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Speakers and Abstracts

**Symposium on Quantum Information, Complexity, and the Physical World
Monday, December 5, 2022 - Princeton University**

**Workshop on Spacetime and Quantum Information **

**Tuesday & Wednesday, December 6-7, 2022 - Institute for Advanced Study**

**Main Menu Scientific Program**

**Monday, December 5**

**Dmitry Abanin**, *University of Geneva*

*Title: *Influence Functionals, Temporal Entanglement, and Quantum Many-Body Dynamics

*Abstract: *We will describe dynamics of a many-body system by its Feynman-Vernon influence functional (IF), which encodes the properties of the system as a quantum bath. In addition to exact solutions in some integrable and non-integrable models, in many relevant cases IF can be efficiently computed using tensor-networks methods, thanks to favourable scaling of its temporal entanglement. Influence-functional approach offers a new lens on non-equilibrium many-body phenomena, both in ergodic and non-ergodic regimes, and connects theory of open quantum systems to quantum statistical physics.

**Ignacio Cirac**, *Max Planck Institute of Quantum Optics*

*Title: *Quantum Computing and Simulation with Errors

*Abstract: *I will discuss the limitations posed by error propagation to gain advantage in quantum computers. I will also discuss how these errors may affect quantum simulators, and some ways to circumvent them.

**Daniel Jafferis**, *Harvard University*

*Title: *Emergent Gravitational Dynamics in Quantum Experiments

**Juan Maldacena**, *Institute for Advanced Study*

*Title: *Gravity and Black Holes from a Quantum System

*Abstract: *This will be mainly a review talk on a quantum mechanical model for certain black holes. It is targeted mainly for people outside the quantum gravity community.

The quantum mechanical model consists of harmonic oscillators and qubits interacting in a very particular way. Over 20 years ago, it was conjectured that a thermal state in this system is described by a black hole in a certain ten dimensional space. We will outline the relation between the two systems and describe some of the evidence for it. We will also describe a sample computation of the quasinormal mode spectrum that exploits some of the peculiar symmetries of the gravity theory.

**Xiao Mi**, *Google Quantum AI*

*Title: *Exploring Many-Body Physics with Superconducting Quantum Processors

*Abstract: *The tremendous advancements in the size, coherence and control accuracy of qubits based on superconducting Josephson junctions over the past decade have led to a flurry of experiments exploring many-body quantum phenomena previously accessible only through theory or numerics. The physics of quantum scrambling, which studies the spread of local quantum observables to the Hilbert space of the entire system, is particularly promising for near-term quantum advantage due to its typically high classical computational complexity. I will describe a recent experiment where we comprehensively characterize operator spreading and operator entanglement in random circuits by measuring the so-called out-of-time-order correlators [1]. We find that the fluctuation of OTOCs between circuit instances serves as a witness to the convergence to quantum chaos. As time permits, I will also describe experimental studies of periodically driven (“Floquet”) models that resist scrambling through mechanisms such as many-body localization [2] and prethermalization [3].

[1] X. Mi, P. Roushan, C. Quintana, K. Kechedzhi et al., Science 374, 1479 (2021).

[2] X. Mi, M. Ippoliti, P. Roushan, V. Khemani et al., Nature 601, 531 (2022).

[3] X. Mi, M. Sonner, D. A. Abanin, P. Roushan et al., Science 378, 785 (2022).

**Umesh Vazirani**, *University of California, Berkeley*

*Title: *Complexity of Random Circuit Sampling

**Edward Witten**, *Institute for Advanced Study*

*Title: *An Observer in de Sitter Space

*Abstract: *In quantum mechanics we usually consider the observer to be external to the system being studied, but in the context of gravity, especially in a closed universe, it may be necessary to explicitly take into account the observer and the observer's gravity. After a general discussion of what an observer can observe, I will describe the example of de Sitter space, where it is necessary to explicitly take into account the gravity of the observer. (This example was analyzed in arXiv:2206.10780 with V. Chandrasekharan, R. Longo, and G. Penington.)

**Tuesday & Wednesday, December 6-7**

**Scott Aaronson**, *University of Texas at Austin*

*Title: *Discrete Bulk Reconstruction

*Abstract: *According to the AdS/CFT correspondence, the geometries of certain spacetimes are fully determined by quantum states that live on their boundaries -- indeed, by the von Neumann entropies of portions of those boundary states. This work investigates to what extent the geometries can be reconstructed from the entropies in polynomial time. Bouland, Fefferman, and Vazirani (2019) argued that the AdS/CFT map can be exponentially complex if one wants to reconstruct regions such as the interiors of black holes. Our main result provides a sort of converse: we show that, in the special case of a single 1D boundary, if the input data consists of a list of entropies of contiguous boundary regions, and if the entropies satisfy a single inequality called Strong Subadditivity, then we can construct a graph model for the bulk in linear time. Moreover, the bulk graph is planar, it has O(N^2) vertices (the information-theoretic minimum), and it's "universal," with only the edge weights depending on the specific entropies in question. From a combinatorial perspective, our problem boils down to an "inverse" of the famous min-cut problem: rather than being given a graph and asked to find a min-cut, here we're given the values of min-cuts separating various sets of vertices, and need to find a weighted undirected graph consistent with those values. Our solution to this problem relies on the notion of a "bulkless" graph, which might be of independent interest for AdS/CFT. We also make initial progress on the case of multiple 1D boundaries -- where the boundaries could be connected wormholes -- including an upper bound of O(N^4) vertices whenever a planar bulk graph exists (thus putting the problem into the complexity class NP).

**Ahmed Almheiri**, *New York University
Title: *Path Integrals for Chords

*Abstract:*I will describe work in progress proposing and analyzing the bulk path integral of double scaled SYK in various bases.

**Netta Engelhardt**, *Massachusetts Institute of Technology
Title: C*omplexity Coarse-Graining in the Black Hole Information Problem

**Bill Fefferman**, *The **University of Chicago
Title: *Quantum Pseudoentanglement

*Abstract:*Quantum pseudorandom states are efficiently preparable states that are indistinguishable from truly Haar random states to an efficient observer. First defined by Ji, Liu and Song, such states have found a wide variety of applications in areas such as quantum gravity and cryptography. A fundamental question is exactly how much entanglement is required to create such states. Haar-random states, as well as t-designs for t ≥ 2, exhibit near-maximal entanglement. Here we provide the first construction of pseudorandom states with only polylogarithmic entanglement entropy across an equipartition of the qubits, which is the minimum possible. Our construction can be based on any one-way function secure against quantum attack. We additionally show that the entanglement in our construction is fully “tunable”, in the sense that one can have pseudorandom states with entanglement Θ(f(n)) for any desired function ω(log n) ≤ f(n) ≤ O(n). More fundamentally, our work calls into question to what extent entanglement is a “feelable” quantity of quantum systems. Inspired by recent work of Gheorghiu and Hoban, we define a new notion which we call “pseudoentanglement”, which are ensembles of efficiently constructable quantum states which hide their entanglement entropy. We show such states exist in the strongest form possible while simultaneously being pseudorandom states. Based on joint work with Adam Bouland, Soumik Ghosh, Umesh Vazirani and Zixin Zhou.

**Henry Lin**, *Stanford University*

*Title: *Algebra and Geometry from Chords

*Abstract: *In double-scaled SYK, the chord diagrams of Berkooz et al. give rise to a bulk algebra and geometry. The algebra is a "quantum deformation" of the JT gravitational algebra that includes a deformation of SL(2,R). The growth of operator size (in particular, its finite temperature, sub-maximal Lyapunov behavior) is governed by this algebra. The deformed SL(2,R) acts in a somewhat unusual manner on the bulk geometry.

**Tamra Nebabu**, *Stanford University*

*Title: *A Generalized Protocol for Bulk Reconstruction from Generalized Free Fields

*Abstract: *Bulk reconstruction is an important step in establishing the dictionary between boundary and bulk quantities in holographic theories. If the starting point is a quantum theory which serves as the putative boundary model, in what cases can one construct a bulk dual description? I will discuss recent work in which we devise a generalized protocol for constructing a bulk theory from any boundary model of generalized free fields. Unlike HKLL reconstruction, the bulk geometry and dynamics are fully emergent. I will discuss the application of our protocol to construct bulk descriptions for various SYK models beyond the conformal limit in which they are known to have a canonical dual. I will show evidence of that some geometric features of the canonical bulk survive in the non-conformal limit, and remark on the ability to extend the protocol to explore emergent bulk physics in more general settings.

**Geoff Penington**, *University of California, Berkeley; Institute for Advanced Study
Title: *The Boundary Algebras in JT Gravity

**Ronak Soni**, *University of Cambridge*

*Title: *Microstates of the 2d Non-Supersymmetric Black Hole

*Abstract: *We identify the microstates of the non–supersymmetric, asymptotically flat 2$d$ black hole in the dual $c=1$ matrix quantum mechanics (MQM). We calculate the partition function of the theory using Hamiltonian methods and reproduce one of two conflicting results found by Kazakov and Tseytlin. We find the entropy by counting states and the energy by approximately solving the Schrödinger equation. The dominant contribution to the partition function in the double-scaling limit is a novel bound state that can be considered an explicit dual of the black hole microstates. This bound state is long-lived and evaporates slowly, exactly like a black hole in asymptotically flat space. Based on arXiv:2110.11493.

**Jonathan Sorce**, *Massachusetts Institute of Technology*

*Title: *Causality and Entanglement in Holography: The Connected Wedge Theorem Revisited

*Abstract: *One puzzling aspect of holography is that it conjectures a duality between a physical theory with a single rigid causal structure (the non-gravitational "boundary theory") and one whose causal structure is state-dependent (the gravitational "bulk theory"). In this talk, I will explain how consistency of holographic quantum gravity can be used to constrain the entanglement structure of a field theory state based on which sets of boundary points admit causal scattering in its gravitational dual. This constraint can be argued for directly in the boundary theory using information-theoretic reasoning, and shown to hold in the bulk theory as a consequence of the quantum extremal surface formula for holographic entanglement entropy. I will also discuss how the gravitational reasoning used in this work suggests theorems about relativistic information processing that can be proved directly in information theory, without any reference to quantum gravity. Based on 1912.05649 and 2210.00018.

**Brian Swingle**, *Brandeis University*

*Title: *Holographic Measurements and Quantum Teleportation

**Gustavo Joaquin Turiaci**, *Institute for Advanced Study
Title: *Black Hole Microstate Counting from Gravity

*Abstract:*Finding a gravitational description of black hole microstates is an important problem in quantum gravity. In this talk we describe how to reproduce the integer number of black hole microstates using the Gibbons-Hawking gravitational path integral. This is done for a specific class of supersymmetric black holes in four dimensions arising from toroidal compactifications of string theory: first by applying localization to the gravitational path integral that computes the index (including a careful evaluation of one-loop determinants), and second by comparing the index with the degeneracy.

**Aron Wall**, *University of Cambridge (remote)
Title: *Cauchy Slice Holography and the Information Paradox

*Abstract:*Cauchy slice holography gives a duality between the wavefunction on a Cauchy slice and the partition function of a T

^{2}deformed field theory. I will review this correspondence and discuss its implications for the black hole information puzzle. The duality implies that postselection plays an important role in the method by which information escapes to the exterior region.

**Zhenbin Yang**, *Stanford University (remote)*

*Title: *Firewalls from Wormholes

*Abstract: *Spacetime wormholes can lead to surprises in black hole physics. We show that a very old black hole can tunnel to a white hole/firewall by emitting a large baby universe. We study the process for a perturbed thermofield double black hole in Jackiw-Teitelboim (JT) gravity, using the lowest order (genus one) spacetime wormhole that corresponds to single baby-universe emission. The probability for tunneling to a white hole is proportional to t^{2}e^{−2S} where *t* is the age of the black hole and *S* is the entropy of one black hole.