Physics and Astronomy Colloquium 2018-2019

Fall 2018 and Winter 2019:  Thursdays, 3:30-4:30 pm

* Spring 2019:  Thursdays, 4:00-5:00 pm

1-434 Physics and Astronomy (map)

Reception from 3:15-3:30
(unless otherwise posted)

Fall 2018

Thursday, October 4, 2018, 3:30-4:30 p.m.

The life and death of turbulence

Nigel Goldenfeld

University of Illinois

Turbulence is the last great unsolved problem of classical physics. But there is no consensus on what it would mean to actually solve this problem. In this colloquium, I propose that turbulence is most fruitfully regarded as a problem in non-equilibrium statistical mechanics, and will show that this perspective explains turbulent drag behavior measured over 80 years, and makes predictions that have been experimentally tested in 2D turbulent soap films. I will also explain how this perspective is useful in understanding the laminar-turbulence transition, establishing it as a non-equilibrium phase transition whose critical behavior has been predicted and tested experimentally. This work connects transitional turbulence with statistical mechanics and renormalization group theory, high energy hadron scattering, the statistics of extreme events, and even population biology.

Thursday, October 11, 2018, 3:30-4:30 p.m.

Magnetism, superconductivity and non-trivial topology in quantum materials

Ni Ni

University of California, Los Angeles

New materials are the driving force for technology innovations and our progressive understanding in condensed mater physics. In the last decade, breakthrough has been made on two material systems: high-temperature superconductors and three-dimensional topological materials. For the former, Fe based superconductors were discovered, joining cuprates and becoming the 2nd member in the high temperature superconductor club. For the latter, the discovery of bulk materials with non-trivial topology has led to rich new emergent phenomena, including Fermi arc surface state, chiral pumping effect, colossal photovoltaic effect, etc.

In this talk, I will present the exciting progress along both directions. I will first go through how the chemical doping and external pressure tune the competing structural, magnetic and superconducting orders in the newly discovered 112 Fe pnictide superconductors. Then I will show the discovery of a “hydrogen atom” topological nodal line semimetal where two nontrivial bulk bands touch along a line and no trivial bands exist at the Fermi level.

Thursday, Octoer 18, 2018, 3:30-4:30

Welcome to the Gravitational-wave Revolution

David Reitze

Executive Director

LIGO Laboratory

California Institute of Technology

The gravitational-wave detections by LIGO and Virgo in the past three years have already revealed breakthrough insights into the high energy cosmos.  Among the new knowledge revealed these detections— black holes can form in binary systems, binary black hole mergers seed the formation of more massive black holes, binary neutron star mergers produce gamma ray bursts, the heaviest elements in the periodic table likely come from the collision of two neutron stars, the radii of neutron stars can be constrained by gravitational-wave emissions, and the Hubble constant can be measured using gravitational-wave sources as standard sirens.

Thursday, October 25, 2018, 3:30-4:30

Investigating the Quantum Measurement Process.

Humphrey J. Maris

Brown University

In quantum mechanics the state of a system is described by the wave function. It is remarkable that according to the quantum theory the wave function changes with time in two seemingly distinct ways. There is a change in time which can be calculated from the time-dependent Schrodinger equation, and also the wave function is believed to change discontinuously as a result of measurements. However, despite much effort what constitutes a measurement and how a measurement causes a change in the wave function remains unclear. I will describe  experiments in which a part of the wave function of an electron is trapped in a box with walls sufficiently thick to prevent escape by tunneling.

Thursday, November 1, 2018, 3:30-4:30

Mergers and Disruptions in Extreme Gravitational Potentials

University of California, Los Angeles

Nuclear star clusters around supermassive black holes are likely the most collisional stellar systems in the Universe and are also embedded in extremely deep gravitational potential. Consequently, unique stellar dynamical processes and interactions are expected to take place. For example, collisions and mergers between stars and compact objects are likely to happen in this environment. I will explore these collisions and mergers and their product and will connect between them and some of the observed puzzles in galactic nuclei and in particular our own Galactic Center. Specifically, I will offer possible connections between those merger products and (1) the perplexing population of young stars that are isotropically distributed (S-stars) in a region that is hostile to star formation, (2)  the new class of cold stars in this same region that are two orders of magnitude larger than typical stars (e.g., the “gas-like cloud” G2), (3) stellar black hole-black hole binary mergers and LIGO observations, and (4) supermassive black-hole merger with a stellar-mass compact objects and future LISA observations. Recent developments in our understanding of the underlining physics of three- to few- body dynamics offer the opportunity to address puzzles at these extreme places in our Universe.

Thursday, November 8, 2018, 3:30-4:30

The International Race For A Quantum Computer

Stephanie Simmons

Simon Fraser University

Silicon transistors, the essential building block of most modern electronic devices, cannot shrink much further without being rendered inoperable by quantum mechanics. This classical-quantum threshold in fact presents a tremendous opportunity: if we harness quantum mechanics, rather than attempt to avoid it, we could build a quantum computer. Quantum computers will open up a world of opportunities — they could accomplish certain computational tasks exponentially faster which would otherwise be forever impractical. During this lecture, Dr. Simmons will discuss various quantum computing approaches, including her own all-silicon approach, how quantum technologies will change our lives in a very fundamental way, and provide a snapshot of the accelerating worldwide race to build a prototype.

Thursday, November 15, 2018, 3:30-4:30

Neutrinos and gamma rays as probes of the extreme universe

Marcos Santander

University of Alabama

In 2013 the IceCube neutrino observatory, a cubic-kilometer particle detector deployed deep within the South Pole glacier, announced the first detection of an astrophysical flux of high-energy neutrinos in the TeV-PeV range. This breakthrough discovery has prompted a wide-ranging observational effort aimed at identifying the sources of the neutrino flux by combining IceCube measurements with observations spanning the entire electromagnetic spectrum. Gamma rays in particular provide a powerful tool to search for neutrino source counterparts as both particles are produced in high-energy hadronic interactions. The detection and study of neutrino sources would not only signify the start of a new form of astronomy, but also solve long-standing questions in high-energy astrophysics such as the origin of high-energy cosmic rays. This talk will introduce the IceCube detector, summarize recent results from multi-messenger searches of neutrino sources and present an overview of current and future gamma-ray follow-up observations, especially with the Cherenkov Telescope Array, a ground-based facility for very-high-energy gamma-ray astronomy currently under construction.

Thursday, November 22, 2018

Thanksgiving Holiday

Thursday, November 29, 2018, 3:30-4:30

Physics after the lab and the desk: Your work in PRL

Samindranath Mitra

Physical Review Letters

In a talk structured to encourage interspersed Q and A, I will discuss the dissemination of your physics results that follows the lab, the keyboard, and the desk. You communicate results through posters, talks, and papers in a cascading sequence that entails interacting with journal editors, referees, conference chairs, journalists, department chairs, deans, funding agencies, and others. I will focus on this post-research collaborative process in physics, now in a state of flux in the age of social media and Google Scholar, primarily through the lens that is Physical Review Letters -- which published its first Letter sixty years ago.

Samindranath (Sami) grew up in Kolkata and Delhi, and received his Ph.D. at Indiana University (Bloomington) in 1994 on theoretical aspects of the quantum Hall effect. After working on chemical physics at the Albert Einstein College of Medicine in New York City, he joined Physical Review Letters. Among his other responsibilities are papers on transport properties in semiconductors, 2D materials, and mesoscopic systems.

Thursday, December 6, 2018, 3:30-4:30

Managing the Complexity of Molecules: Letting Matter Compute Itself

Gregory Kovacs, M.D., Ph.D. (EE)

Chief Technology Officer, SRI International

Professor Emeritus, Stanford University

Person-millenia are spent each year seeking useful molecules for medicine, food, agriculture and other uses. Biomolecules, which are comprised of interchangeable building blocks such as amino acids, represent a near infinite number of combinatorial possibilities. As an example, antibodies, which make up the majority of the top-grossing medicines today, are comprised of 1,100 amino acids chosen from the twenty used by living things. The binding part (variable region) that allows the antibody to recognize other molecules, is comprised of 110 to 130 amino acids, giving rise to at least 10143 possible combinations. However, are apparently only about 1080 atoms in the universe, illustrating the intractability of exploring the entire space of possibility. This is just one example of biological complexity…

Machine learning (ML), artificial intelligence (AI), and “big data” are often put forth as the solutions to all problems, particularly by pontificating TED presenters giving talks dripping with hyperbole. Expecting these methods to provide intelligent de novo prediction of molecular structure and function within our lifetimes is utter rubbish. For example, a neural network trained on daily weather patterns in Palo Alto cannot develop an internal model for global weather. In a similar way, finite and reasonable molecular training sets will not magically cause a generalizable model of molecular quantum mechanics to arise within a neural network, no matter how many layers it is endowed with. Regardless of the algorithms chosen, one simply cannot yet ask a computer to “compute” a drug that cures HIV.

With that provocative preface, we turn to the notion of letting matter compute itself. Massive combinatorial libraries can now be intelligently and efficiently created and mined with appropriate molecular readouts (AKA “the question vector”) at ever-increasing throughputs presently surpassing 1012 unique molecules in a few hours. Once “matter-in-the-loop” exploration is embraced, AI, ML and other methods can be brought to bear usefully in closed-loop methods to follow veins of opportunity in molecular spaces. Several examples of mining massive molecular spaces will be presented, including drug discovery and AI-guided continuous-flow chemical synthesis – all real, all working today.

Winter 2019

Thursday, January 10, 2019, 3:30-4:30 p.m.

Strongly Coupled QCD Matter

Barbara Jacak

UC Berkeley and Lawrence Berkeley National Laboratory

Quantum Chromodynamics predicts a transition from normal hadronic matter to a phase where the quarks and gluons are no longer bound together and can move freely. Quark gluon plasma is now produced regularly in collisions of heavy nuclei at very high energy at both the Relativistic Heavy Ion Collider (RHIC) in the U.S. and at the LHC in Europe.

Quark gluon plasma exhibits remarkable properties. Its vanishingly small shear viscosity to entropy density ratio means that it ﬂows essentially without internal friction, making it one of the most “perfect” liquids known. It is also very opaque to transiting particles including heavy charm and bottom quarks. Determining the transport properties of quark gluon plasma is a key goal of current research, and will be described in this presentation. Recent data suggest that even very small colliding systems may produce a droplet of plasma. It remains a mystery how this plasma emerges from cold, dense gluonic matter deep inside nuclei within 1 fm/c. I will discuss how a future electron-ion collider can help address this question.

Thursday, January 17, 2019, 3:30-4:30 p.m.

Machine Learning Data from Electronic Quantum Matter

Eun-Ah Kim

Cornell University

In recent years, enormous data sets have begun to appear in real-space  visualizations (scanning probes) and reciprocal-space visualizations (scattering probes) of electronic quantum matter. Increasing volume and variety of such data present new challenges and opportunities that are ripe for a new approach: machine learning. However, the scientific questions in the field of electronic quantum matter require fundamentally new approaches to data science for two reasons: (1) quantum mechanical imaging of electronic behavior is probabilistic, (2) inference from data should be subject to fundamental laws governing microscopic interactions. In this talk, I will review the aspects of machine learning that are appealing for dealing with quantum complexity and present how we implemented a machine learning approach to analysis of scanning tunneling spectroscopy data.

Thursday, January 24, 2019, 3:30-4:30 p.m.

The conformal bootstrap: magnets, boiling water, and quantum gravity

David Simmons-Duffin

Caltech

Conformal Field Theory (CFT) describes the long-distance limit of quantum and statistical many-body systems. Often, this limit is so complicated that traditional computational tools (like Feynman

diagrams) fail. However, powerful new techniques for understanding CFTs have emerged in the last decade, based on the old idea of the "conformal bootstrap". I will describe how the bootstrap lets us calculate critical exponents in the 3d Ising Model to world-record precision, how it explains striking relations between magnets and boiling water, and how it can be applied to questions across theoretical physics.

Thursday, January 31, 2019, 3:30-4:30 p.m.

Duality and emergent symmetries in two-dimensional electron systems

Michael Mulligan

University of California, Riverside

Duality is the ability to describe the same physics in two (or more) distinct ways. A dual description can sometimes provide physical insight -- not readily obtained within more conventional approaches -- that is needed to yield a solution to a particular problem. In this talk, I'll describe how emergent symmetries and recently discovered dualities help to understand the surprising metallic behavior found in the two-dimensional electron gas when a strong transverse magnetic field is applied. Duality motivates a picture in which a Fermi sea of neutral Dirac fermions emerges from a gas of interacting electrons. I'll conclude by outlining a few outstanding questions and comment on other physical systems, such as superconducting thin films, where duality may prove useful.

Thursday, February 7, 2019, 3:30-4:30 p.m.

Spin And Charge Transport Through Correlated States In 2D Materials and Heterostructures

Jeanie Lau

The Ohio State University

Low dimensional materials constitute an exciting and unusually tunable platform for investigation of correlated states. Here I will present our results on transport measurements of high quality few-layer graphene and black phosphorus devices. In Bernal-stack trilayer grapehne, we observe tunable integer and fractional quantum Hall states, and quantum parity effect at the charge neutrality point. In tetralayer graphene, we have observed a large intrinsic gap at half filling, up to 80 meV, that arises from electronic interactions in rhombohedral stacking, and multiple Lifshitz transitions in Bernal stacking. Lastly, I will discuss our recent observation of robust long distance spin transport through the antiferromagnetic state in graphene.

Thursday, February 14, 2019, 3:30-14:30 p.m.

States of Space - Searching for a Quantum Spin Liquid

Arthur Ramirez

University of California, Santa Cruz

Two-dimensional quantum spin liquids have been pursued since the 1980s.  They are now thought to be potential hosts for non-Abelian excitations that could form the basis for topological quantum computation.  The materials requirements for such systems rely heavily on discovering materials with favorable spatial attributes for quantum entanglement.  We will review the canonical approaches for creating quantum spin liquids and present a new approach in copper "elpasolite".

Thursday, February 21, 2019, 3:30-4:30 p.m.

Cold and ultracold molecules for quantum information and particle physics

John Doyle

Harvard University

Wide-ranging scientific applications have created growing interest in ultracold molecules.  Heteronuclear bialkali molecules, assembled from ultracold atoms, enabled the study of long-range dipolar interactions and quantum-state-controlled chemistry, and recently have been brought to quantum degeneracy.  Molecules also have a special place in pushing forward precision searches for new particle physics beyond the Standard Model (BSM).  For example, current searches for the electron electric dipole moment probe T-violating physics due to particles with masses up to 30 TeV, which is a mass scale already well beyond the reach of particle colliders.  Polyatomic molecules may be powerful tools in both of the above-mentioned areas of research, quantum information and particle physics. We will describe in this talk our recent results on optical trapping and Λ-enhanced laser cooling and imaging of CaF molecules, the laser cooling of SrOH molecules, and the prospects for laser cooling of larger polyatomic molecules, such as CaOCH3, YbOCH3, CaCCCa, SrOCF2CF2OCa, and CaO(CH2)nOR, where R is a radical termination ligand.  We also will present re-cent progress in the field of electric dipole moment searches using heavy diatomic molecules and future prospects, including the use of polyatomic molecules.

Thursday, February 28, 2019, 3:30-4:30 p.m.

Quantum computation for chemistry and materials

Jarrod McClean

Quantum computers promise to dramatically advance our understanding of new materials and novel chemistry. Recent advances in the technologies related to quantum computing hardware suggest that devices capable of so-called "quantum supremacy" may be available in the next few years.  In this talk I will focus on the application of quantum computers to hard problems in the application area of chemistry and materials, and discuss the challenges and opportunities related to current algorithms. I will begin with an introduction to quantum computation appropriate for individuals with broad backgrounds in physics.  I will then describe one particular method of interest to overcome quantum resource requirements, the variational quantum eigensolver (VQE).  This hybrid quantum-classical method provides a way of solving eigenvalue problems and more generic optimizations on a quantum device leveraging classical resources to minimize coherence time requirements.  I will briefly review the original VQE approach and introduce some extensions requiring no additional coherence time to approximate excited states.  I will close with an outlook on the prospects for near-term experiments and time permitting some connections to quantum neural networks.

Thursday, March 7, 2019, 3:30-4:30 p.m.

Intricacies of the dark universe

Kallia Petraki

NIKHEF, Amsterdam and LPTHE, Paris

Most of the matter in our universe is in the form of some yet unknown particles, known as dark matter. Besides providing firm evidence for the existence of unknown fundamental physics, dark matter is essential in understanding how our universe evolved to be the way we observe it today. The research of the past decades led to the development of two canonical paradigms for the properties of dark matter: the collisionless cold dark matter paradigm, supported by the observed gravitational clustering, and the WIMP paradigm, which provides a well-motivated particle physics framework for collisionless cold dark matter. However, current observational and experimental results motivate looking beyond these scenarios. I will discuss some of this motivation, and new directions and challenges in exploring the phenomenology of dark matter with different particle physics interactions than in the canonical scenario.

Thursday, March 14, 2019, 3:30-4:30 p.m.

Controlling magnetism in a Mott insulator by optical pumping

David Hsieh

California Institute of Technology

Controlling antiferromagnetic (AFM) order with ultrashort optical pulses can lead to both advances in our fundamental understanding of out-of-equilibrium quantum many-body physics as well as to novel AFM-based high-speed and non-volatile information storage and processing technologies. However, optically manipulating AFM order and detecting its out-of-equilibrium behavior 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, and how we have deployed this technique to reveal unusual out-of-equilibrium critical behaviors of AFM order in an optically pumped Mott insulator.

Spring 2019

Thursday, April 4, 2019, 4:00-5:00 p.m

CANCELLED:  There is no Physics and Astronomy Colloquium scheduled for Thursday, April 4, 2019.

Thursday, April 11, 2019, 4:00-5:00 p.m.

SAXON LECTURE

Quantum chaos and black holes
Alexei Kitaev

Ronald and Maxine Linde Professor of Theoretical Physics and Mathematics

Caltech

A prime feature of chaotic dynamics is extreme sensitivity to initial conditions, known as the "butterfly effect". While intuitive and amenable to mathematical study and simulation, this property is hard to test experimentally. Gaining access to the event horizon of a black hole is equally hard: both tasks require precise manipulation of the whole system. I will discuss the quantum butterfly effect and high-energy interactions at the black hole horizon from a purely theoretical point of view. Assuming very little about the system, the relevant information is contained in out-of-time-order correlators (OTOCs). In many cases, the OTOCs are initially small but grow exponentially until saturating at later times. The growth exponent is bounded by 2\pi times the temperature; the latter value is achieved by black holes and some conventional many-body systems, e.g. the Sachdev-Ye-Kitaev model. This similarity is explained by the SYK model having a certain collective mode that is analogous to t'Hooft's "shock waves" at the black hole horizon.

Thursday, April 4, 2019, 4:00-5:00 p.m

CANCELLED:  There is no Physics and Astronomy Colloquium scheduled for Thursday, April 18, 2019.

Thursday, April 25, 2019, 4:00-5:00 p.m.

Nonequilibrium physics and information processing in living systems

Yuhai Tu

IBM T. J. Watson Research Center

Living organisms need to obtain and process information accurately, which is crucial for their survival. Information processing in living systems, ranging from signal transduction in a single cell to image processing in the human brain, are performed by biological circuits (networks), which are driven out of equilibrium. These biochemical and neural circuits are inherently noisy. However, certain accuracy is required to carry out proper biological functions. How do biological networks process information with noisy components? What is the free energy cost of accurate biological computing? Is there a fundamental limit for its performance of the biological functions? What is the optimal design for achieving these information processing tasks? In this talk, we will describe our recent work in trying to address these general questions in the context of two basic cellular computing tasks: sensory adaptation for memory encoding [1,2]; biochemical oscillation for accurate timekeeping [3,4].

[1] “The energy-speed-accuracy trade-off in sensory adaptation”, G. Lan, P. Sartori, S. Neumann, V. Sourjik, and Yuhai Tu, Nature Physics 8, 422-428, 2012.

[2] “Free energy cost of reducing noise while maintaining a high sensitivity”, Pablo Sartori and Yuhai Tu, Phys. Rev. Lett. 2015. 115: 118102.

[3] “The free-energy cost of accurate biochemical oscillations”, Y. Cao, H. Wang, Q. Ouyang, and Yuhai Tu, Nature Physics 11, 772, 2015.

[4] “Design principles for enhancing phase sensitivity and suppressing phase fluctuations simultaneously in biochemical oscillatory systems”, C. Fei, Y. Cao, Q. Ouyang, and Yuhai Tu, Nature Communications, 2018.

Thursday, May 2, 2019, 4:00-5:00 p.m.

Nonlinear Quantum Dynamics in a Disordered Magnet

Thomas F. Rosenbaum

Caltech

Quantum memories depend on encodable, yet minimally-interacting, degrees of freedom. We study a model quantum magnet, Li(Ho,Y)F4, where Ho dipoles in the dilute limit form clusters of several hundred spins that bind together and can be excited resonantly. By analogy to laser excitation of atoms, we use a pump-probe magnetic technique to drive the system out of the linear regime, and study both the nature of the excitations and the coupling of the excitations to the spin bath. By applying a magnetic field transverse to the Ising axis, we are able to tune the dynamics of the quantum degrees of freedom such that localized clusters (the “qubits”) essentially decouple from their environment. In the second part of the talk, I will discuss the case of increased dipole concentration where the ground state of the system is an ordered ferromagnet. In this limit, we drive the system hard and study the magnetic domain dynamics through Barkhausen noise. We characterize the statistics of the magnetic avalanches and reveal the effects of quantum fluctuations on the non-equilibrium response. Together, these experiments demonstrate the new states and new insights that can be derived by driving a quantum system away from equilibrium.

Thursday, May 16, 2019, 4:00-5:00 pm

Quantum Diagnostics: from single cells to single molecules

Prof. Dino Di Carlo

Microfluidic Biotechnology Laboratory

University of California, Los Angeles

The ultimate limits of sensitivity in measuring biological systems occurs at the level of single cells and single molecules. My group leverages microfluidic, micro & nanofabrication technologies to interface at the scale of these single entities. In particular, we make use of the ability to compartmentalize fluid volumes to a sub-nanoliter scale and manipulate cells tens of micrometers in diameter using unique microscale physics. I will discuss progress in using these quantized measurements to develop health monitoring systems that can be cost-effective, scalable, widely distributed, all at the fundamental limits of biology.

Thursday, May 23, 2019, 4:00-5:00 PM

The Hunt for Cosmic Strings and Dark Matter in Quantum Materials

Materials Science Division and Molecular Foundry, Berkeley Lab

Leaping from astronomical scales to the nanoscale might seem a gargantuan task. Common to both, however, is the concept of symmetry breaking and the formation of so-called ‘topological defects’. In the first part of this talk, I will discuss the formation of topological defects in multiferroic materials show how they can be used to study an early-universe theory, the Kibble-Zurek mechanism, in these functional materials. In the second part, I will talk about how new phenomena and materials are key for the search for the elusive dark matter, and discuss new routes to detecting ultra-light dark matter using quantum-mechanical calculations.

Thursday, May 30, 2019, 4:00-5:00 p.m.

Chirality, Vorticity and Magnetic Field in a Subatomic Quantum Fluid

Jinfeng Liao

Indiana University

By colliding heavy ions at high energies, physicists are able to break up''  nuclear particles like protons and neutrons and create a hot subatomic soup'' — a new form of matter called a quark-gluon plasma (QGP). The QGP forms at a temperature of about one trillion degrees or higher, and briefly occupied the baby Universe. Such primordial environment is now replicated in laboratory at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC). Experiments at RHIC and the LHC have opened the door to what I’d call the Quantum-Chromo-Material Science, by revealing and characterizing the fascinating many-body phenomena and properties of strongly interacting matter. Over the past decade, the collider-born QGP is found to be a nearly perfect quantum fluid, sharing many novel behaviors with other strongly correlated quantum systems in astro-, atomic and condensed matter physics. In this talk, I will discuss two recent fascinating examples, arising from the nontrivial interplay of the extreme vorticity and magnetic fields with the spin and chirality of the underlying microscopic particles. The first is the global polarization of particle spin from fluid rotation, demonstrating “fluid spintronics” on the subatomic scale. The second is the anomalous transport phenomenon known as the Chiral Magnetic Effect (CME) that has been enthusiastically studied not only in the subatomic swirls’’ but also in Dirac and Well semimetals as well as in atomic, astrophysical and cosmological systems.

Thursday, June 6, 2019, 4:00-5:00 p.m.

TBA

Past Physics and Astronomy Colloquia