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Speakers and Abstracts

**Workshop on Spacetime and Quantum Information ****Monday-Wednesday, December 11-13, 2023**

**Ahmed Almheiri**, *New York University Abu Dhabi** Title:* Holography on the Quantum Disk

*Motivated by recent observations in double-scaled SYK (DSSYK), this talk will feature work in progress analyzing holography on a non-commutative analogue of the hyperbolic disk known as the quantum disk. I will briefly review hints of non-commutative geometry in the chord description of DSSYK, and point out how it lingers in the continuum limit at finite temperature on the recently studied “fake disk.” I’ll then introduce the quantum disk and derive its non-commutative nature, and analyze the dynamics of fields propagating on the fixed quantum disk background. I will point out some notable features of this system including discreteness of space and UV finiteness. I will also describe the implications of the non-commutativity on properties of its putative boundary dual.*

**Abstract:****Stefano Antonini**, *University of California, Berkeley** Title:* Cosmology from Random Entanglement

*Obtaining a description of cosmology is a central open problem in holography. Studying simple models can help us gain insight on the generic properties of holographic cosmologies. In this talk I will describe the construction of entangled microstates of a pair of holographic CFTs whose dual semiclassical description includes big bang-big crunch AdS cosmologies in spaces without boundaries. The cosmology is supported by inhomogeneous heavy matter and it partially purifies the bulk entanglement of two auxiliary AdS spacetimes. In generic settings, the cosmology is an entanglement island contained in the entanglement wedge of one of the two CFTs. I will then describe the properties of the non-isometric bulk-to-boundary encoding map and comment on an explicit, state-dependent boundary representation of operators acting on the cosmology. Finally, I will discuss an ensemble interpretation of our results and other open questions.*

**Abstract:****Alejandra Castro**, *University of Cambridge***Title: **Keeping Matter in the Loop in 3D Quantum Gravity**Abstract: **In this talk I will discuss a novel mechanism that couples matter fields to three-dimensional quantum gravity. This construction is based on the Chern-Simons formulation of three-dimensional gravity, and it centers on a collection of Wilson loops winding around spacetime. We coin this object a Wilson spool. To construct the spool, we build take advantage of representation theory. To evaluate the spool, we adapt and exploit several known exact results in Chern-Simons theory. Our proposal correctly reproduces the one-loop determinant of a free massive scalar field on S^3 and AdS_3 as G_N->0. Moreover, allowing for quantum metric fluctuations, it can be systematically evaluated to any order in perturbation theory.

**Luca Ciambelli**, *Perimeter Institute** Title:* Symplectic Analysis of Null Raychaudhuri

*Working intrinsically on a null hypersurface, we first show that the Raychaudhuri constraint is the conservation law of a Carrollian stress tensor. After suitably dressing the diffeomorphisms with the internal boost symmetry, we derive the full kinematical Poisson bracket. The diffeomorphism boost charge turns out to be positive and monotonic in the dressing time. We then perform a perturbative analysis in the weak gravity regime, and show how the Raychaudhuri constraint can be interpreted as an equality of CFT stress tensors for the spin-0, spin-2, and matter systems. Finally, we observe that the perturbative spin-0 stress tensor and Poisson bracket behave exactly like a curved beta-gamma CFT.*

**Abstract:****Eugenia Colafranceschi**, *University of California, Santa Barbara** Title:* Type I von Neumann Algebras from Gravitational Path Integrals: Ryu–Takayanagi as Entropy Without Holography

*Recent works by Chandrasekaran, Penington and Witten have shown in various special contexts that the quantum-corrected Ryu-Takayanagi (RT) formula can be understood as computing an entropy on an algebra of bulk observables. These arguments do not rely on the existence of a holographic dual field theory. We show that analogous-but-stronger results hold in any UV-completion of asymptotically anti-de Sitter quantum gravity with a Euclidean path integral satisfying a simple and familiar set of axioms. In particular, the path integral defines type I von Neumann algebras of bulk observables acting on compact closed codimension-2 asymptotic boundaries, as well as entropies on these algebras. Such entropies can be written in terms of standard density matrices and standard Hilbert space traces, and in appropriate semiclassical limits are computed by the RT-formula with quantum corrections. Our work thus provides a Hilbert space interpretation of the Ryu-Takayanagi entropy. Since our axioms do not severely constrain UV bulk structures, they may be expected to hold equally well for successful formulations of string field theory, spin-foam models, or any other approach to constructing a UV-complete theory of gravity.*

**Abstract:****Thomas Faulkner**, *University of Illinois at Urbana-Champaign** Title: *Vacuum Asymptotic Codes

*I will review asymptotically isometric codes - a tool to take the large-N limit in holographic theories, allowing for non-trivial von Neumann algebras to act on the code as well as on the physical Hilbert space. I will then discuss a relationship between the CFT thermal partition function and the properties of entanglement wedges for disjoint boundary regions in the vacuum. Our results suggest a characterization of asymptotic codes with emergent sub-AdS locality.*

**Abstract:****Eanna Flanagan**, *Cornell University** Title: *Horizon Phase Spaces in General Relativity

*We discuss several different definitions of phase spaces associated with horizons in general relativity, and the associated symmetry groups and charges. For stationary horizons we compute the symplectic form of the theory in terms of independent degrees of freedom, by solving the constraint equations on the horizon.*

**Abstract:****Daniel Harlow**, *Massachusetts Institute of Technology** Title: *Gauging Spacetime Inversions

*Spacetime inversion symmetries such as parity and time reversal play a central role in physics, but they are usually treated as global symmetries. In quantum gravity there are no global symmetries, so any spacetime inversion symmetries must be gauge symmetries. In particular this includes CRT symmetry (in even dimensions usually combined with a rotation to become CPT), which in quantum field theory is always a symmetry and seems likely to be a symmetry of quantum gravity as well. I'll discuss what it means to gauge a spacetime inversion symmetry, and explain some of the more unusual consequences of doing this. In particular I'll argue that the gauging of CRT is automatically implemented by the sum over topologies in the Euclidean gravity path integral, that in a closed universe the Hilbert space of quantum gravity must be a real vector space, and that in Lorentzian signature manifolds which are not time-orientable must be included as valid configurations of the theory.*

**Abstract:****Stefan Hollands**, *Leipzig University** Title:* Black Hole Interiors

*Kerr or Reissner-Nordström black holes have inner horizons which delineate the domain of predictability of solutions of wave equation type. Often, though not e.g. for certain (A)deSitter black holes, these inner horizons are classically dynamically unstable, and get turned into some sort of singularity by perturbations, and thus drastically changing the nature of the inner horizons relative to the underlying exact solutions, and relegating the problem of predictability to the real of quantum gravity ("strong cosmic censorship"). Recent work has shown that semi-classical quantum effects dominate classical ones very near the inner horizons, and are expected lead to singularity in cases where the inner horizon is stable classically. I review recent progress surrounding these issues.*

**Abstract:****Hsin-Yuan (Robert) Huang**, *Caltech** Title:* Complexity of Learning and Creating Quantum Systems

*The complexity of a quantum system is a concept of fundamental interest in quantum information, quantum computing, and, more recently, in the study of quantum black holes. In this talk, I will present three notions of complexity for learning and creating quantum systems, including (1) the minimum gates needed to create the state, (2) the minimum number of measurements needed to learn the state, and (3) the minimum computational time needed to learn the state. I will prove how these conceptually different notions closely relate to each other using techniques from random scrambling, learning theory, and cryptography.*

**Abstract:****Luca Iliesiu**, *Stanford University & UC Berkeley** Title:* The Non-Perturbative Hilbert Space of JT Gravity

*We propose a non-perturbative construction of the bulk Hilbert space of JT gravity. Within this Hilbert space, we can non-perturbatively define and study observables that probe the black hole interior. To exemplify the power of this construction, we discuss two related observables for two-sided black holes: one probes the length of the black hole interior, and the other probes the center of mass collision energy between an observer infalling from one side and a shockwave coming from the opposite side. As we shall discuss, both observables are strongly affected by non-perturbative corrections at very late times, serving as useful probes for the presence of firewalls in very old black holes.*

**Abstract:****Clifford V. Johnson**, *University of California, Santa Barbara** Title:* New Non-Perturbative Results from a Random Matrix Model of N=2 JT Supergravity

*Building on the work of Turiaci and Witten who proposed the perturbative (in genus) matrix model of N=2 JT supergravity, a non-perturbative definition has been constructed using scaled orthogonal polynomial technology similar to that used for various N=1 and non-supersymmetric JT cases. The analysis allows for the excavation of the underlying microscopic physics. The spectrum of non-BPS states with zero threshold can be studied quite readily in this framework, while the non-zero threshold sector requires more care. BPS states are naturally incorporated into the description due to the natural properties of the governing "string equation".*

**Abstract:****David Kolchmeyer**, *Massachusetts Institute of Technology** Title:* Gravitational Algebras in (A)dS

_{2}

*To construct interesting algebras in quantum gravity, one can dress local field operators to features of the spacetime. I will discuss this first in AdS JT gravity with matter, where matter operators may be dressed to the right or left AdS boundaries. Then, I will consider a scalar field theory in dS*

**Abstract:**_{2}together with two entangled observers. Each observer carries a clock. As in the AdS example, matter operators may be dressed to either of the two observers. The observers' degrees of freedom are exactly quantized. The algebra of dressed operators teaches us about a putative quantum-mechanical dual theory.

**Mikhail Lukin**, *Harvard University** Title:* Scrambling and Quantum Error Correction Frontier

**Abstract:****Geoff Penington**, *University of California, Berkeley**(Zoom only)** Title: *Islands Far Outside the Horizon

*Information located in an entanglement island in semiclassical gravity can be*

**Abstract:**nonperturbatively reconstructed from distant radiation, implying a radical

breakdown of effective field theory. We show that this occurs well outside of

the black hole stretched horizon. We compute the island associated to

large-angular momentum Hawking modes of a four-dimensional Schwarzschild black

hole. These modes typically fall back into the black hole but can be extracted

to infinity by relativistic strings or, more abstractly, by asymptotic boundary

operators constructed using the timelike tube theorem. Remarkably, we find that

their island can protrude a distance of order $\sqrt{\ell_p r_{\rm hor}}$

outside the horizon. This is parametrically larger than the Planck scale

$\ell_p$ and is comparable to the Bohr radius for supermassive black holes.

Therefore, in principle, a distant observer can determine experimentally

whether the black hole information information paradox is resolved by

complementarity, or by a firewall.

**Eric Perlmutter**, *IPhT Saclay/IHES** Title:* AdS3/RMT2 Duality

*We introduce a framework for quantifying random matrix behavior of 2d CFTs and AdS3 quantum gravity. This is anchored by a 2d CFT trace formula, analogous to the Gutzwiller trace formula for chaotic quantum systems. We use this framework to understand the Cotler-Jensen torus wormhole of AdS3 pure gravity from a microscopic CFT point of view. Factorizing the wormhole generates a new, non-perturbative piece of the AdS3 pure gravity path integral with torus boundary, containing fine-grained spectral data of pure gravity black hole microstates.*

**Abstract:****Jinzhao Wang**, *Stanford University** Title:* What Exactly Does Bekenstein Bound?

*The Bekenstein bound posits a maximum entropy for matter with finite energy confined to a spacetime region. It is often interpreted as a fundamental limit on the information that can be stored by physical objects. In this work, we test this interpretation by asking whether the Bekenstein bound imposes constraints on a channel's communication capacity, a context in which information can be given a mathematically rigorous and operationally meaningful definition. We study specifically the Unruh channel that describes a stationary Alice exciting different species of free scalar fields to send information to an accelerating Bob, who is confined to a Rindler wedge and exposed to the noise of Unruh radiation. We show that the classical and quantum capacities of the Unruh channel obey the Bekenstein bound that pertains to the decoder Bob. In contrast, even at high temperatures, the Unruh channel can transmit a large number of zero-bits, which are quantum communication resources that can be used for quantum identification and many other primitive information processing protocols. Therefore, unlike classical bits and qubits, zero-bits and their associated information processing capability are not constrained by the Bekenstein bound. Time permitting, I’ll discuss what if one also restrains the encoder Alice. (Based on the joint work with Patrick Hayden https://arxiv.org/abs/2309.07436.)*

**Abstract:****Edward Witten**, *Institute for Advanced Study** Title:* Background Independent Algebra for an Observer

*I consider the operator product algebra along an observer's worldline as a background independent algebra in quantum gravity.*

**Abstract:****Shunyu Yao**, *Stanford University** Title:* Scramblon Loops

*Out of time ordered correlator(OTOC) is one signature of quantum chaos. In certain large N systems, emergent scramblon-exchange modes dominate, while scramblon interactions are suppressed by 1/N. However, we are going to discuss certain types of scramblon loop corrections, which cause a dramatic breakdown of scramblon exchange approximation, much earlier than naive expectation. Interestingly, while these effects are significant in high temperature/saddle(stringy) dominant/incoherent systems, they get cut off in low temperature/pole(graviton) dominant/coherent systems by ballistic growth. This suggests a sharp distinction between coherent/incoherent scrambling could appear by studying quantum fluctuations. We will also comment on their relation to wavefront broadening phenomena and gravitational high energy scattering.*

**Abstract:****Ying Zhao**, *Kavli Institute for Theoretical Physics** Title: *Closed Cosmologies in Two Dimensional Gravity

*We study closed cosmologies in simples models of two dimensional gravity. We show that there are stark contrast as well as connections between semi-classical and non-perturbative aspects of the theory of closed universes.*

**Abstract:**