Past Colloquia: 2019-2020

Spring 2020

Recent developments in non-perturbative quantum gravity

January 28, 2020

Kristan Jensen, San Francisco State University

Host: Matthew Headrick

Abstract: A theory of “Quantum gravity” refers to a unification of quantum mechanics with general relativity. We have good reason to believe that our world is described by just such a theory, although known models (meaning string theories) predict new physics at extremely high energy, well outside of the reach of experiments for the foreseeable future. That being said, from a purely theoretical perspective, known models of quantum gravity are still incredibly rich and complicated, and much of what is known comes from low orders in perturbation theory. In this colloquium I will discuss recent theoretical progress in simple, “stripped-down” models of quantum gravity in two and three spacetime dimensions. These models are much simpler than gravity in our four-dimensional universe, but from the point of view of a theorist they have the appealing feature of being calculable, sometimes in a perturbative expansion and sometimes exactly as a function of the gravitational interaction. I will summarize some known facts about these models, as well as what they are teaching us about black holes and quantum cosmology. Along the way I will also point out connections with the Sachdev-Ye-Kitaev models and random matrix theory.

Journey to the Limits of Quantum Field Theory

January 30, 2020

Grant Remmen, University of California Berkeley

Host: Matthew Headrick

NOTE: This colloquium will start at 4:15 p.m.

Abstract: Hints of new physics are encoded as higher-dimension operators in effective field theory. The couplings of these operators, "dial settings" in the low-energy laws of physics, can be bounded using methods independent of the ultraviolet completion, relying only on principles like causality, unitarity of quantum mechanics, and properties of scattering amplitudes. In this talk, I will show how such infrared consistency techniques can be used to constrain a wide variety of quantum field theories, from the effective field theory of the Standard Model to quantum corrections of Einstein gravity. These constraints have important implications, in contexts ranging from near-term experimental observations to the long-term behavior of black holes, and provide us with powerful means to test the tenets of quantum field theory itself.

Entangled Butterflies: Chaos in Complex Quantum Systems

February 10, 2020

Brian Swingle, University of Maryland

Host: Matthew Headrick

NOTE: This colloquium will start at 4:30 p.m.

Abstract: I will describe recent progress in the study of chaos in complex and highly entangled quantum systems. Building on the rich history of quantum chaos, this recent activity is driven by new experimental possibilities and by a remarkable convergence of ideas in many-body physics, quantum information, and gravity. The focus will be on the quantum butterfly effect and its relation to the spreading of entanglement and to emergent speed limits. I will also describe how this physics is being probed in experiments, with a particular focus on nuclear magnetic resonance experiments.

Thermalization through the lens of entanglement entropy

February 11, 2020

Mark Mezei, Simons Center for Geometry and Physics

Host: Matthew Headrick

Abstract: Thermalization in an isolated system is a foundational problem in statistical physics. The time evolution of entanglement entropy provides insight into this process. Most conventional methods are not applicable to this problem, however gauge/gravity duality allows for progress by mapping special out-of-equilibrium quantum systems to collapsing black holes in general relativity. In this talk, I derive an effective theory for entanglement dynamics from gauge/gravity duality that is akin to hydrodynamics, and present strong evidence that it is universally valid in chaotic systems. I discuss the interplay between this effective theory and hydrodynamics and chaotic operator growth. I conclude with some outstanding problems this new theory may help to solve.

Anomalies: From Magnets to the Nuclear Force

February 13, 2020

Shu-Heng Shao, Institute for Advanced Study

Host: Matthew Headrick

Abstract: Anomaly matching is one of the few universally applicable tools to constrain the dynamics of strongly coupled quantum systems. I will give an overview of 't Hooft anomalies and their applications. Anomalies arise in a wide range of physical systems, ranging from pedagogical quantum mechanical models to Heisenberg spin chains to quantum chromodynamics. I will then discuss implications of anomalies in gapless phases described by conformal field theory. Using the conformal bootstrap techniques, I will prove rigorous constraints on the spectrum from anomalies.

Fall 2019

September 10, 2019

Alexander Hensley, "Measuring crystal nucleation and growth of DNA-grafted colloidal particles"

Abstract: Crystallization is a phase transition in which a fluid of particles forms a solid lattice. In nature it produces diamonds and snowflakes, while in industry it leads to the crystalline silicon used in electronic circuits. Micrometer scale particles can also form crystals. For example particles coated in DNA can form a variety of crystals whose structures and properties can be prescribed by the choice of DNA sequences. Is the crystallization of these artificial particles governed by the same physics as the crystallization of diamonds and silicon? I will present progress on an experimental study of the nucleation and growth of colloidal crystals due to DNA hybridization. Specifically, I will describe a microfluidics-based approach in which we produce hundreds of monodisperse, isolated droplets filled with colloidal particles and then track the formation of crystals within each drop as a function of time. We find that the initial nucleation of crystals from a supersaturated solution involves overcoming a free-energy barrier. Furthermore, we find that free-energy barrier to crystallization decreases drastically with increasing temperature and exhibits evidence of heterogeneity in the barrier height, which we hypothesize results from variations in the interaction strengths between particles. We also find that once nucleated, the crystals grow at a rate that is limited by the diffusive flux of colloidal particles to the growing crystal surface. These findings may help us to devise strategies to tune the nucleation rates and crystal growth kinetics independently, which will be helpful as we try to engineer higher quality or more complex self-assembled structures.

Luke Korley, "Lux-Zeplin: The Search for Dark Matter"

Abstract: The Lux-Zeplin experiment will search for dark matter particle interactions with 7 active tonnes of liquid xenon surrounded by 17.5 tonnes of Gd doped organic scintillator, 4850 feet deep in the Black Hills. I will introduce the LZ detector; and give a brief overview of ongoing efforts in the mine and at home in preparation for the first science run.

Gravity, entanglement, and bit threads

September 17, 2019

Matthew Headrick, Brandeis University

Abstract: In trying to understand quantum gravity at a fundamental level, one of the most confusing questions is where the degrees of freedom are. So-called holographic dualities help with this question, by showing that certain quantum gravity theories are equivalent to conventional quantum field theories, in which we understand in principle where the degrees of freedom are and how they interact. Using such dualities, a new way of understanding entanglement in quantum gravity, involving so-called “bit threads”, has recently been developed. From this point of view, space becomes a kind of channel for carrying entanglement of fundamental degrees of freedom. We will explain what holographic dualities are, what bit threads are, and what they might tell us about the nature of space in quantum gravity.

Crowd Control: manipulating group dynamics in living, cellular systems

September 24, 2019

Daniel J. Cohen, Princeton University

Host: Guillaume Duclos

Abstract: Tissues are wonderfully complex communities of soft, living agents. Emergent behaviors arising from cell-cell interactions contribute to critical behaviors such as collective motion, healing, and coordinated growth. Tools that can harness or bias these processes allow us a new way to manipulate living tissue dynamics with broad fundamental and biomedical applications. Our work draws from soft matter biophysics, swarm dynamics, and engineering to develop new experimental approaches to programmatically bias key collective cell behaviors — effectively allowing us to ‘herd’ cells. I will discuss two of our core approaches — Outside-In steering of collective cell migration, and Inside-Out guidance of cellular activity. In the case of Outside-In control, we apply an external stimulus in the form of a DC electric field to a population of cells to induce migration through an obscure phenomenon known as "electrotaxis." Our latest electric bioreactor allows us to literally program collective cell migration. We hope to someday use this to accelerate healing processes. For Inside-Out control, we flip the paradigm and focus on mimicking an agent in the group (e.g. a cell). Here, we use a cell-mimetic biomaterial that can infiltrate a tissue to manipulate growth dynamics or spy on the tissue from within. So far, we have demonstrated successful integration with epithelial and cardiac tissues, and we are now exploring the broader space of cell-cell interactions for new targets that we hope will help improve implant integration and efficacy.

Architecting soft functional materials with instabilities

October 8, 2019

Pierre-Thomas Brun, Princeton University

Host: Guillaume Duclos

Abstract: The talk is concerned with the directed control of fluidic instabilities to program shapes. While instabilities are traditionally regarded as a route towards failure in engineering, I aim to follow a different path; taming fluidic instabilities and harnessing the patterns and structures they naturally form. This methodology capitalizes on the inherent periodicity, scalability, versatility and robustness of mechanical instabilities. This new design paradigm — building with instabilities — calls for an improved understanding of instabilities and pattern formation in complex media. While stability analysis is a classic topic in mechanics, little is known on the so called inverse problem: finding the optimal set of initial conditions and interactions that will be transmuted into a target shape without direct external intervention. While the epicenter of the research is fundamental, utilizing instabilities to structure soft materials opens new research directions in the study of the behavior and deformations of architected soft materials, inspired by natural soft-materials that self-assemble into well defined structures to display remarkable properties. More broadly, the talk is rooted on the basis of recognizing model experiments as a valuable and powerful tool for discovery and exploration, in turn seeding the development of formal and predictive models.

Introduction to Quantum Error Correction: Schrödinger meets Maxwell’s Demon

October 22, 2019

Steven Girvin, Yale University

Host: Matthew Headrick

Abstract: A "second quantum revolution" is underway based on our new understanding of how information can be stored and manipulated using quantum hardware. Even more remarkable than the concept of quantum computation is the concept of quantum error correction. We know that measurement ‘back action’ disturbs a quantum state when we observe it. Nevertheless, it is possible to store an unknown quantum state and, if it develops errors due to imperfect hardware, we can measure and correct such errors to recover the original unknown state. Crucially, we must be able to do this without ever learning anything about that unknown state. This talk will present an elementary introduction to the basic theoretical concepts underlying quantum error correction for discrete systems (qubits) as well as for continuous-variable systems (harmonic oscillators).

Prediction in immune repertoires

October 29, 2019

Aleksandra Walczak, École Normale Supérieure, Paris

Host: Guillaume Duclos

Abstract: Living systems often attempt to calculate and predict the future state of the environment. Given the stochastic nature of many biological systems how is that possible? I will show that even a system as complicated as the immune system has reproducible observables. Yet predicting the future state of a complex environment requires weighing the trust in new observations against prior experiences. In this light, I will present a view of the adaptive immune system as a dynamic Bayesian machinery that updates its memory repertoire by balancing evidence from new pathogen encounters against past experience of infection to predict and prepare for future threats.

Work and Dissipation in the Cell Cytoskeleton

November 5, 2019

Michael Murrell, Yale University

Host: Bulbul Chakraborty

Abstract: Living cells generate and transmit mechanical forces over diverse time-scales and length-scales to determine the dynamics of cell and tissue shape during both homeostatic and pathological processes, from early embryonic development to cancer metastasis. These forces arise from the cell cytoskeleton, a scaffolding network of entangled protein polymers driven out-of-equilibrium by enzymes that convert chemical energy into mechanical work. However, how molecular interactions within the cytoskeleton lead to the accumulation of mechanical stresses that determine the dynamics of cell shape is unknown. Furthermore, how cellular interactions are subsequently modulated to determine the shape of the tissue is also unclear. To bridge these scales, our group in collaboration with others, uses a combination of experimental, computational and theoretical approaches. On the molecular scale, we use active gels as a framework to understand how mechanical work is produced and dissipated within the cell cytoskeleton. On the scale of cells and tissues, we abstract mechanical stresses to surface tension in a liquid film and draw analogies between the dynamics of wetting and the dynamics of simple tissues. Together, we attempt to develop comprehensive description for how cytoskeletal stresses translate to the physical behaviors of cells and tissues with significant phenotypic outcomes such as epithelial wound healing.  

Virus biophysics as a basis for developing Hepatitis B Virus-specific antivirals

November 12, 2019

Adam Zlotnick, Indiana University

Host: Michael Hagan

Abstract: Many viruses have an icosahedral shell, or capsid, that spontaneously assembles from dozens to hundreds of subunits. We argue that identifying mechanisms for dysregulating assembly may be an effective way to engineer direct-acting antivirals. Hepatitis B Virus (HBV) has an icosahedral capsid constructed from 120 dimeric subunits. We observe that assembly of purified protein follows the characteristics predicted by computational assembly models: weak interaction energy between multivalent subunits and assembly kinetics limited by allosteric regulation lead to accumulation of few intermediates and many capsids. However, assembly can be thrown off track by strengthening association energy, leading to kinetically trapped intermediates and defective particles. We use our understanding of assembly to identify small molecules, putative drugs, to accelerate and misdirect HBV capsid assembly. These molecules lead to over-initiation of assembly, poor geometry between subunits, and failure to form regular capsids in the test tube. They appear to lead to the same defects and suppress virus production in vivo. The path laid out theoretically and in practice with HBV should be applicable to many other viruses.

Physics of Breaking

December 3, 2019

Alain Karma, Northeastern University

Host: Guillaume Duclos

Abstract: Crack propagation is the main model of materials failure. Despite their practical relevance and apparent similarities to other instabilities in condensed-matter physics and materials science, dynamical instabilities of propagating cracks remain poorly understood. Instabilities spontaneously break the translation-invariant property of simple planar propagation and generically yield complex crack paths in two and three dimensions. In this talk, I will discuss fundamental progress to understand such instabilities in varied contexts using a theoretical framework that bridges micro and macro scales and offers unique capabilities to predics arbitrarily complex crack paths without ad hoc path selection criteria. Examples will illustrate how this framework has shed light on long-standing puzzles in materials failure from dynamic instabilities of ultra-high-speed cracks to crack-front segmentation in mixed-mode tensile-shear fracture to crack kinking in biological composites.

Nuclear Physics from the Standard Model

December 10, 2019

Phiala Shanahan, MIT

Host: Matthew Headrick

Abstract: I will discuss the status and future of numerical lattice Quantum Chromodynamics (QCD) calculations for nuclear physics. With advances in supercomputing, we are beginning to quantitatively understand nuclear structure and interactions directly from the fundamental quark and gluon degrees of freedom of the Standard Model. Recent studies provide insight into the neutrino-nucleus interactions relevant to long-baseline neutrino experiments, double beta decay, and nuclear sigma terms needed for theory predictions of dark matter cross-sections at underground detectors. I will also address new work constraining `exotic glue’ in nuclei, which will be measurable for the first time at a future electron-ion collider, and explain how machine learning tools are providing new possibilities in this field.