Quantum and Gravitational Theory Group

January 18, 2022

January 25, 2022

"Protecting quantum information with sparse nonlocal interactions"

Entanglement is a valuable resource, but it tends to be very fragile. Recent work driven by Clifford circuit simulations, however, has demonstrated that strong scrambling can generate volume-law entanglement that persists despite the presence of ongoing continuous measurement. In this talk I describe how fast scrambling dynamics generated by long-range and sparse nonlocal interactions can be used to improve a system’s robustness to measurement, allowing for many-body entanglement to survive at substantially higher rates of measurement compared to circuits featuring only local interactions. Further, I will show that these long-range interactions also improve the code properties (code rate, code distance) of the dynamically-generated error-correcting codes which underpin the volume-law phase. Nonlocal interactions of this type can be implemented in near-term quantum simulators using cavities with a multifrequency drive or in Rydberg chains with tweezer-assisted shuffling.

February 1, 2022

"An introduction to decomposition"

In this talk I will review recent work on `decomposition,' a property of 2d theories with 1-form symmetries and, more generally, d-dim'l theories with (d-1)-form symmetries. Decomposition is the observation that such quantum field theories are equivalent to ('decompose into’) disjoint unions of other QFTs, known in this context as "universes." Examples include two-dimensional gauge theories and orbifolds with matter invariant under a subgroup of the gauge group. Decomposition explains and relates several physical properties of these theories -- for example, restrictions on allowed instantons arise as a "multiverse interference effect" between contributions from constituent universes. First worked out in 2006 as part of efforts to understand string propagation on stacks, decomposition has been the driver of a number of developments since. In the first half of this talk, I will review decomposition; in the second half, I will focus on the recent application to anomaly resolution of Wang-Wen-Witten in two-dimensional orbifolds.

February 8, 2022

"Scrambling and the black hole atmosphere"

We argue that the scrambling time is the same, up to a numerical factor in three or more spacetime dimensions, as the time for the atmosphere to fall across the horizon or escape, to be replaced by new atmosphere. We propose that these times agree because the physical scrambling process is part and parcel of the atmosphere refreshment process. We provide some support for this relation also in two dimensions, but the atmosphere is not as localized, so the argument is less justified.

February 15, 2022

"Ocean turbulence from space"

In this talk I will first outline some basic regimes of ocean dynamics, organized by spatial and temporal scales. I will then describe the use of satellite altimetry to observe ocean dynamics for ``balanced" motions in which the Coriolis and gravitational forces dominate the Navier-Stokes equations. Finally, I will discuss a current theory of the energization of the submesoscale, at which vertical motion becomes important, and give evidence from altimetry that this picture is correct. This talk is based on work with Jörn Callies at Caltech; a preprint can be found at https://arxiv.org/abs/2201.09136.

February 22, 2022

March 1, 2022

"AdS-Kasner and Path Integral Complexity"

Over many years, AdS-Kasner backgrounds have been used to probe signatures of space-like singularities in the field theory dual. In this talk I will first briefly review aspects of earlier work in this direction. I will then describe some recent work on the behavior of path integral complexity and its universality in these backgrounds.

March 8, 2022

"The Principles of Deep Learning Theory"

Deep learning is an exciting approach to modern artificial intelligence based on artificial neural networks. The goal of this talk is to provide a blueprint — using tools from physics — for theoretically analyzing deep neural networks of practical relevance. This task will encompass both understanding the statistics of initialized deep networks and determining the training dynamics of such an ensemble when learning from data. Borrowing from the "effective theory" framework of physics and developing a perturbative 1/n expansion around the limit of infinite hidden-layer width, we will find a principle of sparsity that will let us describe effectively-deep networks of practical large-but-finite-width networks.

This talk is based on a book, "The Principles of Deep Learning Theory," co-authored with Sho Yaida and based on research also in collaboration with Boris Hanin. It will be published this summer by Cambridge University Press.

March 15, 2022

"Fun with Quantum Lego"

The quantum gravity community has benefited immensely from quantum information theoretic concepts in the past decade. In this talk, we take the reverse direction and look at how progress in AdS/CFT can return the favour. Inspired by the holographic codes, I introduce a flexible and graphically intuitive framework that constructs complex quantum error correction codes from simple codes or states, generalizing code concatenation. More specifically, we represent the complex code constructions as tensor networks built from the tensors of simple codes or states in a modular fashion. Using a set of local moves known as operator pushing, one can derive properties of the more complex codes, such as transversal non-Clifford gates, by tracing the flow of operators in the network. To highlight the framework's range of capabilities and to provide a tutorial, I lay out some examples where we construct and customize non-trivial codes using simple quantum lego blocks. Surprisingly, we also find that the surface code is equivalent to the 2d Bacon-Shor code after ``dualizing'' its tensor network encoding map.

March 25, 2022

March 29, 2022

"The Holographic Fault Tolerance Threshold is the Hawking-Page Phase Transition"

We study the error threshold properties of holographic quantum error-correcting codes. We demonstrate that holographic CFTs admit an algebraic threshold, which is related to the confinement-deconfinement phase transition. We then apply geometric intuition from holography and the Hawking-Page phase transition to motivate the CFT result, and comment on potential extensions to other confining theories.

April 5, 2022

April 12, 2022

"Quantum Simulation of Z2 lattice gauge theories with dynamical matter in (2+1)D"

Gauge fields coupled to dynamical matter are a universal framework in many disciplines of physics, ranging from particle to condensed matter physics, but remain poorly understood at strong couplings. In the past years a new perspective has emerged through analog quantum simulation platforms which have become a powerful tool to study interacting quantum many-body systems in a highly controllable fashion. Here we propose a scheme, in which a Z2 gauge structure emerges from local two- and one-body interactions in two spatial dimensions. The scheme is suitable for Rydberg atom arrays and enables to experimentally study both (2+1)D Z2 lattice gauge theories coupled to dynamical matter (Z2 mLGT) and quantum dimer models on the honeycomb lattice, for which we derive effective Hamiltonians. We discuss ground state phase diagrams of the effective Z2 mLGT for U(1) and quantum Z2 matter featuring deconfined phases. Further, we present experimental perspectives and show signatures of disorder-order free localization as well as the Schwinger effect in (2+1)D using small-scale exact diagonalization studies. Our proposed scheme allows to experimentally study not only longstanding goals of theoretical physics, such as Fradkin and Shenker's conjectured phase diagram, but also go beyond currently accessible numerical regimes.

April 19, 2022

April 26, 2022

"Enstrophy and black hole supertranslations"

Enstrophy is an approximately conserved charge in 2+1 dimensional nonrelativistic fluids that implies an inverse energy cascade in turbulent flows. In this talk, I will present an algorithm on how to construct an enstrophy current for generic fluid flows (relativistic and non). In addition, I will show how a subset of certain horizon symmetries of 3+1 dimensional AdS black holes also leads to enstrophy conservation in the dual holographic fluid theory.

June 7, 2022

"Measurement-induced criticality and charge-sharpening transitions"

Monitored quantum circuits (MRCs) exhibit a measurement-induced phase transition between area-law and volume-law entanglement scaling as a function of the measurement rate. In this talk, I will first review our understanding of such measurement-induced phase transitions. I will argue that MRCs with a conserved charge additionally exhibit two distinct volume-law entangled phases that cannot be characterized by equilibrium notions of symmetry-breaking or topological order, but rather by the non-equilibrium dynamics and steady-state distribution of charge fluctuations. These include a charge-fuzzy phase in which charge information is rapidly scrambled leading to Luttinger-liquid-like spatial fluctuations of charge in the steady state, and a charge-sharp phase in which measurements collapse quantum fluctuations of charge without destroying the volume-law entanglement of neutral degrees of freedom. I will present some statistical mechanics and effective field theory approaches to such charge-sharpening transitions.