# Recorded Group Seminars

Recent recorded seminars are listed below. For a playlist of all recorded seminars, see here.

## Past talks

### Isha Kotecha, OIST - 18 Feb 2021

**Title:** Generalised Gibbs States and Application in Discrete Quantum Gravity

**Abstract:** Equilibrium Gibbs states are undeniably important in statistical descriptions of macroscopic systems with many underlying microscopic degrees of freedom. They are expected to be important also in discrete quantum gravity approaches, where classical continuum spacetime is thought to emerge from the collective physics of some underlying quantum degrees of freedom. However, what equilibrium even means in a background independent context is a foundational open issue. In this talk, I discuss a generalisation of Gibbs states potentially suitable for such contexts, and emphasise on a thermodynamical characterisation based on the maximum entropy principle. The resultant setup is then applied to discrete quantum gravity, by modelling a quantum spacetime as a many-body system of candidate quanta of geometry, and utilising their field theoretic formulation of group field theory (GFT). This leads to the construction of several different concrete examples of quantum gravitational generalised Gibbs states. I then present a class of inequivalent thermal representations induced by a class of these generalised Gibbs states. The corresponding non-perturbative thermal vacua are thermofield double states. An interesting class of condensates, thermal coherent states, encoding fluctuations in quantum geometry are defined in this thermal Hilbert space. Finally, thermal coherent states associated with a spatial volume operator are applied in GFT cosmology, to extract an effective flat FLRW relational dynamics from a class of free GFT models. Friedmann equations are recovered at late times, with a bounce and accelerated expansion at early times. The expansion phase admits an increased number of e-folds as a direct consequence of using thermal condensates, instead of pure (zero temperature) condensates as done in past studies.

### Fabio Mele, OIST - 11 Feb 2021

**Title:** Cosmological and Black Hole Singularities in Effective Loop Quantum Gravity

**Abstract:** The fate of gravitational singularities in a theory surpassing classical General Relativity are puzzling questions that have generated a great deal of interest among various quantum gravity approaches. Among them, recent efforts have been devoted in the framework of Loop Quantum Gravity (LQG) to construct effective symmetry-reduced models of cosmological and black hole spacetimes where quantum corrections to the geometry are captured by a phase space regularisation motivated by the LQG polymer quantum representation. In the resulting effective spacetime, quantum effects induce an upper bound on curvature invariants, the classical singularity is resolved by a quantum bounce, and classical geometry is recovered at low curvatures. In this talk, after briefly reviewing how the procedure works for a simple cosmological example, I will present a new effective model for Schwarzschild black holes based on new canonical variables directly related to curvature. Dirac observables, structure of the resulting effective spacetime, and (if time allows) quantum corrections to the relevant thermodynamic quantities will be then discussed.

### Qi Hu, Perimeter - 23 Dec 2020

**Title:** Emergent universality in critical quantum spin chains: entanglement Virasoro algebra

**Abstract:** Entanglement entropy and entanglement spectrum have been widely used to characterize quantum entanglement in extended many-body systems. Given a pure state of the system and a division into regions A and B, they can be obtained in terms of the Schmidt values, or eigenvalues λα of the reduced density matrix ρ_A for region A. In this paper we draw attention instead to the Schmidt vectors, or eigenvectors |vα⟩ of ρ_A. We consider the ground state of critical quantum spin chains whose low energy/long distance physics is described by an emergent conformal field theory (CFT). We show that the Schmidt vectors |vα⟩ display an emergent universal structure, corresponding to a realization of the Virasoro algebra of a boundary CFT (a chiral version of the original CFT). Indeed, we build weighted sums Hn of the lattice Hamiltonian density h_{j,j+1} over region A and show that the matrix elements ⟨vα |Hn |vα′ ⟩ are universal, up to finite-size corrections. More concretely, these matrix elements are given by an analogous expression for (Ln + L−n)/2 in the boundary CFT, where Ln’s are (one copy of) the Virasoro generators. We numerically confirm our results using the critical Ising quantum spin chain and other (free-fermion equivalent) models.

### Gabriel Wong, Fudan - 23 Dec 2020

**Title:** Entanglement edge modes, extended TQFT, and generalized entropy

**Abstract:** In semi-classical Euclidean gravity, black holes behave like thermal objects with a generalized entropy that satisfies the second law of thermodynamics. It has been conjectured that generalized entropy can be identified with entanglement entropy in quantum gravity, with the leading area term arising from the entropy of entanglement edge modes. In this talk, we review an approach to understanding this conjecture using the framework of extended topological quantum field theory (TQFT). We use two-dimensional Yang Mills as a paradigmatic example in which a notion of generalized entropy can be defined that is analogous to generalized entropy in Euclidean gravity. Applying extended TQFT techniques, we define a factorization of the Hilbert space which leads to entanglement edge modes whose entropy reproduces the area term for the generalized entropy. Time permitting we will discuss applications of this frame work to topological string theory and conformal field theory.

### Pushkal Shrivastava, IIS Bengaluru - 23 Dec 2020

**Title:** Holographic encoding of information in asymptotically flat spacetimes

**Abstract:** Quantum gravity is widely expected to be holographic. In fact, holography in asymptotically anti-de-Sitter spacetimes has been studied extensively in the context of AdS/CFT correspondence for over two decades. However, our understanding of holography in asymptotically flat spacetimes is still rudimentary. In this seminar, I will explore the holographic encoding of information in 4-d asymptotically flat spacetimes. I will argue that all information about massless excitations can be obtained from an infinitesimal neighborhood of the past boundary of future null infinity. This result is in stark contrast to local quantum field theories, where the measurements over all of the null infinity are required to determine the state. Finally, I will discuss the implications for the black hole information paradox.

### Yuri Lensky, Stanford - 22 Dec 2020

**Title:** Tuning the dual boundary in large-q SYK

**Abstract:** The AdS/CFT duality has taught us much about how to embed gravitational physics in quantum mechanical models. Many questions in quantum gravity can be given concrete formulations and answers in this framework, but one aspect that remains mysterious is the region behind the black hole horizon. We try to take a step in understanding the spacetime in this region by a detailed analysis of a 2D "toy" version of this problem; a coupled model where the two sides of the black hole are put in causal contact at late time (using ideas from [Gao-Jafferis-Wall] and [Maldacena-Qi]). We first give a full solution of the boundary model, a pair of time-dependently coupled Sachdev-Ye-Kitaev (SYK) dots. We then give bulk interpretations to our solution, and find important corrections to the naive bulk description are necessary in the region causally disconnected from the 2D "black hole" boundaries.

### Stefan Eccles, UT Austin - 21 Dec 2020

**Title:** Holographic Complexity as Volume

**Abstract:** In the context of the AdS/CFT correspondence, the Complexity = Volume conjecture posits that the volume of a certain extremal bulk Cauchy slice is dual to the quantum circuit complexity of a corresponding CFT state. In this talk I will review the motivation for this conjecture and discuss its strengths and challenges. I’ll cover various geometric results about the behavior of maximal bulk slices in black hole spacetimes, and view the conjecture in light of a “volume flow current” which can be defined, given a bulk foliation by maximal volume slices.

### Ronak Soni, Stanford - 17 Dec 2020

**Title:** Seeing the Entanglement Wedge

**Abstract:** We study the problem of revealing the entanglement wedge using simple operations. We ask what operation a semiclassical observer can do to bring the entanglement wedge into causal contact with the boundary, via backreaction. In a generic perturbative class of states, we propose a unitary operation in the causal wedge whose backreaction brings all of the previously causally inaccessible `peninsula' into causal contact with the boundary. This class of cases includes entanglement wedges associated to boundary sub-regions that are unions of disjoint spherical caps, and the protocol works to first order in the size of the peninsula. The unitary is closely related to the so-called Connes Cocycle flow, which is a unitary that is both well-defined in QFT and localised to a sub-region. Our construction requires a generalization of the work by Ceyhan & Faulkner to regions which are unions of disconnected spherical caps. We argue that this cocycle should be thought of as naturally generalizing the non-local coupling introduced in the work of Gao, Jafferis & Wall.

### Juan Margalef Bentabol, Penn State - 16 Dec 2020

**Title:** Geometric formulation of covariant phase methods with boundary

**Abstract:** In physics, one standard way to study and understand a theory is through its dynamical formulation. Whenever possible, this is obtained by considering some initial conditions and evolving them through the dynamical equations of the theory. One gets then a curve over the space of initial conditions which codifies the evolution. This approach is useful in many settings, including General Relativity (ADM, numerical relativity, gravitational waves...), however, it also has some limitations. Namely, to understand some non-local concepts such as black holes and their properties (e.g. spin, energy, or entropy) one runs into some complications. Another approach is to study the space of solutions where each point represents a whole solution of the theory. For well-posed problems, this space is equivalent to the space of initial conditions (each initial condition gives rise to one and only one solution) although in general there would be some gauge degeneracy (the solution is determined up to some gauge transformation). In this talk, I will present this latter approach in what is known as the Covariant Phase Space methods. In particular, I will show how to construct a presymplectic structure over the space of solutions canonically associated with the action of the theory. The novelty of our work is that we consider the manifold with boundary, which adds several difficulties that had not been solved before.

### Andreas Blommaert, Stanford - 14 Dec 2020

**Title:** Wormholes and cluster decomposition

**Abstract:** We discuss the role of wormholes and branes in reconciling geometry with quantum mechanics. This will be based on recent developments in JT gravity, an analytically solvable model of two-dimensional quantum gravity. First, we will see how wormholes enable geometry to accurate capture averaged properties of late time correlators of chaotic quantum systems. Then we explain based on general intuition the effects of wormholes on large distance correlators in quantum gravity. We reproduce these effects by carefully defining diff invariant bulk matter observables in JT gravity and computing the corresponding amplitudes. Finally, we mention that microstructure of the dual quantum mechanics is represented in bulk JT gravity by some background spacetime branes.

### Andrea Di Biagio, Sapienza Universita di Roma - 4 Dec 2020

**Title:** Can We Think Timelessly About Causation?

**Abstract:** We often say that quantum mechanics allows to calculate the probability of future events. In fact, quantum mechanics does not discriminate between predicting the future or postdicting the past. I will present the results of a recent work by Rovelli, Donà and me, where we address the apparent tension between the time symmetry of elementary quantum mechanics and the intrinsic time orientation of the formulations of quantum theory used in the quantum information and foundations communities. Additionally, I will sketch a way to think time symmetrically about causality in quantum theory by using the new notion of a causal-inferential theory recently proposed by Schimd, Selby and Spekkens.

### ChunJun Cao, University of Maryland - 2 Dec 2020

**Title:** Towards emergent space-time in approximate quantum error correction codes

**Abstract:** In AdS/CFT, the bulk space-time geometry and gravitational interactions can emerge from the boundary CFT. In this talk, I will touch on two related topics on how space-time and gravity can emerge from approximate quantum error correction codes. We will first construct an efficiently decodable holographic quantum code that reproduces certain properties of AdS/CFT, such as the Ryu-Takayanagi formula and subregion duality, much like other known holographic codes. However, the code becomes approximate when "coherent noise" is injected, allowing it to capture features analogous to those of gravity, such as back-reaction, subspace-dependence, and approximate bulk locality. I will then explain how entanglement data extracted from such kind of systems can be used to determine whether the bulk has a well-defined emergent geometry. When the bulk is "geometric", we show that one can explicitly reconstruct the spatial metric tensor through numerical methods.

### Alex May, University of British Columbia - 25 Nov 2020

**Title:** An Operational Approach to Holography

**Abstract:** Quantum tasks are quantum computations which have inputs and outputs that occur at designated spacetime locations. Understanding when tasks are possible to complete, and what resources are required to complete them, captures spacetime-specific aspects of quantum information. In this talk we explore how quantum tasks can be used to capture operational implications of the holographic principle. In the context of the AdS/CFT correspondence we find this operational approach leads to a novel connection between causal features of bulk geometry and boundary entanglement.