## Quantum/Gravity Seminar Series

Unless otherwise noted, seminars take place at 11:15 am on Thursdays in Abelson 333.

### Fall 2024 Seminars

September 5, 2024

September 12, 2024

**Title:**Gravitational back-reaction is magical.

**Abstract:**We study the interplay between magic and entanglement in quantum many-body systems. We show that non-local magic, which is supported by the quantum correlations is lower bounded by the non-flatness of entanglement spectrum and upper bounded by the amount of entanglement in the system. We then argue that a smoothed version of non-local magic bounds the hardness of classical simulations for incompressible states. In conformal field theories, we conjecture that the non-local magic should scale linearly with entanglement entropy but sublinearly when an approximation of the state is allowed. We support the conjectures using both analytical arguments based on unitary distillation and numerical data from an Ising CFT. If the CFT has a holographic dual, then we prove that the non-local magic vanishes if and only if there is no gravitational back-reaction. Furthermore, we show that non-local magic is approximately equal to the rate of change of the minimal surface area in response to the change of cosmic brane tension in the bulk.

September 19, 2024

**Title: **Supersymmetry breaking, anomalies, and the spectrum of black holes from SYK to M-theory

**Abstract: **The Bekenstein–Hawking formula gives a coarse-grained count of the number of microstates of a black hole, and it is remarkable that it may sometimes be reproduced from a microscopic count in string theory. However, the standard approach (which we will briefly review) typically relies on supersymmetry and AdS/CFT by counting special BPS states in free CFT, while the bulk is treated semiclassically. This neglects quantum effects which lead to pathologies for both supersymmetric and non-supersymmetric black holes; further, it obscures the fact that a black hole is a highly chaotic quantum system.

For black holes embedded in string theory, we will explain how a more careful treatment of the gravitational path integral using tools from JT gravity leads to predictions about the spectrum of BPS black holes (which agree with the microscopic counting), as well as the spectrum of near-BPS black holes (which obey a version of random matrix universality). Surprisingly, we find explicit examples in M-theory in which neither the Bekenstein-Hawking formula nor the supersymmetric index account for the entropy. This and other features arise from genuinely quantum mechanical aspects of black hole physics, including supersymmetry breaking, 't Hooft anomalies, and the Witten effect. Many of these observations have an analog in SYK models, so we can see how a more conventional quantum system can reproduce the general expectations of supergravity with or without unbroken supersymmetry.

September 26, 2024

**Title: **Topological recursion for closed string field theory.

**Abstract: **Closed string field theory is a field theory which correctly reproduces closed string scattering amplitudes. Unfortunately, the Lagrangian for it has an infinite number of interaction terms. I'll sketch how, using ingredients from Mirzakhani's recursion applied to the recently discovered hyperbolic vertices, one can write all of the interaction terms in the Lagrangian in terms of the cubic term. Time-permitting, I will also comment on potential relations to dualities between low-dimensional string theories and matrix models.

October 3, 2024

October 10, 2024

**Title: **Emergent Holographic Forces from Quantum Circuits and Criticality.

**Abstract: **I will explain a model of the AdS/CFT correspondence that can be implemented on contemporary quantum simulators. In particular, using extensive numerics and analytics, I will show that a specific 1+1 critical quantum spin chain has a duality with a 2+1 bulk theory possessing long-range forces that quantitatively match AdS gravity. This duality is instantiated by a simple quantum circuit which maps the 1+1 theory to the 2+1 theory. We will explore implications for quantum gravity and discuss the role of quantum simulation.

October 16, 2024

Note unusual day

**Title: **Gravity at finite cutoff.

**Abstract: **I will describe thermodynamics of gravitational systems with conformal boundary conditions, where the conformal class of boundary metrics and the trace of the extrinsic curvature K are kept fixed. The main focus is on comparing the series of subextensive terms in the free energy to predictions from thermal effective field theory. We observe that while there is agreement in terms of the high temperature expansion structure, the first subextensive correction to the free energy is negative. This violates a conjectured bound on this coefficient in quantum field theory, which we interpret as a signal that gravity does not fully decouple in the putative boundary dual. I will also discuss cosmic-type solutions corresponding to negative K, and explore them further in the Einstein-Maxwell theory. These solutions are necessary for consistency with thermal effective field theory on the boundary.

October 24, 2024

October 31, 2024

November 7, 2024

November 14, 2024

November 21, 2024

November 28, 2024

December 5, 2024