (2014).

For more information about colloquia this semester, contact: Markus Raschke.

Colloquia are Wednesdays at 4:00 p.m. in DUAN G1B20, unless otherwise noted.

Coffee, tea and cookies will be available before regular colloquia beginning at 3:45 p.m. in DUAN G1B31.

January 15 — "Making a Clock"

• Presenter: Jun Ye, University of Colorado Boulder, JILA
• Host: Murray Holland
• Abstract: The relentless pursuit of spectroscopy resolution has been a key drive for many scientific and technological breakthroughs over the past century, including the invention of laser and the creation of ultracold matter. State-of-the-art lasers now maintain optical phase coherence over many seconds and provide this piercing resolution across the entire visible spectrum. The new capability in control of light has enabled us to create and probe novel quantum matters via manipulation of dilute atomic and molecular gases at ultralow temperatures. For the first time, we control the quantum states of more than 1000 atoms so precisely that we achieve a more accurate and more precise atomic clock than any existing atomic clocks. With the clock accuracy and stability both reaching the 10-18 level, we now realize a single atomic clock with the best performance in both key ingredients necessary for a primary standard. We are also on the verge of integrating novel many-body quantum states into the frontiers of precision metrology, aiming to advance measurement beyond the standard quantum limit. Such advanced clocks will also allow us to test the fundamental laws of nature and find applications among a wide range of technological frontiers.

January 22 — "What to Do Now? How Individual Choices Add Up to Efficient Labor in Honeybee Colonies"

• Presenter: Michael Breed, Department of Ecology and Evolutionary Biology, University of Colorado Boulder
• Host: John Cumalat
• Abstract: Social insects are among the most spectacularly successful animals. There are more individual social insects in terrestrial habitats than all other types of multicellular animals taken together. Key to their success is the ability to cooperate and efficiently perform tasks like nest construction, brood care, and food collection. Division of labor among worker social insects facilitates efficient task performance. Work in honeybee colonies is regulated by complex interactions among genetics, age, physiological state, and spatial location within the nest. Our work has focused on colony defense and on the work of middle-aged bees. Our findings give a window of insight into the how the social organization of honeybees generates efficient solutions for division of labor within colonies.

January 29 — "Solar Neutrinos and the Planets"

• Presenter: Wick Haxton, UC Berkeley
• Host: Paul Romatschke
• Abstract: A problem in the standard solar model has arisen recently -- a disagreement between tests of surface metalicity (photospheric absorption lines) and interior metalicity (helioseismology). The discrepancy has an interesting connection to certain solar neutrino experiments (Borexino and especially SNO+), which may have the “reach” necessary to settle this question by directly measuring the amount of C and N in the Sun’s core. Such a measurement is important, as the discrepancy may be connected to a very interesting stage of solar system formation -- the last few million years of the nebular disk, when the process of planetary formation scrubbed between 50 and 100 earth masses of metal from the remaining nebular gas. The implications range from planet hunting to decoding the Sun’s structure. I will describe recent observations of “solar twins” that have made speculations of a planetary connection particularly interesting.

February 5 — "Mars Climate Change and the MAVEN Mission to Mars"

• Presenter: Bruce Jakosky, LASP, University of Colorado Boulder
• Host: Mihaly Horanyi
• Abstract: The Mars Atmosphere and Volatile Evolution (MAVEN) spacecraft launched on November 18, 2013, and will arrive at Mars and go into orbit on Sept. 22 this year. The goal of the mission is to study the Martian upper atmosphere composition and structure, and to measure the rate of escape of atmospheric gas to space. These observations will allow us to determine how much of the atmosphere has been lost to space throughout Martian history, and to better understand processes that relate to the history of the planet's climate and of its potential habitability by microbes. With the compelling observations of the geology and geochemistry of the surface that show evidence for a change in the Martian climate throughout time, the MAVEN observations will fill a large gap in our understanding of the Martian volatile and climate system. I'll talk about the development of the mission concept and hardware, current status of the spacecraft (as it's operating in space en route to Mars), and the planned operations and anticipated science results.

February 12 — "Pilot-Wave Hydrodynamics

• Presenter: John Bush, MIT
• Host: Leo Radzihovsky
• Abstract: Yves Couder and coworkers have recently discovered that droplets walking on a vibrating fluid bath exhibit several features previously thought to be peculiar to the microscopic, quantum realm. These walking droplets propel themselves by virtue of a resonant interaction with their own wave field. Thus, the system is reminiscent of the double-wave pilot-wave dynamics envisaged by Louis de Broglie. New theoretical developments provide rationale for the complex behavior of the bouncing droplets, and yield a trajectory equation for the walking droplets. Experimental and theoretical results in turn reveal and rationalize the emergence of quantization and wave-like statistics from pilot-wave dynamics in a number of settings.

February 19 — "One, Two, Three, Many: Creating Quantum Systems One Atom at a Time"

• Presenter: Selim Jochim, University of Heidelberg
• Host: Cindy Regal
• Abstract: Experiments with ultracold gases have been extremely successful in studying many body systems, such as Bose Einstein condensates or fermionic superfluids. These are deep in the regime of statistical physics, where adding or removing an individual particle does not matter. For a few-body system this can be dramatically different. This is apparent for example in nuclear physics, where adding a single neutron to a magic nucleus dramatically changes its properties. In our work we deterministically prepare generic model systems containing up to ten ultracold fermionic atoms with tunable short range interaction. In our bottom-up approach, we have started the exploration of such few-body systems with a two-particle system that can be described with an analytic theory. Adding more particles one by one we enter a regime in which an exact theoretical description of the system is exceedingly difficult, until the particle number becomes large enough such that many-body theories provide an adequate approximation.
  Our vision is to use our deterministically prepared tunable few-body systems as microscopic building blocks to prepare model systems that might help to gain insight into complex many-body systems in condensed matter or even QCD.

February 26 — "A Tabletop-scale Probe for TeV Physics: The Electric Dipole Moment of the Electron"

• Presenter: David DeMille, Yale University
• Host: Eric Cornell
• Abstract: Time-reversal (T) symmetry is observed to be broken in K- and B-meson systems, in a manner consistent with the Standard Model (SM) of electroweak interactions. Violation of T-invariance makes it possible for elementary particles such as the electron to have an electric dipole moment (EDM) along their spin axis. Although the SM prediction for the electron EDM is too small to detect, extensions to the SM frequently predict EDMs within a few orders of magnitude of the current limits. I will describe the ACME experiment, which uses methods of atomic and molecular physics to detect the electron's EDM. We recently completed the most sensitive search for this quantity, finding a result consistent with zero but setting a limit an order of magnitude smaller than previous work. Remarkably, the result of this tabletop-scale experiment sets strong constraints on the existence of T-violating phenomena well above the TeV scale being probed at the Large Hadron Collider, and has a substantial impact on theories of physics beyond the Standard Model.

March 5 — "The Double Pulsar, Aurora Borealis and Testing Theories of Gravity"

• Presenter: Maxim Lyutikov, Purdue University
• Host: Dmitri Uzdensky
• Abstract: The Double Pulsar - a system of two neutron stars in which both companions emit pulsed radio signals - is an excellent astrophysical tool to probe theories of gravity, stellar evolution and plasma physics in extreme conditions. Periodic eclipses seen in the system provided the first test of relativistic spin precession in strong gravity regime. A number of methods used in studying the interaction of the Solar wind with planetary magnetospheres can be directly applied to this amazing system.

March 12 — "Helium Rain and The State of Water Ice in Giant Planets Predicted with Ab Initio Simulations"

• Presenter: Burkhard Militzer, University of California, Berkeley
• Host: Shijie Zhong
• Abstract: The Kepler satellite has detected over 3400 extrasolar planet candidates. Many of them are gas and ice giants, and some are even larger than Jupiter. This talk introduces ab initio simulations and discusses the state of matter at high temperature and pressure conditions that prevail in the interiors of giant planets. We describe how data from the Galileo mission to Jupiter has been combined with simulation results to demonstrate that helium rain occurs on this planet. We characterize water ice at megabar pressures and describe the phase transitions that we recently predicted. After formation of a giant planet, its core is exposed to metallic hydrogen at extreme pressure-temperature conditions and different core materials may dissolve. We were able to show that the cores of Jupiter and Saturn have been eroded. We conclude by discussing future directions in the field of ab initio computer simulations with applications in Earth and exoplanet science.

March 19 — "Signal Processing at Light Speed: Ultrashort Optical Pulse Generation with Arbitrary Waveforms

• Presenter: Andrew Weiner, Purdue University
• Host: Markus Raschke
• Abstract: Lasers capable of generating picosecond and femtosecond pulses of light are now firmly established and widely deployed. Going beyond simple pulse generation, the programmable shaping of ultrafast laser fields into arbitrary waveforms has resulted in substantial impact, both enabling new ultrafast science and influencing practical applications in high-speed lightwave signal transmission. The lecture begins with a brief introduction to ultrafast optics and then specifically addresses methods permitting shaping of ultrafast laser fields on time scales too fast for direct electronic control, selectively surveying the field of pulse shaping from its inception more than twenty-five years ago until the present. Several examples illustrating a new area of science — in which researchers worldwide have used shaped laser pulses as tools to manipulate nanoscopic and quantum mechanical processes, including simple photochemical reactions — will be described. The final section of the lecture focuses on recent work from the Weiner laboratory in which pulse shaping is applied to investigate broadband frequency comb spectra generated through nonlinear optics inside chip-scale microresonators and to manipulate the wave packets of correlated photons.

March 26 — Spring Break; No Colloquium

April 2 — "MINOS + MINOS+ + CHIPS : Neutrino Oscillations at Work"

• Presenter: Jennifer Thomas, University College, London
• Host: Stephen Wagner
• Abstract: The MINOS experiment has been the work horse of the Fermilab neutrino program and over the last 7 years has contributed to a number of seminal measurements of neutrino oscillation parameters. MINOS+ is starting now, a new phase of the experiment, to search for any-new-effect-it-can-find in the neutrino oscillation spectrum, but will also continue to contribute to the "standard parameter" measurements worldwide. Finally, a new vision for the Fermilab neutrino program could include further exploitation of this NuMI neutrino beam, the most powerful in the world. CHIPS is one such experiment, planned to measure delta-CP before anyone else does!

April 9 — "The 2013 Chelyabinsk Airburst Event"

• Presenter: Mark Boslough, Sandia National Lab
• Host: John Cumalat
• Abstract: On Feb. 15, 2013, a small asteroid exploded about 40 km to the south of the Russian city of Chelyabinsk. Its proximity to a population center led to many injuries and widespread blast damage, but also yielded a plethora of serendipitous data in the form of video footage from security and dashboard cameras. Combined with seismic, infrasound, and satellite records, this data provides a rich and multi-faceted means to determine the projectile size and entry parameters, and develop a self-consistent model of the airburst.
  The best estimate of the kinetic yield (explosive energy) is 400-500 kilotons, making Chelyabinsk the most powerful such event observed since the 1908 Tunguska explosion (3-5 megatons). Analysis of video combined with subsequent on-site stellar calibrations enable precise estimates of entry velocity (19 km/s), angle (17° elevation) and altitude of peak brightness (29 km). This implies a pre-entry diameter of ~ 20 m and mass of ~12,000 tonnes.
  Hydrodynamic models can now be initialized with extremely accurate energy depositions at correct locations, and results can be compared to observations (such as timing and distribution of blast energy at the surface, and evolution of the trail) to validate the models and better understand the physical phenomena associated with airbursts. According to observation-based size/frequency curves, Chelyabinsk is approximately a once-per-century event Tunguska is about once-per-millennium. These two outliers suggest that the curves underestimate the frequency of large airbursts. Models suggest that they are more damaging than nuclear explosions of the same yield (traditionally used to estimate impact risk). The risk from airbursts is therefore greater than previously thought.
  Sandia is a multiprogram laboratory operated by Sandia Corporation, a Lockheed Martin Company, for the United States Department of Energy under Contract DE-AC04-94AL85000.

For more information about colloquia this semester, contact: Tobin Munsat.

April 16 — "THz Investigations of Exotic Quantum States of Matter"

• Presenter: Peter Armitage, Johns Hopkins University
• Host: Dan Dessau
• Abstract: “The underlying physical laws necessary for the mathematical theory of a large part of physics and the whole of chemistry are ... completely known…” ...or so was claimed in 1929 by P.A.M. Dirac shortly after the Schrodinger equation had been verified for few electron systems like H2 and He. Dirac continued that the difficulty in extending this success to larger systems is “only that the exact application of these laws leads to equations much too complicated to be soluble”. One could not have anticipated in 1929 that it is precisely this complexity and the resultant effects of 1024 particles acting in quantum mechanical unison that gives rise to a host of beautiful and striking phenomena in materials like superconductivity and magnetism. Like waves on the sea, these are collective phenomena with elementary excitations not easily reducible to the properties of the underlying electrons. Almost a century after Dirac, we know better; to paraphrase P. W. Anderson, more really IS different.
  The occurrence of exotic quantum phenomena that emerges on long length scales heightens the need for new experimental tools that probe finite, yet long time scales (compared to bare electronic ones). This talk will review recent advances in the area of THz spectroscopy and its application to exotic quantum states of matter. I will give examples of its use on material systems as diverse as high-temperature cuprate superconductors, 1D quantum spin chains, “heavy-fermion” magnets, and topological insulators. From the observations of quarks and meson-like bound states in spin-chains to Dirac strings and to nematic electronic liquid crystals, these systems are host to phenomena which are found repeated across the diverse length and time scales of physics. A desire to characterize materials in a novel fashion and answer specific scientific questions is driving the THz technology forward, while new technology is changing the kinds of questions we think to ask.

April 23 — "Testing Gravity via Lunar Laser Ranging"

• Presenter: Tom Murphy, UC San Diego
• Host: Tobin Munsat
• Abstract: Forty years ago, Apollo astronauts placed the first of several retroreflector arrays on the moon. Laser range measurements between the earth and the moon have provided some of our best tests to date of general relativity and gravitational phenomenology--including the equivalence principle, the time-rate-of-change of the gravitational constant, the inverse square law, and gravitomagnetism. A new effort called APOLLO (the Apache Point Observatory Lunar Laser-ranging Operation) is now collecting measurements at the unprecidented precision of one millimeter, which will produce order-of-magnitude improvements in a variety of gravitational tests, as well as reveal more detail about the interior structure of the moon. This talk will include an overview of the science, a description of the instrument and its performance, evidence for dust accumulation on the lunar surface, re-discovery of a lost Soviet reflector, and an outlook for advancing the state of gravity tests in the solar system.

April 30 — "Quantum Computing and the Entanglement Frontier"

• Presenter: John Preskill, Caltech
• Host: Victor Gurarie
• Abstract: The quantum laws governing atoms and other tiny objects seem to defy common sense, and information encoded in quantum systems has weird properties that baffle our feeble human minds. John Preskill will explain why he loves quantum entanglement, the elusive feature making quantum information fundamentally different from information in the macroscopic world. By exploiting quantum entanglement, quantum computers should be able to solve otherwise intractable problems, with far-reaching applications to cryptology, materials science, and medicine. Preskill is less weird than a quantum computer, and easier to understand.
  John Preskill is the Richard P. Feynman Professor of Theoretical Physics at Caltech.

Colloquia are Wednesdays at 4:00 p.m. in DUAN G1B20, unless otherwise noted.

Coffee, tea and cookies will be available before regular colloquia beginning at 3:45 p.m. in DUAN G1B31.

August 28 — "Ultracold Polar Molecules"

• Presenter: Deborah Jin, JILA, University of Colorado Boulder
• Host: Murray Holland
• Abstract: Gases of atoms can be cooled to temperatures close to absolute zero, where intriguing quantum behaviors such as Bose-Einstein condensation and superfluidity emerge.  A new direction in experiments is to try to produce an ultracold gas of molecules, rather than atoms.  In particular, polar molecules, which have strong dipole-dipole interactions, are interesting for applications ranging from quantum information to modeling condensed matter physics.  I will describe experiments that produce and explore an ultracold gas of polar molecules. 

September 4 — "Superposition, Entanglement, and Raising Schrödinger’s Cat"

• Presenter: David Wineland, NIST, University of Colorado Boulder
• Host: Jun Ye
• Abstract: Research on precise control of quantum systems occurs in many laboratories throughout the world, for fundamental research, new measurement techniques, and more recently for quantum information processing. I will briefly describe experiments on quantum state manipulation of atomic ions at NIST, which serve as examples of similar work being performed with many other atomic, molecular, optical (AMO) and condensed matter systems across the world. This talk is in part the “story” of my involvement that I presented at the 2012 Nobel Prize ceremonies.

September 11 — "Distributed Information Processing in Materials that Think"

• Presenter: Nikolaus Correll, University of Colorado, Boulder
• Host: Tobin Munsat
• Abstract: Materials that think are enabled by recent advances in smart polymers, desktop manufacturing systems and miniaturization of computers.  These materials tightly integrate sensing, actuation, computation and communication in a periodic, amorphous fashion, which might enable revolutionary new composites with fully programmable capabilities.  The challenges in creating materials that think lie at the intersection of material science engineering and computer science, and - from a CS perspective - require advances in distributed algorithms for signal processing, control and routing of information.  I illustrate these challenges and recent advances by our group using three experimental case studies, all using identical computational infrastructure: (1) a soft robotic skin that can locate and classify textures by locally sampling, processing and classifying vibrations and route relevant information to a CPU using multi-hop networking; (2) A modular building block for creating intelligent walls and facade systems that can recognize complex gestures; and (3) variable-stiffness composites that can assume arbitrary shapes using simple actuation and local feedback control.  Albeit serving different functions at different scales, material and computational properties can be designed using an unified mathematical framework based on abstracting the computer network as continuous amorphous medium.

September 18 — "Quantum Phases of Matter"

• Presenter: Michael Hermele, University of Colorado Boulder
• Host: Paul Beale
• Abstract: Much of condensed matter physics is based on the idea that two different systems can be in the same phase of matter, with its own characteristic properties.  Some phases of matter are intrinsically quantum in nature, including metals, band insulators and fractional quantum Hall liquids.  In recent years, it has become clear that we lack a satisfactory understanding even of the most basic question: "What constitutes a distinct quantum phase of matter?" This colloquium will describe my recent contributions toward an answer.

September 25 — "Topological Soft Matter: From Mathematical Theorems to Self-Assembly"

• Presenter: Ivan Smalyukh, University of Colorado Boulder
• Host: Paul Beale
• Abstract: Topologically nontrivial fields and vortices frequently arise in superstring and quantum field theories, plasmas, optics, elementary particles, cosmology, condensed matter and atomic systems, etc. Their complex structures are expected to follow predictions of topological theorems and mathematical theories, such as the knot theory, but are rarely accessible to direct experimental visualization. On the other hand, soft condensed matter systems, such as colloids and liquid crystals, offer complexity in degrees of freedom and symmetries that allow for probing analogous phenomena on completely different scales, ranging from kinetics of atoms in glasses to cosmic strings in the early Universe. In my lecture, I will show examples of how CU students extend these possibilities by developing soft matter model systems to probe the scale-invariant interplay of topologies of surfaces, fields, and defects. This combination of topology and self-assembly paradigms emerges as an interdisciplinary scientific frontier of topological soft matter, potentially enabling scalable fabrication of composite materials with unusual properties.

October 2 — "Manybodypedia, Cluster-by-Cluster"

• Presenter: Mackillo Kira, Universität Marburg, Germany
• Host: Steven Cundiff
• Abstract: Several seemingly unrelated research efforts are approaching the same central question: how does the quantum physics of interacting many-particle systems control macroscopic phenomena? This would be easy to answer if one only were able to solve the many-body Schrödinger equation exactly, a task that seems unsolvable for decades to come. The number of theoretic approaches and experimental setups are steadily increasing, which also makes the knowledgebase scattered. Therefore, it seems that one should start a thorough learning process, pedia, to combine and develop common many-body knowhow.
  In this talk, I present a tutorial “manybodypedia” by comparing semiconductors, atom condensates, and degenerate Fermi gases. I analyze them with correlated clusters that rigorously identify the elementary particle configurations, quasi-particles, within many-body systems. It turns out that the clusters also reformulate quantum optics in a form suitable for many-body investigations. The clusters can therefore be viewed as the “alphabets” combining the knowhow in quantum-optics and various many-body studies. As a synthesis of these concepts, I introduce the quantum-optical spectroscopy that produces unprecedented access to unexplored many-body physics.

October 9 — "Fierce Competition in a Correlated World"

• Presenter: Kyle McElroy
• Host: Paul Beale
• Abstract: One of the most important states of matter we see around us is the Landau-Fermi liquid. In fact, this is the state of matter electrons have in metals and semiconductors which we rely on so much in our technology. Over the last several decades we have seen that introducing strong correlations and destroying this traditional conducting state leads to new and dramatic macroscopic states of matter. One playground for these new behaviors is doped Mott insulators, where Coulomb repulsion localizes electrons and competes with the itinerancy of introduced carriers. Such competition leads to strong atomic scale variation in the electronic properties which are a key component in understanding the resulting macroscopic phenomena.
  We have developed microscopic techniques for measuring the electronic structure on the atomic scale. With these probes we have investigated several materials in which atomic scale disorder plays a huge role in the electronic structure. I will discuss several examples where these probes have let us uncover the true microscopic nature of these macroscopic phases that are hidden on the bulk scale.

October 16 — "Crystallization Mechanisms in Biominerals"

• Presenter: Pupa Gilbert, University of Wisconsin - Madison
• Host: Meredith Betterton
• Abstract: Biominerals include mollusk shells and the skeletons of algae, sponges, corals, sea urchins and most other animals. The function of biominerals are diverse: mechanical support, attack, defense, grinding, biting, and chewing, gravitational and magnetic field sensing, light focusing, and many others. The exquisite nanostructure of biominerals is directly controlled by the organisms, which have evolved to master the chemico-physical aspects of mineralization. By controlling the inorganic precursor nanoparticle size, packing, and phase transitions, organisms efficiently fill space, produce tough and hard structures, with micro- or macroscopic morphology optimized for their functions. Specifically, this talk will show the complex architecture of mollusk shell nacre, or mother-of-pearl, and how it is formed by animal-controlled self-assembly.

October 23 — "Black Holes–the Harmonic Oscillators of the 21^st Century"

• Presenter: Andrew Strominger, Harvard University
• Host: Oliver DeWolfe
• Abstract: In the twentieth century, many problems across all of physics were solved by perturbative methods which reduced them to harmonic oscillators. Black holes are poised to play a similar role for the problems of  twenty-first century physics. They are at once the  simplest and most complex objects in the physical universe. They are maximally complex in that the number of possible microstates, or entropy, of a black hole is believed to saturate a universal bound. They are maximally simple in that, according to Einstein's theory, they are featureless holes in space characterized only by their mass, charge and angular momentum. This dual relation between simplicity and complexity, as expressed in black holes, has recently been successfully applied to problems in a disparate variety of physical systems. I will give an introduction to the subject intended for a general audience.

October 30 — "Assessing the Cumulative Impact of Humans on the Landscape"

• Presenter: James Syvitski, University of Colorado Boulder
• Host: Tobin Munsat
• Abstract: Humans are changing the Earth’s biophysical system — atmospheric and ocean climatology and chemistry, extent of snow cover, permafrost and sea-ice, glacier, ice-sheet and ocean volume, and indeed the hydrological cycle.  Some changes are truly global, represented by similar temporal trends — atmospheric greenhouse gases, global surface temperatures, nitrogen fluxes to the coastal zone, and species extinctions. 

  Striking is the extent and rate at which humans have modified Earth’s land surface; as just one example, humans are now the largest force in the movement of sediment — greater than ice, wind and water.  The traces of humanity (e.g. petroleum wells, geotechnical boreholes, mining-exploration holes, and deep-water wells) will last millions of years.  Historical deforestation and land clearing have greatly impacted soil erosion, hill slope failure and downstream sedimentation. 

  In this talk, I will discuss how, by any measure, we have entered a new geological era (labeled the Anthropocene), unique to the history of our planet. Some of these changes have crept up on us; others have gone unrecognized until recently. Global sustainability involves facing our risks both global and local and aligning governance with stewardship.

November 6   — "Poincaré's Tangle: The Topology of Chaos in State Space"

• Presenter: Richard Kautz, NIST
• Host: David Bartlett
• Abstract: In 1889, while investigating the notorious problem of three gravitationally attracting bodies, Henri Poincaré discovered a tangled topological structure that we now recognize as the mathematical heart of chaotic motion. Poincaré would later write of his discovery, "One is struck by the complexity of this picture, which I do not even attempt to draw." Using the driven pendulum as an example and graphics as our primary tool, we will explore the topology of state space, where the trajectories of a system are visible as the streamlines of a flow. Saddle orbits within a flow are the keys to its topology and led Poincaré and his successors to understand that the trajectories of seemingly simple systems can be entwined in infinitely complex patterns. In 1960 Steve Smale introduced the horseshoe map as perhaps the simplest example of a state-space tangle and applied symbolic dynamics to demonstrate how chaos can be "as random as a series of coin flips."

November 13 — "Brilliant Blunders: From Darwin to Einstein – Colossal Mistakes by Great Scientists That Changed Our Understanding of Life and the Universe"

• Presenter: Mario Livio, Hubble Space Telescope Science Institute
• Host: Tobin Munsat
• Abstract: Charles Darwin, William Thomson (Lord Kelvin), Linus Pauling, Fred Hoyle, and Albert Einstein were all brilliant scientists. Each made groundbreaking contributions to his field—but each also stumbled badly. For example, Darwin’s theory of natural selection shouldn’t have worked, according to the prevailing beliefs of his time. Not until Gregor Mendel’s work was known would there be a mechanism to explain natural selection. How could Darwin be both wrong and right? Astrophysicist Fred Hoyle dismissed the idea of a “Big Bang” origin to the universe. And Albert Einstein, whose name is synonymous with genius, speculated incorrectly about the forces that hold the universe in equilibrium—and that speculation opened the door to brilliant conceptual leaps. These scientists expanded our knowledge of life on earth, the evolution of the earth itself, and the evolution of the universe, despite and because of their errors. In this talk, I will discuss how the scientific process advances through error. Mistakes are essential to progress.

November 20 — "Mission 'Impossible': Exploring the Properties of Hot QCD Matter"

• Presenter: Berndt Mueller, Duke University
• Host: Paul Romatschke
• Abstract: This lecture presents an overview of the status of the investigation of the properties of the quark-gluon plasma using relativistic heavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC). My lecture will focus on the insights that have been gained to date by the comparison experimental data from both facilities with theoretical calculations, highlight some of the present challenges, and end with an outlook on the experimental investigations that are planned for the next decade.

November 27 — Fall Break; No Colloquium

December 4 — "The National Solar Observatory — The Coming Decade of Discovery"

• Presenter: Mark Rast, LASP, University of Colorado Boulder
• Host: Tobin Munsat
• Abstract: The move of the headquarters of the National Solar Observatory to the Boulder campus is underway, as is the construction of the four meter Advanced Technology Solar Telescope (ATST). This talk will focus on how these events promise, over the coming years, to transform solar physics and how it is taught at the university. The mission of the National Solar Observatory is to advance our knowledge of the Sun in the contexts of both stellar astrophysics and solar influences. The ATST will contribute by providing diffraction limited spectropolarimetric observations of the solar photosphere, chromosphere, and corona, resolving for the first time the fundamental interactions between solar magnetic fields and the dynamic plasma on scales below 0.1 arcsec. The aim is to elucidate the magnetohydrodynamic underpinnings of the solar dynamo as well as the reconfiguration of magnetic fields during flare and coronal mass ejection events. Since research in solar physics requires a knowledge base that is both wide-ranging and carefully focused, it poses challenges to effective graduate education. We will describe these and ongoing efforts to address them through a distributed program.

December 11 — "Science with 800,000 Collaborators: Tales from the Zooniverse"

• Presenter: Chris Lintott, University of Oxford, UK
• Host: Paul Romatschke
• Abstract: The Zooniverse is the world's largest and most successful scientific crowdsourcing platform, engaging more than 800,000 volunteers in tasks including classifying galaxies, discovering planets and mapping star formation in the Milky Way. This talk will present highlights from the last six years, including the serendipitous discovery of galaxy-scale light echoes, and explain how an unusual set of bulgeless spiral galaxies identified by Galaxy Zoo volunteers is informing models of galaxy formation and feedback. The talk will also set out the future for this massively distributed effort in the world of future facilities such as the LSST and SKA.

For more information about colloquia this semester, contact: Tobin Munsat.

Colloquia are Wednesdays at 4:00 p.m. in DUAN G1B20, unless otherwise noted.

Coffee, tea and cookies will be available before regular colloquia beginning at 3:45 p.m. in DUAN G1B31.

January 16 — "Small Modular Nuclear Reactors: A Better Way to Go?"

• Presenter: Jerry Peterson, University of Colorado Boulder
• Abstract: About 13% of global electricity is derived from nuclear fission, and many new plants are under construction around the world. These tend to be very large, producing over a GW of electricity, costing billions, and taking many years to construct, piece-by-piece, on site. Popular opposition has led some nations to plan to discontinue their reliance on such facilities. There is also a large market, in principle, for small nuclear power plants, for remote communities, military bases, mines, and oilfields. Many firms are now developing Small Modular Reactors (SMR), which could be mass produced and shipped as sealed units, with high resistance to meltdown and proliferation concerns. The physics and engineering principles were demonstrated half a century ago, and modern methods indicate a bright future for SMR. This talk will emphasize the context and opportunities, but not the detailed designs, since the market is just opening.

January 23 — "Atoms in Optical Lattices: Cold Atomic Gases as Condensed Matter"

• Presenter: William D. Phillips, Quantum Measurement Division, Physical Measurement Laboratory, National Institute of Standards and Technology
• Host: Jun Ye
• Abstract: An atomic-gas Bose-Einstein Condensate, placed in the periodic light-shift potential of an optical standing wave, exhibits many features that are similar to the familiar problem of electrons moving in the periodic potential of a solid-state crystal lattice. Among the differences are that the atoms in an optical lattice offer possibilities for measurement and control that are not readily available in traditional condensed systems. Experiments that are difficult or impossible with solids are often straightforward with optical lattices, sometimes with surprising results. Among these are single-particle effects like Bloch oscillations and many-body problems like the Hubbard Model and the Mott insulator transition. Cold atomic gases hold the promise of being quantum simulators of calculationally intractable problems of both fundamental and practical interest.

January 30 — "The Physical Genome"

• Presenter: Rob Phillips, Caltech
• Host: Leo Radzihovsky
• Abstract: The ability to read the sequence of genes and genomes has transformed the face of biology. However, as part of the copying, storage, transfer and reading of the genetic information they harbor, genomes are subject to constant physical manipulation. In this talk, I will describe two case studies that illustrate the rich interplay between the information content of genomes and their behavior as physical objects. The first example will focus on how the genomes of viruses are packed within the tiny confines of the viral capsid and how that DNA comes out again once the virus infects a host. The second example will focus on the statistical mechanics of how cells make decisions by manipulating their genomes through protein binding.

February 6 — "Knots in Light and Fluids"

• Presenter: William Irvine, Physics, University of Chicago
• Host: Leo Radzihovsky
• Abstract: To tie a shoelace into a knot is a relatively simple affair. Tying a knot in a field is a different story, because the whole of space must be filled in a way that matches the knot being tied at the core. The possibility of such localized knottedness in a space-filling field has fascinated physicists and mathematicians ever since Kelvin’s 'vortex atom' hypothesis, in which the atoms of the periodic table were hypothesized to correspond to closed vortex loops of different knot types. Perhaps the most intriguing physical manifestation of the interplay between knots and fields is the existence of knotted dynamical excitations. I will discuss some remarkably intricate and stable topological structures that can exist in light fields whose hydrodynamic-like evolution is governed entirely by the geometric structure of the field. I will then turn to experimental hydrodynamics: how to make knotted vortex loop configurations in fluids and how they evolve once made.

February 13 — "Irreversibility and the Second Law of Thermodynamics at the Nanoscale"

• Presenter: Christopher Jarzynski, University of Maryland
• Host: Leo Radzihovsky
• Abstract: What do the laws of thermodynamics look like, when applied to microscopic systems such as optically trapped colloidal particles, single molecules manipulated with laser tweezers, and biomolecular machines? In recent years it has become apparent that small systems far from thermal equilibrium satisfy a number of strong and unexpected laws. In particular, a proper accounting of fluctuations allows us to rewrite familiar inequalities of macroscopic thermodynamics as equalities. I will describe some of this progress, and will argue that it has refined our understanding of the second law and the thermodynamic arrow of time.

February 20 — "A Close-Up of Synthetic Quantum Matter"

• Presenter: Markus Greiner, Harvard University
• Host: Leo Radzihovsky
• Abstract: Ultracold atoms in optical lattices enable experimenters to create and study synthetic quantum matter, opening a window into the fascinating world of many-body quantum physics. With quantum gas microscopy we are now able to take the control of atoms in an optical lattice to the next and ultimate level of high fidelity addressing, manipulation and readout of single particles. I will present microscopic studies of strongly correlated quantum matter and the first realization of quantum magnetism in an optical lattice. This work opens a wide range of new possibilities and brings the realization of exotic states of matter within experimental reach.

February 27 — "Twisting Atomic Gases into Novel Quantum Phases"

• Presenter: Nigel Cooper, Cambridge University
• Host: Victor Gurarie
• Abstract: One of the most important techniques in the ultracold atom toolbox is the optical lattice: a periodic scalar potential formed from standing waves of light. Optical lattices are central to the use of atomic gases to explore strong-correlation phenomena related to condensed matter systems. I shall describe how optical coupling of internal states of an atom can be used to create new forms of optical lattice, which cause neutral atoms to behave as charged particles in a strong effective magnetic field. The resulting Lorentz force will twist the trajectories of atoms into tight circles, completely altering the collective behaviour of the gas. I shall explain the origin of this Lorentz force, and the possibilities these lattices offer to explore novel topological many-body phases using ultracold gases.

March 6 — "Splitting the Higgs Boson: Composite Models at the High-Energy Frontier"

• Presenter: Ethan Neil, Fermilab
• Host: Anna Hasenfratz
• Abstract: The recent discovery of the Higgs boson at the Large Hadron Collider represents a triumph for the Standard Model of particle physics. However, from a theorist's perspective several puzzles remain, among them the stability of the Higgs mass and the identity of dark matter. Models in which the Higgs itself is a composite bound state can provide elegant solutions to these puzzles, but these composite systems are inherently strongly coupled, and as such they can be challenging to investigate. Numerical field theory, which has enjoyed enormous success as a tool for study of the strong nuclear force, can provide new insights and deeper understanding. In this talk, I will briefly discuss the most promising models of Higgs compositeness, and present recent large-scale simulation results targeting such models, with an emphasis on connections to experiment.

March 13 — "Cancer Risks from Low Doses of Radiation, or BEIR and Blight"

• Presenter: Tom Johnson; Radiation Biology, Colorado State University
• Host: Jerry Peterson
• Abstract: Ionizing radiation is unavoidable, and has been a particular issue in Boulder due to the proximity of the Rocky Flats plant. The physics community should be a source of factual information and context for the public. Recently the National Academy of Sciences released the “Biological Effects of Ionizing Radiation Report VII” (BEIR) in 2006. The BEIR report provides a comprehensive examination of the risks of “low” levels of ionizing radiation. The goal of this talk is to provide a brief review of radiation risks within the context of background radiation and occupational radiation exposures. We will examine how the BEIR numerical estimates of cancer risks (blight) have been cited, used to set policy, as well as abused. Additionally, a brief review of background radiation, regulatory limits, and high dose effects will be provided. Next, an overview of the BEIR committee recommendations and the source of the recommendations will be presented. Recommendations on how to best use and abuse the risk estimates will round out the lecture.

March 20 — "Confronting the Dark Energy Crisis in Fundamental Physics"

• Presenter: Christopher Stubbs, Harvard University
• Host: Konrad Lehnert
• Abstract: The discovery of the accelerating expansion of the Universe has precipitated a crisis in fundamental physics that lies along the intellectual fault line between quantum mechanics and gravitation. We have given the name "Dark Energy" to the origin of the gravitational repulsion that is driving this accelerating expansion. Dark Energy evidently comprises about 75% of the cosmic inventory. We are living through an intellectual revolution that has many parallels with the advent of quantum mechanics, but it's unclear when we will achieve a breakthrough in our understanding. I will review the current state of Dark Energy observations, and sketch out some possible scenarios for the future.

March 27 — Spring Break; No Colloquium

April 3 — "How Might a Fermi Surface Die?"

• Presenter: Senthil Todadri, Massachusetts Institute of Technology
• Host: Michael Hermele
• Abstract: In the last many years a number of metallic solids have been studied that defy understanding within the principles of conventional textbook solid state physics. The most famous are the cuprate high temperature superconductors though many other examples have been found. In this talk I will argue that the mysterious properties of many such materials arises from an imminent `death' of their Fermi surfaces. I will discuss some theoretical ideas on how to kill a Fermi surface, and their implications for experiments.

April 10 — "Plasma Nuclear Science: Nuclear Science in Hot, Dense and Dynamic Laboratory Plasmas"

• Presenter: Dennis McNabb, Livermore National Laboratory
• Host: Jerry Peterson
• Abstract: Harnessing the energy of the sun and stars has been a science goal for the past ~60 years. The National Ignition Facility aims to be the first inertial confinement fusion facility to demonstrate a self-sustaining fusion burn, required to provide enough energy gain to make fusion energy feasible. The plasma environments created using laser-driven inertial confinement fusion implosions at the National Ignition Facility [1] and OMEGA Laser Facility [2] closely resemble the burning core of a star where the reactants are ionized and the electrons are in continuum states. The fusion reactions in these plasmas can also lead to an extremely high neutron brightness, ~10 orders of magnitude higher than produced in conventional accelerator and reactor facilities. The brightness is potentially high enough that a significant number of nuclei could sequentially undergo to two nuclear reactions within ~10 ps, providing a capability to study reactions on short-lived excited states. These capabilities offers some intriguing opportunities to improve our understanding of how the elements originated and the stars, stellar objects and stellar events that produce the elements. However, these facilities also present challenges because the plasma environment is a complex and dynamic system that is difficult to model. I will discuss some of these challenges and recent progress that has been made nuclear scientists to disentangle these complexities and exploit them to better understand the reactions that power the stars and create the elements that we observe in our world today.

April 17 — "Slippery Wave Functions"

• Presenter: Leo Kadanoff, The James Franck Institute, University of Chicago
• Host: Leo Radzihovsky
• Abstract: Superfluids and superconductors are ordinary matter that show a very surprising behavior at low temperatures. As its temperature is reduced, materials of both kinds can abruptly fall into a state in which they will support a persistent, essentially immortal, flow of particles. These flows differ from anything in classical physics in that they produce neither friction nor resistance. A major accomplishment of Twentieth Century physics was the development of an understanding of this very surprising behavior via the construction of partially microscopic and partially macroscopic quantum theories of superfluid helium and superconducting metals. Such theories come in two parts: a theory of the motion of particle-like excitations, called quasiparticles, and of the persistent flows themselves via a huge coherent excitation, called a condensate. The quasiparticles are described by giving the quasiparticle energy as a function of the quasiparticle momentum. The condensate is described by giving its quantum wave function.
  
  Two people, above all others, are credited with for the construction of the quasi-particle side of the theories of these very special low-temperature behaviors: Lev Landau and John Bardeen.  Curiously enough, they partially ignored and partially downplayed the importance of the condensate wave function.  In both cases, this partial neglect of the actual superfluid or superconducting flow interfered with their ability to assess the subsidiary advances that occurred immediately after their celebrated work on quasiparticles.
  
  Some speculations are offered about the source of this unevenness in the judgments of these two leading scientists.

April 24 — "Dead Dinosaurs and Nuclear Wars"

• Presenter: Brian Toon, Laboratory for Atmospheric and Space Physics, University of Colorado Boulder
• Host: Paul Beale
• Abstract: Sixty six million years ago a mountain sized chunk of rock, traveling at more than 10 times the muzzle velocity of an assault rifle, slammed into the shallow sea covering what is now the Yucatan Peninsula of Mexico. Shortly thereafter the 5th of the Earth’s great mass extinctions occurred. The energy released by the impact was comparable to having a 1-megaton nuclear explosion spaced every 5 km over the Earth’s surface. Massive tidal waves and earthquakes swept the Gulf of Mexico, and rock ejected from the crater carpeted the land as far away as Wyoming. However, a greater danger lay in the vaporized asteroid and coral from the bottom of the sea, which flew back into space in a massive fireball. After the material in the fireball recondensed into drizzle-sized droplets of rock it reentered the atmosphere over the entire Earth as a massive swarm of shooting stars. The shooting stars heated the upper atmosphere until it glowed like the heating bar in an electric broiler oven. The radiation from this hot gas and rock ignited all the land biomass on Earth, creating firestorms on a global scale, and broiled the dinosaurs alive. Then it became pitch-black, to dark even for cats to see, and much too dark for photosynthesis. Temperatures on the land fell below freezing. In the few years before the Earth recovered many species of plants and animals that were not broiled alive were driven to extinction by the cold weather. The food chain in the ocean collapsed because phytoplankton could no longer metabolize without sunlight, and the fish, ammonites and other larger creatures that depended on plankton starved to death. Many of these same phenomena may occur if there is a nuclear war. Nuclear explosions in cities would cause urban firestorms, killing large numbers of people. For example, 20 million Indians and Pakistanis might die directly from nuclear bomb blasts in a war involving 2/3 of their current arsenals. North Korea could cause as many casualties as the U.S. experienced in World War II, by exploding only 3 small atomic weapons over U.S. cities. The smoke from these firestorms would rise into the upper atmosphere, block sunlight, and cool the planet. The resulting cold temperatures would reduce or eliminate food production. Even a war between smaller powers, such as India and Pakistan, could cool the Earth to lower temperatures than any in thousands of years, damaging agriculture and triggering mass starvation, perhaps killing a billion people. A war between Russia and the US would likely create sub-ice age temperatures and kill the majority of the human population by starvation. Destruction of agriculture, starvation and large losses of life would sweep the planet even in places not attacked. Unfortunately, at present, we are not capable of stopping an asteroid from hitting the planet. It remains to be seen if we can prevent a nuclear conflict.

May 1 — "Theories and Models in Physics and Finance"

• Presenter: Emanuel Derman, Director of Financial Engineering Program, Columbia University
• Host: Markus Raschke
• Abstract: Theories are attempts at an accurate description of some part of the world. Models are metaphors, analogies that explain the parts we don’t understand in terms of the parts we do. Models tell you what something is more or less like; theories try to tell you what something is. Models stand on someone else's feet. Theories stand on their own. Financial models are not theories; they are analogies, idealizations that always sweep dirt under the rug. Good models and good modelers have an obligation to make the the dirt explicit.
  
  Bio: EMANUEL DERMAN is Co-Head of Risk at Prisma Capital Partners and a professor at Columbia University, where he directs their program in financial engineering. His latest book is Models.Behaving.Badly: Why Confusing Illusion with Reality Can Lead to Disasters, On Wall Street and in Life, one of Business Week’s top ten books of 2011. He is also the author of My Life As A Quant, also one of Business Week's top ten of 2004, in which he introduced the quant world to a wide audience. He was born in South Africa but has lived most of his professional life in Manhattan. He obtained a PhD in theoretical physics from Columbia University, thereafter doing research on unified theories of elementary particle interactions. At AT&T Bell Laboratories in the 1980s he developed programming languages for business modeling. From 1985 to 2002 he worked on Wall Street, running quantitative strategies research groups in fixed income, equities and risk management, and was appointed a managing director at Goldman Sachs & Co. in 1997. The financial models he developed there, the Black-Derman-Toy interest rate model and the Derman-Kani local volatility model, have become widely used industry standards. In his 1996 article Model Risk Derman pointed out the dangers that inevitably accompany the use of models, a theme he developed in My Life as a Quant. Among his awards and honors, he was named the SunGard/IAFE Financial Engineer of the Year in 2000. He writes a fortnightly column called Models Behaving Badly for the Frankfurter Allgemeine Zeitung.

For more information about colloquia this semester, contact: Leo Radzihovsky

Colloquia are Wednesdays at 4:00 p.m. in DUAN G1B20, unless otherwise noted.

Coffee, tea and cookies will be available before regular colloquia beginning at 3:45 p.m. in DUAN G1B31.

August 29 — "Ultracold Fermions with Repulsive Interactions and Bragg Scattering in an Optical Lattice

• Presenter: Wolfgang Ketterle, John D. MacArthur Professor of Physics, MIT-Harvard Center for Ultracold Atoms, Massachusetts Institute of Technology
• Host: Jun Ye
• Abstract: For a gas of fermions with repulsive interactions, a transition to a ferromagnetic phase has been predicted – the so-called Stoner model for itinerant ferromagnetism. However, any strong short-range repulsive potential necessarily features a weakly bound paired state. Our experiments show that the ferromagnetic transition does not take place, but is rather preempted by rapid pair formation. Our experimental study can be regarded as a quantum simulation of a Hamiltonian, for which even the qualitative phase diagram cannot be reliably obtained by computational methods. Second, I will show how light scattering can reveal the structure of atoms in optical lattices, analogous to X-ray or neutron scattering off solids.

September 5 — "Deliberate Discovery of Missing Materials and the 'Inverse Problem'"

• Presenter: Alex Zunger
• Host: Dan Dessau
• Abstract: Condensed matter physics and material science have historically often proceeded via trial-and-error or even accidental discoveries of materials with interesting physical properties. One wonders if it makes sense instead to first declare the physical property you really want, then find which structure/material has this property? I will describe recent steps taken in this new field of "Inverse Design " where electronic structure theory is combined with biologically- inspired generic search strategies ("Material Genome Initiative"). Examples for nanostructures, magnetism, semiconductors and spectroscopy will be mentioned. Given that many materials that can be expected to exist, are in fact missing from the compilations of all materials previously made , one wonders if they are missing for a good reason (i. e, they are intrinsically unstable), or did people did not get around to making them yet, but they could have interesting properties? I will describe the way modern "first principles thermodynamics" can address this question of "Missing Materials".

September 12 — "Quantum Electro-Mechanics: A New Quantum Technology"

• Presenter: Konrad Lehnert, JILA, University of Colorado Boulder
• Host: Leo Radzihovsky
• Abstract: That an object can be in two distinct places simultaneously is a consequence of quantum theory and a fact routinely invoked to account for the behavior of electrons and atoms. Nevertheless, these superpositions are in conflict with our everyday experience. What is the largest and most tangible object that can be prepared in such a superposition? This question has motivated researchers to fabricate micron-scale mechanical resonators and coax them towards the regime of quantum behavior. Indeed micro-mechanical devices recently reached the quantum regime. In this talk, I will describe how we use electricity to achieve the exquisite control and measurement of micro-mechanical resonators necessary to reach the quantum regime. Having entered this regime, we are now able to pursue many exciting ideas. We endeavor to use mechanical resonators as long-lived memories for the quantum states of electrical circuits. In addition, we are developing the technology to transfer quantum states between two incompatible systems via a mechanical intermediary. In the future, it may even be possible to test quantum theory itself in an unexplored region of mass and size scales.

September 19 — "Neutrino Mass and Oscillations: An Experimental View"

• Presenter: Eric Zimmerman, University of Colorado Boulder
• Host: John Cumalat
• Abstract: The past fifteen years have seen a revolution in our understanding of the properties of the neutrino. A large set of experiments has observed two oscillation modes, indicating that there are three distinct mass states. In the last year, a third oscillation mode has been discovered, opening up future probes of new phenomena including CP violation. Despite the recent progress, however, much remains unknown about neutrinos and several experimental results remain difficult to reconcile with the simplest models. This talk will give an overview of what is known, what we're learning in this exciting era of measurements, and what we may be able to learn in the next decade and beyond.

September 26 — "Flatland with Cold Atoms"

• Presenter: Jean Dalibard, Laboratoire Kastler Brossel, Département de physique de l'Ecole normale supérieure
• Host: Eric Cornell
• Abstract: In his world-famous novel "Flatland" published in 1884, the English writer Edwin Abbott imagined a social life in a two-dimensional world. With a very original use of geometrical notions, E. Abbott produced a unique satire of his own society. Long after Abbott's visionary allegory, microscopic physics has provided a practical path for the exploration of low-dimensional worlds.
  
  With the realization of quantum wells for example, it has been possible to produce two-dimensional gases of electrons. The properties of these fluids dramatically differ from the standard three-dimensional case, and some of them are still lacking a full understanding.
  During the last decade, a novel environment has been developed for the study of low-dimensional phenomena. It consists of cold atomic gases that are confined in tailor-made electromagnetic traps. With these gases, one hopes to simulate and understand more complex condensed-matter systems. The talk will discuss some aspects of this research, both from an experimental and a theoretical perspective. Connections with other domains of 2D many-body physics, such as the Quantum Hall phenomenon, will also be addressed.

October 3 — "Black Holes as Holograms of Strong Interactions"

• Presenter: Oliver DeWolfe, University of Colorado Boulder
• Host: Tom DeGrand
• Abstract: Physical systems with strong interactions have long frustrated theorists, who are unable to turn to the traditional tools of perturbation theory for a solution. Recently, string theory — a theory designed to unify gravity with quantum mechanics — has produced evidence that black holes living in higher dimensions can provide a "holographic" description of certain strongly coupled phenomena, including systems related to quantum chromodynamics (the theory of the strong nuclear force) and the "strange metals" that include high-temperature superconductors, opening up a new avenue to exploring these fascinating systems. We describe these developments and some hopes for the future.

October 10 — "Bohr and his Atom a Century Later"

• Presenter: Tom DeGrand, University of Colorado Boulder
• Host: Leo Radzihovsky
• Abstract: We are in the centenary year of the birth of what we still recognize as "modern physics," and it's worth spending a sentimental hour recalling how it all started. That's what the talk will be about. It should be no surprise, that what Bohr actually did was a little different than what textbooks say he did. Life is always more complicated than people want to think it is, and almost nobody finds the shortest path the first time out. To prepare yourself for the talk, think of an experiment you could do, with 1912 technology, which would tell you how many electrons hydrogen actually has.

October 17 — "The Higgs Particle, 40 Years of Searches: Is It Over?" **NOTE LOCATION CHANGE - G1B30**

• Presenter: Tiziano Camporesi, CERN
• Host: William T. Ford
• Abstract: On July 4th the whole scientific world was aroused by news from CERN that the search for the Higgs boson might be over. In this colloquium we will introduce the reasons why this particle plays such a fundamental role in what is known as the Standard Model description of the subatomic world. We'll go through the challenges faced by the CMS experiment in its investigation of the various channels used to search for this elusive object, and why we are convinced of having found a new particle. The discovery ends a phase of the search and gives rise to new questions. The most urgent is whether the new particle has all the characteristics expected of the Higgs Boson. We'll conclude with an outline of what is being done to answer this question and what are the next fundamental questions raised by this discovery.

October 24 — "The National Ignition Facility: Pathway to Energy Security and the Physics of the Cosmos"

• Presenter: Ed Moses, Lawrence Livermore National Laboratory
• Host: Margaret Murnane
• Abstract:The National Ignition Facility (NIF), at Lawrence Livermore National Laboratory in Livermore, California, is the world’s most energetic laser system. NIF is capable of producing over 1.8 MJ and 500 TW of ultraviolet light, 100 times more than any other operating laser. Completed in March 2009, it is maturing rapidly and transitioning into the world’s premier high-energy-density science experimental facility, while supporting its strategic security, fundamental science, and energy security missions.
  
  By concentrating intense laser energy into target only millimeters in length, NIF can, for the first time, produce conditions emulating those found in planetary interiors and stellar environments and creating fusion energy to power our future. The extreme conditions of energy density, pressure, and temperature will enable scientists to pursue fundamental science experiments designed to address a range of scientific questions, from observing new states of matter to exploring the origin of ultrahigh-energy cosmic rays. Early experiments have been successfully completed in support of materials equations of state, materials strength, and radiation transport in extreme temperature and pressure conditions.
  
  The National Ignition Campaign, an international effort pursued on the NIF, aims to demonstrate fusion burn and generate more energy output than the laser energy delivered to the target. Achieving this ignition goal will validate the viability of inertial fusion energy (IFE) as a clean source of energy. A laser-based IFE power plant will require advances in high-repetition-rate lasers, large-scale target fabrication, target injection and tracking, and other supporting technologies. These capabilities could lead to an operational prototype IFE power plant in 10 to 15 years. LLNL, in partnership with academia, national laboratories, and industry, is developing a Laser Inertial Fusion Energy (LIFE) baseline design concept and examining technology choices for developing a LIFE prototype power plant.This talk will describe the unprecedented experimental capabilities of the NIF, its role in strategic security and fundamental science, and the pathway to achieving fusion ignition to create a clean and secure energy future.

October 31 — "Exotic Quantum Critical Points: Beyond the Landau Paradigm"

• Presenter: Matthew Hastings, Duke University
• Host: Leo Radzihovsky
• Abstract: The standard paradigm for describing phase transitions is the Landau paradigm of symmetry breaking. For example, the freezing phase transition between water and ice is associated with a breaking of translational symmetry due to the periodic lattice in the solid phase, while in a ferromagnet there is a phase transition associated with breaking of spin rotation symmetry as the local magnetic moments all line up, producing a macroscopic magnetic field. If the phase transition is second order, these symmetry properties determine the properties of the critical point between the two phases. However, it was proposed in the last decade that a very different class of "exotic" critical points may occur in quantum systems. These critical points are associated with the appearance of new particles with exotic properties: for example, particles that have a charge that is a fraction of the charge of an electron. I'll begin by reviewing the Landau symmetry breaking paradigm, and then turn to quantum systems and talk about how fractional charge can occur, before discussing recent numerical work with Melko and Isakov on these exotic points at the end.

November 7— "Five Decades of Lasers, Six Decades of Progress, and a Proposed Space Experiment to Test Einstein’s Assumptions"

• Presenter: John Hall, University of Colorado, Boulder; JILA Fellow
• Host: Jun Ye
• Abstract: Even though this is the 51^st year of the Laser, progress in its control and application in precision measurements is still accelerating. The Optical Frequency Comb technology exploded in 1999-2000 from the synthesis of advances in independent fields of Laser Stabilization, UltraFast Lasers, and NonLinear Optical Fibers, enabling a thousand-fold advance in optical frequency measurement, and searches (in the 16th digit) for time-variation of physical "constants". Current advances in ultra-precise locking are making possible stable optical frequencies defined by length and the speed of light, as well as by locking lasers to the resonant frequency of atoms. These two “clocks” represent our current prototypes of the clocks postulated by Einstein in 1905 in formulating the theory of Special Relativity, which can now be tested into the 18th decimal in a proposed Space-based experiment now being planned by our Space-Time Asymmetry Research collaboration (STAR).

November 14 — "Glacial Earthquakes and Glacier Dynamics in Greenland"

• Presenter: Meredith Nettles, Columbia University
• Host: Michael Ritzwoller
• Abstract: Approximately half of the ice-mass loss currently occurring in Greenland is the result of dynamic processes leading to the export of calf ice from the Greenland Ice Sheet's marine margins. Rapid changes in glacier flow speeds, thinning rates, and terminus positions have been observed during the past decade, but remain poorly understood. Many of Greenland's largest glaciers also produce seismic signals equivalent in amplitude to those from earthquakes of magnitude ~ 5. The earthquakes show a strong seasonal signal, with most events occurring in late summer, as well as a secular variation, with 3-6 times as many earthquakes occurring in each recent year as in the early 1990s. We combine observations of globally recorded seismic signals with local geodetic observations at a large Greenland outlet glacier to obtain insight into the glacial-earthquake source process and the response of outlet glaciers to changing environmental conditions, including ice loss at the terminus and varying surface meltwater input. Systematic analysis of glacial earthquakes across Greenland shows that spatio-temporal trends in earthquake occurrence are closely linked to changes in ice dynamics, and allows us to use the earthquakes as a remote-sensing tool.

November 21— Fall Break; No Colloquium

November 28 — "Atomic and Molecular Processes in Strong Fields at the LCLS X-ray Laser"

• Presenter: Phil Bucksbaum, Marguerite Blake Wilbur Professor in Natural Science; Director, Stanford PULSE Institute SLAC National Accelerator Laboratory, Stanford University
• Host: Markus Raschke
• Abstract: The Stanford Linac Coherent Light Source (LCLS) at the SLAC National Accelerator Laboratory is the world’s first hard x-ray free electron laser. Atomic and molecular physics experiments at LCLS have concentrated on studies of fundamental processes of photoionization and subsequent relaxation. The significant dynamical time scales involved are on the order of a few femtoseconds. Several strong-field and nonlinear effects have now been documented, and I will review the early results as well as the status of ongoing work.

December 5 — "Quantum is Different: Topology and Deconfinement in Solids"

• Presenter: Ashvin Vishwanath, University of California, Berkeley
• Host: Leo Radzihovsky
• Abstract: The idea that states of matter are governed by new emergent laws – ‘More is Different’ – has been the guiding principle in the theory of solids.Largely, this was exemplified by symmetry breaking states such as superfluids and ferromagnets, which are ultimately described by a classical order parameter, despite being composed of quantum mechanical particles. However, experimental results over the last several years have impelled us to look for new forms of emergence that are intrinsically quantum mechanical. I will briefly discuss topological insulators, a newly discovered class of solids with strong spin orbit-interactions where topology forms the basis of a subtle distinction from ordinary insulators. Subsequently, I will talk about strongly interacting systems like frustrated magnets that realize even more exotic phases and how quantum entanglement can serve as a fingerprint of these novel states.

December 12 — "Defects with Character -- Majorana States in Condensed-Matter Systems"

• Presenter: Bert Halperin, Harvard University
• Host: Leo Radzihovsky
• Abstract: Theory predicts the existence of some peculiar phases of quantum condensed matter systems, which have multiple degrees of freedom with very low energy, when localized “defects” are introduced. We shall focus on a class of these phases where each defect has half of a conventional degree of freedom, and the defects may be considered as sites for localized zero-energy states of a “Majorana fermion”. Such defects would also exhibit the intriguing property of “non-Abelian statistics” -- i.e., if various defects can be moved around each other, or if two identical defects can be interchanged, the result is a unitary transformation on the quantum mechanical state that depends on the order in which operations are performed but is insensitive to many other details.
  In my talk, I will try to explain these various concepts and discuss the attempts to realize them in condensed matter systems.

For more information about colloquia this semester, contact: Leo Radzihovsky

Colloquia are Wednesdays at 4:00 p.m. in DUAN G1B20, unless otherwise noted.

Coffee, tea and cookies will be available before regular colloquia beginning at 3:45 p.m. in DUAN G1B31.

For more information about colloquia this semester, contact: Leo Radzihovsky

January 18 — "picoSpin: Miniature NMR Spectrometers for Fun and Profit"

• Presenter: John Price, Physics, University of Colorado Boulder, picoSpin, LLC
• Abstract: Nuclear magnetic resonance spectroscopy has long been the preferred analytical tool of the organic chemist and its role is expanding into biochemistry. Every significant research organization in these fields hosts at least one high-resolution NMR spectrometer. Modern instruments are large and complex, they rely on liquid helium for cooling a superconducting magnet, and they cost from $0.5M to $5M each. Consequently, analytical applications of NMR spectroscopy have generally been confined to research laboratories. picoSpin is a Boulder start-up that is conducting a two-fold experiment: First, can we build a drastically smaller, cheaper, easier to use and less expensive high-resolution NMR spectrometer? Second, with its more limited capabilities, will this instrument find applications and a significant market? The answer to the first question is definitely yes. I will describe a $20k shoe-box sized NMR spectrometer and demonstrate during the talk that it can collect high-resolution proton spectra of organic liquids. The answer to the second question is still unfolding, but possible applications include chemical education, chemical manufacturing, quality control, bench use by research chemists and even field inspection of chemical fluids. I’ll also share some reflections on entrepreneurship from the perspective of an academic physicist.

January 25 — "Magnetic Resonance Physics Through a Cognitive Neuroscientist's Eyes"

• Presenter: Marie Banich, Director of the Institute of Cognitive Science
• Host: Jerry Peterson
• Abstract: Magnetic resonance imaging has transformed the ability of neuroscientists to make linkages between behavior, brain structure, and brain function. In this talk, I will provide a very broad overview of what types of information these techniques can apply (and a rudimentary discussion of how they work) and why they have been so transformative for our field. In addition, I will speak to some limitations of these methods. Finally, a description on the new human magnetic resonance imaging system on the Boulder campus will be provided with an eye for how it might be a resource for the Physics community on campus.

February 1 — "From Viscous Fluids to Fermi Surfaces: The Lore of Anti-De Sitter Black Holes"

• Presenter: Steve Gubser, Princeton
• Host: Oliver DeWolfe
• Abstract: Through the magic of string dualities, black holes in negatively curved spacetimes capture phenomena as diverse as viscous fluid dynamics and Fermi surfaces. I will give an overview of what these black holes look like and how they are used in applications to heavy ion collisions and aspects of condensed matter theory. Time permitting, I will discuss not only viscous fluid dynamics and Fermi surfaces, but also heavy quark energy loss and holographic superconductors.

February 8 — "Particle Physics Today"

• Presenter: Lisa Randall, Harvard
• Host: Oliver DeWolfe
• Abstract: New developments in physics have the potential to radically revise our understanding of the world: its makeup, its evolution, and the fundamental forces that drive its operation. The Large Hadron Collider, which houses a 27 km ring accelerating protons to enormously high energies 100 meters underground, contains the most extensive and elaborate experiments ever built. In this lecture I will explore what theories predict and the nature of the experiments that study these tiny distances.

February 15 — "Magnetic Dynamos in the Lab: Progressing from Liquid Metal to Plasmas

• Presenter: Cary Forest, University of Wisconsin
• Host: Tobin Munsat
• Abstract: Every astrophysical plasma is, as far as we can measure, magnetized and turbulent. How these magnetic fields spontaneously self-generate, through a process called the dynamo, and then act back on their surroundings is a central question in plasma astrophysics. Dynamos occur in plasmas that range from small and dense stellar plasmas to diffuse plasmas in galaxy clusters, and can have impacts that range from making life possible on Earth to controlling accretion onto black holes.
  Laboratory experiments on dynamos have been pursued for the last decade using large volumes of fast flowing, electrically neutral liquid metals to create conditions in which magnetic induction dominates resistive dissipation of electrical currents. In some cases, this creates a self-excited magnetic dynamo. This talk begins with an overview of the basics of dynamos, also demonstrating how lab experiments relate to natural dynamos in the Earth (liquid metal), and the Sun (plasma). As one example of this, our group has recently measured the turbulent electromotive force (through correlated fluctuations of velocity and magnetic field) in a 200 horsepower, 1 meter diameter liquid sodium experiment. We have directly observed how turbulence, on average, enhances the effective resistivity of the liquid metal and more rapidly transports magnetic flux. Recent experiments on a novel plasma device will then be described that establish the feasibility for creating a large, steady-state, fast flowing, weakly magnetized, hot plasma, exhibiting all of the critical parameters for dynamo studies. Remarkably, by changing plasma composition and density, the fluid Reynolds number can be independently controlled. Finally, we describe a much larger device, now under construction, which is projected to extend accessible parameter space to a regime much closer to astrophysical dynamos than liquid metals.

February 22 — "Quantum Networks of Trapped Atomic Ions"

• Presenter: Chris Monroe, Physics, JQI and University of Maryland
• Host: Jun Ye
• Abstract: Trapped atomic ions are standards for quantum information processing, with each atom storing a quantum bit (qubit) of information in appropriate internal electronic levels. All of the fundamental quantum operations have been demonstrated between small numbers of atoms, and the central challenge now is how to scale the system to larger numbers of interacting qubits. The Coulomb interaction between trapped ions allows entangling operations through the collective motion of the ion crystal, which can be excited through the state-dependent optical dipole forces. When such an force is applied globally, an effective spin-spin interaction emerges whose sign and range can be precisely controlled with the laser, and any possible spin correlation function can be measured with standard state-dependent fluorescence techniques. This allows the quantum simulation of interesting spin models that possess nontrivial ground states for the investigation of quantum phase transitions, quantum frustration, and the emergence of spin liquid behavior. Such a quantum network may be limited in size by the stability and coherence of the motion of larger ion crystals, and current efforts are devoted to multiplexing to even bigger systems by shuttling ions through complex ion trap structures or mapping qubits onto photons that can allow the probabilistic entanglement between remotely-located atoms. Work on all of these fronts will be reported, including quantum simulations of magnetism with N=16 atomic qubits as well as progress on operating deterministic gates between atoms separated by macroscopic distances.

February 29 — "How Symmetric is the Electron? Looking for Out-of-roundness of 10^-15 Femtometers"

• Presenter: Eric Cornell, JILA, University of Colorado Boulder
• Host: Leo Radzihovsky
• Abstract: The electron's electric dipole moment (eEDM) will be sensitive to particle physics beyond the standard model. We make use of the extreme electric fields found within a molecular bond to pursue an experiment to set a new limit on eEDM at a level that should severely constrain supersymmetric models.

March 7 — The Geodynamo from a Whole Earth Perspective

• Presenter: Peter Olson, Johns Hopkins University
• Host: Shijie Zhong
• Abstract: The geomagnetic field is generated through self-sustaining dynamo action in the Earth's core. The first part of this talk surveys the main components of the geodynamo, focusing on the roles of convection in the fluid outer core, solidification of the inner core, and heat transfer to the mantle in the dynamo process. The second part of this talk examines a defining property of the geodynamo, the phenomenon of magnetic polarity reversals, which represents the most extreme form of geomagnetic variability. Progress in resolving the history of geomagnetic reversals and the structure of the geomagnetic field during polarity transitions has been matched by recent advances in modeling the reversal process in the core using first-principles numerical dynamos. Numerical dynamos reveal that geomagnetic reversals are sensitive to convection in the core, Earth's rotation, and the interactions between the core and the mantle. Although individual geomagnetic reversals appear to be stochastic, their long-term sequencing includes time periods with frequent reversals alternating with long-lasting stable magnetic polarity superchrons over approximately 200 million year cycles. A proposed explanation for these ultra-low frequency cycles is the modulation of heat transport from the core to the mantle on geologic time scales.

March 14 — "Transport Experiments on Surface States of Topological Insulators"

• Presenter: N.P. Ong, Princeton
• Host: Minhyea Lee
• Abstract: The topological surface states in 3D materials predicted by Kane, Mele and Fu were rapidly detected by surface sensitive spectroscopy (ARPES and STM). Transport experiments to investigate their properties have been more challenging. Following a brief introduction, I will survey experiments in which the surface states were first detected by magnetoresistance oscillations in Bi2Se3 and Bi2Te3. More recently, the hybrid Bi2Te2Se has emerged as the most attractive candidate for transport experiments. I will describe a recent experiment showing that the occupation of the surface Dirac cone can be tuned by liquid gating.

March 21 — "Once Upon a Time in Kamchatka: The Extraordinary Search for Natural Quasicrystals"

• Presenter: Paul Steinhardt, Princeton
• Host: Leo Radzihovsky
• Abstract: A quasicrystal is an exotic state of matter with an atomic structure analogous to a Penrose tiling, exhibiting symmetries forbidden to crystals, such as five-fold symmetry in the plane and icosahedral symmetry in three dimensions. The concept of quasicrystals was first introduced twenty-eight years ago and over a hundred types have been synthesized in the laboratory by now. But could Nature have beaten us to the punch? The campaign to answer that question makes for one of the stranger scientific stories you are ever likely to hear.

March 28 — Spring Break; No Colloquium

April 4 — "Wiring up Quantum Systems: Fun with Artificial Atoms and Microwave Photons "

• Presenter: Steve Girvin, Yale
• Host: Leo Radzihovsky
• Abstract: A revolution is underway in the construction of ‘artificial atoms’ out of superconducting electrical circuits. These macroscopic ‘atoms’ have quantized energy levels and can emit and absorb quanta of light (in this case microwave photons), just like ordinary atoms. Unlike ordinary atoms, the properties of these artificial atoms can be engineered to suit various particular applications, and they can be connected together by wires to form quantum ‘computer chips.’ This so-called ‘circuit QED’ architecture has given us the ability to test quantum mechanics in a new regime using electrical circuits and to construct rudimentary quantum computers which can perform certain tasks that are impossible on ordinary classical computers. [1] ‘Wiring up quantum systems,’ R.J. Schoelkopf and S.M. Girvin, Nature 451, 664 (2008).

April 11 — "A Little Big Bang: Strong Interactions in Ultracold Fermi Gases"

• Presenter: Martin Zwierlein, Physics, MIT
• Host: Leo Radzihovsky
• Abstract: Fermions, particles with half-integer spin such as electrons, protons and neutrons, are the building blocks of matter. When fermions strongly interact, complex phenomena emerge, for example high-temperature superconductivity or superfluidity in neutron stars. Ultracold Fermi gases of atoms are a new type of strongly interacting fermionic matter that can be created and studied in the laboratory with exquisite control. For example, we can study the collision of "spin up" and "spin down" Fermi gases with the strongest interactions allowed by quantum mechanics. In equilibrium, direct absorption images of the trapped atomic gas reveal the entire thermodynamics of the system, including the transition into the superfluid state. Scaled to the density of electrons, superfluidity would occur far above room temperature. We were recently able to follow the evolution of fermion pairing from three to two dimensions, connecting quite directly to models of layered superconductors. Our measurements in and out of equilibrium provide benchmarks for current many-body theories and will help to understand other strongly interacting Fermi systems, such as high-temperature superconductors and neutron stars.

April 18 — "Topological Order and Long Range Quantum Entanglements -- From Origins to New Quantum States of Matter"

• Presenter: Xiao-Gang Wen, Physics, MIT
• Host: Leo Radzihovsky
• Abstract: What is the origin of fractional charges and fractional statistics in FQH states? What is the origin of light? It turns out that long range entanglement is the reason why fractional charges and fractional statistics can appear FQH state. Long range entanglement is also the reason why waves that satisfy Maxwell equation can appear in some qubit (spin) systems. Long range entanglement also lead to a deeper understanding of gapped quantum phases. It allows us to obtain a classification of interacting topological insulators/superconductors, as well as the much more general symmetry protected topological phases, and intrinsic topological phases.

April 25 — "Redefining Snake Oil: Lessons Learned from Pythons that Could Benefit People"

• Presenter: Leslie Leinwand, MCDB, University of Colorado Boulder
• Host: Meredith Betterton
• Abstract: The major research interests of Dr. Leinwand’s laboratory are the biology of inherited diseases of the heart and how gender and diet modify the heart. Recently, her work has focused on the extreme biology of the Burmese python and how this biology might be translated to therapeutics for human heart disease. In the wild pythons do not eat very often but when they do eat, they eat enormous meals that can equal their body mass. To digest such a meal, almost all organs in the body grow very rapidly and then regress in size just as rapidly. Her laboratory’s investigations into the mechanisms responsible for the increase in heart mass in Burmese pythons after a large meal have revealed information that may be applicable to the mammalian heart. They found that heart growth in pythons is characterized by cellular enlargement in the absence of cell proliferation and by activation of beneficial signaling pathways much like the process by which highly conditioned athletes increase the sizes of their hearts. Despite extremely high levels of circulating lipids, which would be toxic to the heart, the post-fed python heart does not accumulate fats. Instead, there is robust activation of pathways of fatty acid transport and oxidation combined with increased expression and activity of a cardioprotective enzyme. They also identified a specific combination of three fatty acids in python plasma that promotes beneficial heart growth when injected into either pythons or mice. The long term goal is to promote heart health using the biology of the python.

May 2 — "The Physical Basis for Biological Morphogenesis"

• Presenter: L Mahadevan, Harvard University
• Host: Leo Radzihovsky
• Abstract: The range of shapes in the plant (and animal) world is "enough to drive even the sanest man mad", wrote Darwin. Motivated by qualitative and quantitative biological observations, I will show that there is a "method in the madness" - using examples of growth and form in cells, tissues and organs such as a freely growing pollen tube, undulating fringes on a leaf or petal, the growth of floral spurs, the looping of the gut and the coiling of tendrils. In each case, we will see how a combination of biological and physical experiments, mathematical models and computations allow us to unravel the quantitative basis for the diversity and complexity of biological form, with tantalizing links to evolutionary canalization, biomimetic technologies, and new aspects of geometry and analysis.

Fall 2011 Colloquium Schedule

Colloquia are Wednesdays at 4:00 p.m. in DUAN G1B20, unless otherwise noted.

Coffee, tea and cookies will be available before regular colloquia beginning at 3:45 p.m. in DUAN G1B31.

August 24 — "First Galaxies, First Stars, and the Reionization Epochs of H and He"

• Presenter: Michael Shull, Astrophysical and Planetary Science, University of Colorado at Boulder
• Host: Paul Beale
• Abstract: The universe is thought to have originated in a hot fireball (Big Bang) whose expansion in the presence of dark matter and baryons led to cooling, formation of large-scale structure, the "Dark Ages", and eventually to the first stars and galaxies. I will describe theoretical studies of the expectations for the earliest light in the universe, and the impact of these first galaxies on the high-redshift intergalactic medium. I will then discuss the remarkable new observations by the Hubble Space Telescope of high-redshift galaxies and quasars that probe the predicted epochs of reionization of hydrogen (at a redshift factor z = 7) and singly-ionized helium (at redshift z = 3). We are now learning that the universe was teeming with star-formation activity during its first 500 Myr of existence.

August 31 — "Nonlinear Waves: What You Wanted to Know but..."

• Presenter: Mark Ablowitz, Applied Math, University of Colorado at Boulder
• Host: Leo Radzihovsky
• Abstract: The study of localized waves has a long history dating back to the discoveries in the 1800s describing water waves in shallow water. In fluid dynamics and nonlinear optics there has been considerable interest in various aspects of localized waves. Perhaps surprisingly, nonlinear waves in water and optics have many similar features. After some introductory and historical remarks, topics that will be briefly discussed include: a novel formulation of classical water waves, some new properties of gravity-capillary waves, ultra-short pulse dynamics in mode-locked lasers and nonlinear waves in `photonic graphene'.

September 7 — "The Biology, Chemistry & Physics of RNA Nano-machines"

• Presenter: Tom Cech, MCDB, University of Colorado at Boulder Director, Colorado Initiative in Molecular Biotechnology Nobel Laureate (Chemistry, 1989)
• Host: Paul Beale
• Abstract: In this colloquium geared for physicists, Prof. Cech will review the fundamental information flow in biology (DNA --> RNA --> Protein) and tell the story of the discovery of catalytic RNA. He will explain how X-ray crystallography has been used to determine atomic structures of catalytic RNA molecules. He will then move to studies of the RNA-protein machine telomerase, which is required to replicate the very ends of chromosomes. Recent work indicates that the RNA once again plays an active role in catalysis, this time helping to move the telomerase template through the active site of the enzyme.

September 14 — "Unfinished Business in Particle Physics"

• Presenter: Jonathan Rosner, Physics, University of Chicago
• Host: Eric Zimmerman
• Abstract: The past fifty years have seen an incredible consolidation of results in particle physics into a unified picture known as the Standard Model. In one view, only one piece of this puzzle remains - the Higgs boson. I will argue that, on the contrary, we are seeing only the tip of a very large iceberg, with many exciting discoveries to come in the coming decades.

September 21 — "The Light, and Sound, Fantastic: Radiation Pressure at the Nanoscale"

• Presenter: Oskar Painter, Applied Physics, California Institute of Technology
• Host: Jun Ye
• Abstract: In the last several years, rapid advances have been made in the field of cavity optomechanics, in which the usually feeble radiation pressure force of light is used to manipulate, and precisely monitor, mechanical motion.Amongst the many new geometries studied, coupled phononic and photonic crystal structures (dubbed optomechanical crystals) provide a means for creating integrated, chip-scale, optomechanical systems. Applications of these new nano-opto-mechanical systems include all-optically tunable photonics, optically powered RF and microwave oscillators, and precision force/acceleration and mass sensing. Additionally there is the potential for these systems to be used in hybrid quantum networks, enabling storage or transfer of quantum information between disparate quantum systems. A prerequisite for such quantum applications is the removal of thermal excitations from the low-frequency mechanical oscillator. In this talk I will describe our recent efforts to optically cool and measure the quantum mechanical ground-state of a GHz oscillator (see figure below), and to demonstrate efficient translation between light and sound quanta.

September 28 — "Tracing Attosecond Dynamics of Electrons in Molecules"

• Presenter: Andreas Becker, Physics, JILA
• Abstract: In the past time-resolved experiments and theoretical analysis explored molecular rotation and vibration as well as chemical reactions on the time scale of atomic motion. Recent advances in laser science led to the development of attosecond laser pulses (1 atomic unit = 24 attoseconds) which can uncover new insights in the inner dynamics of atoms and molecules on the natural time scale of electronic motion. In my talk I will present physical concepts behind attosecond laser technology and my perspective on the current status of attosecond science. I will then discuss ongoing theoretical and experimental efforts to monitor and control the dynamics of an electron in the chemical bond of a molecule, using nature's most simplest molecule as an example.

October 5 — "Flight of the Fruit Fly: Life at Intermediate Reynolds Numbers"

• Presenter: Itai Cohen, Physics, Cornell University
• Host: Leo Radzihovsky
• Abstract: There comes a time in each of our lives where we grab a thick section of the morning paper, roll it up and set off to do battle with one of nature's most accomplished aviators - the fly. If however, instead of swatting we could magnify our view and experience the world in slow motion we would be privy to a world-class ballet full of graceful figure-eight wing strokes, effortless pirouettes, and astonishing acrobatics. After watching such a magnificent display, who among us could destroy this virtuoso? How do flies produce acrobatic maneuvers with such precision? What control mechanisms do they need to maneuver? More abstractly, what problem are they solving as they fly? Despite pioneering studies of flight control in tethered insects, robotic wing experiments, and fluid dynamics simulations that have revealed basic mechanisms for unsteady force generation during steady flight, the answers to these questions remain elusive. In this talk I will discuss our strategy for investigating these unanswered questions. I will begin by describing our automated apparatus for recording the free flight of fruit flies and a new technique called Hull Reconstruction Motion Tracking (HRMT) for backing out the wing and body kinematics. I will then show that these techniques can be used to reveal the underlying mechanisms for flight maneuvers, wing actuation, and flight stability. Finally, I will comment on the implications of these discoveries for investigations aimed at elucidating the evolution of flight.

October 12 — "Controlling Molecular Interactions with Electric Fields"

• Presenter: Heather Lewandowski, Physics, JILA, University of Colorado
• Abstract: The process of breaking one chemical bond and forming another is challenging to understand at a quantum mechanical level. This basic understanding is important to any molecular reaction. However, the complicated quantum nature of these processes is difficult to explore experimentally because full control over all degrees of freedom is required. In particular, controlling interactions between cold molecules using external electric and magnetic fields can elucidate the detailed role of quantum mechanics in molecular collisions.
  We introduce a versatile platform for investigating atom-molecule interactions at temperatures of 100 mK and demonstrate that an electric field can strongly affect cold atom-molecule collisions. These results show that even when only one of the colliding species is polar, electric fields can have a major effect on the collision dynamics at millikelvin temperatures. These experiments represent a launching pad to understanding and controlling cold chemical reactions.

October 13 (NOTE: Thursday colloquium, different location) — "Revisiting Blackbody Radiation at the Nanoscale"

• Presenter: Jean-Jacques Greffet, Laboratoire Charles Fabry, Institut d'Optique, CNRS
• Location: JILA Auditorium
• Host: Markus Raschke
• Abstract: When reducing the size of systems down to the nanoscale, usual macroscopic laws are often no longer valid. In this talk, we will show that blackbody radiation is dramatically modified at the nanoscale. We will show that blackbody radiation can be coherent in the near field. We will also show that the energy flux can be orders of magnitude larger than the standard Stefan-Boltzman σT⁴ law. All these effects cannot be understood in the framework of radiometry. We will briefly discuss how thermal radiation can be described using an electrodynamics framework.

October 19 — "Results from the CMS Experiment at the LHC"

• Presenter: Kevin Stenson, University of Colorado at Boulder
• Abstract: The Large Hadron Collider has been colliding protons at a world-record energy of 7 TeV for 18 months. The data collected by the four LHC experiments has expanded our understanding of the most fundamental forces of nature. Following a description of the accelerator and experiment, I will present a selection of results from the CMS experiment. These results include searches for the Higgs boson and for new physics such as supersymmetry. I will also provide an outlook of the physics still to come from the LHC as the luminosity and energy increase.

October 26 — "What if we don't find the Higgs?"

• Presenter: Adam Martin, Fermilab
• Host: Anna Hasenfratz
• Abstract: With the first full year of LHC running nearly over, the Higgs boson has yet to be seen. While there are a few places left where it can hide, they are getting fewer by the day. In this talk I will review the role the Higgs boson plays in the standard model and discuss how the theory can be adjusted to 'hide' the Higgs. These adjustments range from slight tweaks to the Higgs decay channels to removing the Higgs boson completely!

October 31 (Special Physics Colloquium) — "A Multi-disciplinary Discussion of the CNGS/OPERA Neutrino Speed Anomaly

• Presenters: Alysia Marino, Eric Zimmerman, Neil Ashby, Judah Levine, Physics, University of Colorado Boulder
• Abstract: The apparent greater-than-c speed of neutrinos between the CERN accelerator and LGNS OPERA detector at Gran Sasso in Italy is the subject of enormous interest and attention in the scientific community as well as in the media. This seminar will discuss how the measurements were made, concepts of world-wide clock synchronization using GPS coordinate time, and the details of the time difference measurements made in the experiment.

November 2 — "Three-Dimensional Magnetic Field Line Reconnection"

• Presenter: Walter Gekelman, Physics, UCLA
• Host: Tobin Munsat
• Abstract: Magnetic field line reconnection is a processes in which magnetic field energy is converted to particle energy and heating accompanied by changes in the magnetic field topology. It occurs near the surface of the sun and is thought to be responsible for coronal heating. Images and data from the sun and several laboratory experiments indicate that reconnection is a fully three-dimensional process. Reconnection events often involve complex structures called magnetic flux ropes, which are helical magnetic fields with pitch that varies with radius. We describe experiments in the 17-meter long large plasma device (LAPD) at UCLA in which up to three magnetic flux ropes are generated from adjacent pulsed current channels. The flux ropes exert mutual magnetic forces causing them to twist about each other and merge, while the currents associated with the ropes exhibit dynamic behavior and break up into filaments following the merging process. Volumetric space-time data show multiple reconnection sites with time-dependent locations. We describe the concept of the quasi-separatrix layer, a tool to understand and visualize how magnetic field lines reconnect in 3D, observed for the first time in our laboratory. Its three dimensional development will be shown in movies made from the data. Other phenomena such as merging of current sheets leading to filamentation will also be presented.

November 9 — "Pairing in Unusual Places — Stretching the Realm of Superconductivity"

• Presenter: Randall Hulet, Physics, Rice University
• Host: Leo Radzihovsky
• Abstract: Ultracold atoms are powerful tools for the investigation of complex many-body phenomena. This is partly a consequence of the ability to vary parameters such as interaction strength and dimensionality. I will discuss experiments on the pairing of spin-polarized ⁶Li atoms in both 3D and 1D geometries. Spin-polarization of ultracold atoms is accomplished by creating an imbalanced population of two hyperfine levels, a scenario with direct correspondence to magnetized superconductors, and perhaps with the physics of neutron stars. Spin-polarized ultracold atomic gases are excellent candidates for creating the elusive Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) modulated superfluid state, which is characterized by pairs with non-zero center of mass momentum.

November 16 — "Physics Education Research: A Resource for Educational Transformation at a Critical Time"

• Presenter: Noah Finkelstein, University of Colorado Boulder
• Abstract: After decades of research into student learning, assessments, and curriculum design, physics is considered one of the leading fields engaged in discipline-based educational research (DBER). Simultaneously, unprecedented national attention is now being paid to the outcomes of and needs for DBER. After framing the national scale scene of physics education, and how physics education research (PER) is positioned to contribute to the national dialog, I will review the growth of our own program at CU, and particularly my own research that examines several of the critical scales of focus in physics education. This work develops a new theoretical line of inquiry in PER through experimental work on student reasoning in physics at the level of the individual, the course, and the departmental scales. I will present samples of these scales reviewing: novel work on student use of representation and analogy in physics learning, demonstration of the impacts of teaching interpretive themes on student learning of quantum mechanics in our modern physics courses, and conclude with how subtle faculty choices influence something as canonical as clicker use in our introductory physics sequence.
• View a video of the lecture ( and ).

November 23 — Fall Break; No Colloquium

November 30 — "Einstein's Next Test"

• Presenter: Neil Cornish, Montana State University
• Host: Peter Bender
• Abstract: When Einstein was asked how he would have reacted if Eddington's expedition to measure the bending of light by the Sun had conflicted with the predictions of general relativity, he replied "Then I would feel sorry for the dear Lord. The theory is correct anyway." Over the ensuing century Einstein's theory has survived a wide array of precision experimental tests. In the coming decade the detection of gravitational waves will allow us to test dynamical, strong field gravity for the first time, including such basic predictions that gravitational waves propagate at the speed of light and come in two transverse polarizations, and that black holes have event horizons. In the next five years the advanced LIGO-Virgo detectors will be online, and the galactic scale gravitational wave detector formed by an array of milli-second pulsars should also be sensitive enough to make a detection. Next decade we hope to fly some variant of the space based LISA detector. I will describe how these instruments can be used to perform unique tests of Einstein's theory.

December 7 — "Search for the Chimera" NOTE LOCATION CHANGE: DUAN G1B30

• Presenter: James Randi, James Randi Educational Foundation
• Host: Leo Radzihovsky
• Abstract: James Randi exposes popularly-accepted fakery by discussing with his audience everything from ESP to dowsing, from academic frauds to faith healers. Randi reveals what really took place in the last three decades in the labs of various prestigious "think tanks" that verified a series of simple magicians tricks as genuine miracles, thus launching a world-wide plague of misinformation, the repercussions of which can still be felt today. This unique and provocative lecture is not only educational but also highly entertaining. It attracts persons of all educational and social backgrounds and provides a rational perspective on the seemingly paranormal and otherwise unexplained happenings in our day-to-day life.

For more information about colloquia this semester, contact: Leo Radzihovsky