Recorded Group Seminars

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

Past talks

Hong Zhe Chen, Perimeter Institute - 6 Dec 2022

Title: Holography, the black hole information paradox, and branes / Asymptotic symmetries, entanglement, and holography

Abstract: This first half of my presentation will focus on the black hole information paradox --- the question of whether black holes destroy information. I will begin by introducing entanglement entropy and demonstrate how the information paradox arises semiclassically. Then, I will explain the exciting breakthroughs found recently (around 2019) in using AdS/CFT to (partially) resolve this paradox. This came with the discovery of `island' regions in black holes which are encoded by Hawking radiation. Finally, I will discuss the doubly-holographic brane world model studied in two of my papers [Chen:2020uac, Chen:2020hmv], which provide the first setups in which islands can be calculated analytically in dimensions greater than two. The second half of my presentation will discuss work in progress on asymptotic symmetries in asymptotically flat spacetime. After a motivational discussion in connection to other infrared phenomena in gauge theories, I will introduce asymptotic symmetries and their associated charges in the simplest context-free Maxwell theory. I will show how such long-distance concepts can be conformally mapped to more natural, or better studied short-distance ideas, namely: how asymptotic charge conservation is equated to a smoothness constraint on the trajectories of `image' sources passing through spacelike infinity; and how fluctuations of asymptotic charge restricted to a subregion of asymptotic null infinity are really just edge modes (which contribute to the entanglement entropy) of that region. Finally, I discuss how asymptotic charge conservation and the asymptotic entanglement structure of flat spacetime can be studied using holography (celestial and AdS/CFT)

Aidan Chatwin-Davies, University of British Columbia - 5 Dec 2022

Title: Information Theoretic Predictions for Quantum Gravitational Signals from Inflation

Abstract: The huge separation between the Planck scale and typical laboratory scales makes it extremely difficult to detect quantum gravitational effects; however, the situation is in principle much more favourable in cosmology. In particular, the Planck and Hubble scales were likely only separated by about 5 to 6 orders of magnitude during inflation. This motivates looking for present-day signatures of Planck-scale physics from the early universe. The question, then, is what quantum gravitational effects should we look for, and what are their observational signatures? Here I will discuss predictions for how an information theoretic, quantum gravity-motivated, natural UV cutoff manifests in primordial power spectra. The cutoff is model-independent, both in the sense that it does not rely on a particular UV completion of quantum gravity, nor does it assume a particular model of inflation. The predicted signature consists of small oscillations that are superimposed on the conventional primordial power spectra, where the template waveform is parameterized by the location of the cutoff between the Planck and Hubble scales. This will allow experiments to place new rigorous bounds on the scale at which quantum gravity effects become important.

Victor Godet, ICTS Bangalore - 5 Dec 2022

Title: Holography in (A)dS and the Wheeler-DeWitt equation

Abstract: In the first part of this talk, I will explain how the Wheeler-DeWitt equation implies a semi-classical version of holography in AdS. In the second part, I will describe how it can be solved for asymptotically de Sitter universes and its implications for de Sitter holography.

Eyoab Dejene Bahiru, SISSA - 2 Dec 2022

Title: Locality vs. perturbative quantum gravity

Abstract: Locality is a well established concept in quantum field theory. In general relativity, even though there are some subtleties, one can still understand it clearly. We will talk about tensions involving locality and perturbative quantum gravity. These tensions can be traced back to the gauge redundancies of general relativity and we will try to resolve them by defining local, diffeomorphism invariant operators in the context of AdS/CFT. This has several implications some of which will be discussed in the talk.

Simon Lin, University of Illinois Urbana-Champaigne - 1 Dec 2022

Title: Reflected entropy in random tensor networks

Abstract: Reflected entropy is a quantum informatic measure that is associated with the canonical purification, a generalization of the thermal field double. It is conjectured to be dual to twice the area of entanglement wedge (EW) cross-sections for holographic theories. In this talk I will discuss our attempt to verify the conjecture in random tensor networks (RTNs), a tensor network toy model for AdS/CFT. I will show that analytical calculations can be performed for simple networks consisting of one or two random tensors. These results are exact in the sense that they sum up the non-perturbative effects that smooths out the sharp jump of the reflected entropy near EW phase transition. I will also provide evidence that the conjecture is true for arbitrary networks, at least semiclassically. Finally, I will touch on the emergence of a type-II von Neumann algebra that arises during the computation of two random tensors, and the possible role it plays in the emergent bulk of the canonical purification.

Batoul Banihashemi, University of Maryland - 29 Nov 2022

Title: Quasilocal thermodynamics of cosmological horizons

Abstract: The entropy of de Sitter space was derived long ago by Gibbons and Hawking via a gravitational partition function. Since there is no boundary at which to define the temperature or energy of the ensemble, the statistical foundation of their approach deserves further study. To place the statistical ensemble on a firm footing we introduce an artificial “York boundary”, with either canonical or microcanonical boundary conditions, as has been done previously for black hole ensembles. The partition function and the density of states are expressed as integrals over paths in the constrained, spherically reduced phase space of pure 3+1 dimensional gravity with a positive cosmological constant. In the first part of my talk I briefly discuss issues related to the domain and contour of integration in the path integral, explain thermodynamic phases and (in)stability, and mention an evolving reservoir model that can stabilize the cosmological horizon in the canonical ensemble. In the second part of the talk I turn to the Gibbons-Hawking first law for the static patch of de Sitter space, according to which the entropy of the cosmological horizon is reduced by the addition of Killing energy. This raises the question how the thermodynamics of the static patch should be understood. I argue that the confusion arises because of a mistaken interpretation of the matter Killing energy as the total internal energy, and the puzzle is resolved when a proper thermodynamic ensemble is specified, again by introducing a system boundary. Finally, I explain how in the shrinking boundary limit the first law as well as the Gibbons-Hawking partition function on the 4-sphere is understood. I will also briefly discuss an upcoming work on deriving the horizon entropy from the Lorentzian path integral.

Jyotirmoy Mukherjee, Indian Institute of Science in Bangalore - 25 Nov 2022

Title: Entanglement entropy of gravitational edge modes

Abstract: We introduce the importance of edge modes in the evaluation of entanglement entropy of subregions in U(1) gauge theory. We evaluate the contribution of these modes towards the universal logarithmic coefficient of entanglement across a spherical spatial region in U(1) gauge theory in even d dimensions. We show that this agrees with the logarithmic divergent part of the edge Harish-Chandra character of the theory on the d-sphere. We then consider the theory of linearized gravitons in 4 space-time dimensions. Quantizing the theory in tensor spherical harmonics we evaluate the contribution of the edge modes of the graviton towards the entanglement of a spatial region. We observe that this coefficient coincides with that extracted from the edge Harish-Chandra character of the massless spin-2 field on the S^4.

Andrew Rolph, University of Amsterdam - 21 Nov 2022

Title: Quantum bit threads

Abstract: In this talk, I will describe a flow-based (bit thread) formulation of holographic entanglement entropy which is valid to all orders in $1/N$. The goal is to give a prescription that is equivalent to the quantum extremal surface prescription and the motivation is to understand developments in bulk entanglement, islands and black holes through the lens of bit threads. I will introduce and explain the proposal, give the proof of equivalence to the quantum extremal surface formula, describe how bit threads behave in doubly holographic models, and explain in what sense islands and bulk spacetime itself are emergent from the quantum bit thread perspective. Lastly, I will outline work in progress on covariant quantum bit threads, a generalisation to non-time-reflection-symmetric states, and applications of threads to entanglement in de Sitter and cosmology.

Francesco Sartini, ENS Lyon - 16 Nov 2022

Title: Hidden symmetries in cosmology and black holes

Abstract: Cosmological models and black holes belong to classes of space-time metrics defined in terms of a finite number of degrees of freedom, for which the Einstein–Hilbert action reduces to a one-dimensional mechanical model. We investigate their classical symmetries and the algebra of the corresponding Noether charges. These dynamical symmetries have a geometric interpretation, not in terms of spacetime geometry, but in terms of motion on the field space. Moreover, they interplay with the fiducial scales, introduced to regulate the homogenous model, suggesting a relationship with the boundary symmetries of the full theory. Finally, the existence of these symmetries unravels new aspects of the physics of black holes and cosmology. It opens the way towards a rigorous group quantization of the reduced models, to the study of their holographic properties and might have significant consequences on the propagation of test fields and the corresponding perturbation theory.

Yuki Yokokura, Riken - 15 Nov 2022

Title: Entropy-Area Law from Interior Semi-classical Degrees of Freedom

Abstract: In this talk, we focus on the interior of a most typical black hole and consider it as a bound state consisting of many semi-classical degrees of freedom. The distribution of their information determines the interior metric through the semi-classical Einstein equation. Then, the interior is a continuous stacking of AdS_2 times S^2 without horizon or singularity and behaves like a local thermal state. Evaluating the entropy density from thermodynamic relations and integrating it over the interior volume, the area law is obtained with the factor 1/4 for any interior degrees of freedom. Here, the dynamics of gravity plays an essential role in changing the entropy from the volume law to the area law. This should help us clarify the holographic property of black-hole entropy. [arXiv: 2207.14274]

Rifath Khan, Cambridge - 14 Nov 2022

Title: Asymptotic structure of GR

Abstract: In the Canonical theory of Quantum Gravity (CQG), states are given by the superposition of geometries on a Cauchy slice, called the Wheeler-DeWitt (WDW) states. On the other hand, the Holographic principle states that quantum gravity in d+1 spacetime dimensions is the same as a quantum field theory in d spacetime dimensions. In this talk, I will briefly review both of these and will explain how to reformulate CQG as a holographic theory by defining a new holographic dictionary that maps any state of the boundary field theory to a bulk WDW state. This dictionary is an isomorphism between the Hilbert space of CQG and holographic CFT. This also reformulates the holographic principle in a way that the dual field theory now lives on Cauchy slices of the bulk, hence applicable to dS and flat spacetimes too. I will then explain why this is a manifestly background independent theory of “effective” quantum gravity. Time permitting, I will also discuss UV completion of quantum gravity, emergence of classical spacetime from WDW states and its possible implications for the black hole information paradox and holographic cosmology. Based on work with Goncalo Araujo-Regado and Aron C. Wall: arXiv:2204.00591 and an upcoming paper.

Marc Geiller, ENS Lyon - 24 Oct 2022

Title: Cauchy Slice Holography

Abstract: I will review some recent developments in the investigation of the asymptotic structure of general relativity, and in particular the various extensions of the asymptotic symmetry group which have been proposed in the literature.

Steffen Gielen, University of Sheffield - 17 Oct 2022

Title: Unitarity and clock dependence in quantum cosmology

Abstract: I will discuss a clash between the notions of unitarity in quantum mechanics and general covariance in general relativity, using the example of a simple cosmological model. General relativity tells us that time evolution should be phrased in relational terms, by describing the change in a given variable relative to a "clock" which is itself a dynamical variable. There is then in general a "multiple choice problem" of which out of several candidates is to be selected as the clock; the cosmological model used here contains (at least) three possible clocks. Requiring unitary time evolution can lead to very different physical consequences depending on which clock is used. In our model this affects questions such as whether the classical Big Bang singularity is "resolved", or whether the Universe has a maximal size.

Bianca Dittrich, Perimeter Institute - 12 Oct 2022

Title: Progress and challenges for the Lorentzian quantum gravity path integral

Abstract: Euclidean quantum gravity approaches have a long history but suffer from a number of severe and persisting issues. This gives a strong motivation to push the development of Lorentzian approaches. The Lorentzian path integral allows to tackle many open and highly interesting questions about Lorentzian quantum space times and quantum causal structures. But Lorentzian path integrals in general come with taxing computational obstacles, and highly interesting, if challenging, conceptual questions. I will present progress in overcoming the computational obstacles and first results that address open conceptual questions.

Luca Marchetti, LMU Munich - 20 Sep 2022

Title: Emergent Cosmology from (T)GFT Condensates

Abstract: I will highlight some of the challenges related to the extraction of continuum physics from quantum gravity and I will discuss how they can be addressed with effective methods. More concretely, applying these techniques to the (T)GFT approach to quantum gravity, I will show how (homogeneous and slightly inhomogeneous) cosmological physics emerges from the collective behavior of microscopic quantum geometric degrees of freedom.

Slides for Luca's talk can be found here.

Miguel Campiglia, Universidad de la Rebública - 13 Sep 2022

Title: Symmetries of asymptotically flat spacetimes

Abstract: In this talk I will review the link between symmetries of asymptotically flat spacetimes and the low frequency limit of gravitational radiation. Building on well-established results, I will present some open problems and current efforts towards their resolution.

Jordan Francois, Univerity of Mons - 13 Jun 2022

Title: Presymplectic structure of gauge theories: boundaries, edge modes, variational connections and all that…

Abstract: The boundary problem is the failure to associate a symplectic structure to a gauge theory over a bounded region of spacetime. Two strategies to circumvent this problem have been much discussed in the recent literature: the edge modes approach by Donnelly & Freidel, and the connection approach by Gomes & Riello. Relying on the bundle geometry of field space, we attempt to make them more systematic, so as to facilitate comparison and to shed some light on conceptual aspects. We illustrate the general results with the standard examples of Yang-Mills theory, the Cartan formulation of General Relativity, and Chern-Simons theory -- thereby reproducing several results of the literature.

Alexandre Belin, CERN - 19 Apr 2022

Title: Quantum chaos, OPE coefficients and wormholes

Abstract: In this talk, I will discuss the statistical distribution of OPE coefficients in chaotic conformal field theories. I will present the OPE Randomness Hypothesis (ORH), a generalization of ETH to CFTs which treats any OPE coefficient involving a heavy operator as a pseudo-random variable with an approximate Gaussian distribution. I will then present some evidence for this conjecture, based on the size of the non-Gaussianities and on insights from random matrix theory. Turning to the bulk, I will argue that semi-classical gravity geometrizes these statistical correlations by wormhole geometries. I will show that the non-Gaussianities of the OPE coefficients predict a new connected wormhole geometry that dominates over the genus-2 wormhole.

Manus Visser, University of Geneva - 5 Apr 2022

Title: De Sitter Entropy and the Gravitational Path Integral

Abstract: Gibbons and Hawking famously derived the de Sitter horizon entropy from the Euclidean gravitational path integral in the saddle point approximation. This result is a crucial hint for quantum gravity in de Sitter space, as it suggests that the Hilbert space is finite dimensional. In this talk we extend this result in two different ways. First, we compute the entropy of de Sitter black holes from the on-shell Euclidean action, and take their contributions in the gravitational path integral into account using the formalism of constrained instantons. We use this to calculate the pair creation rate of arbitrary mass black holes in de Sitter space. Second, in two-dimensional de Sitter space we show how the generalized entropy (the sum of the classical gravitational entropy and matter entanglement entropy) can be obtained from an on-shell action for semiclassical dilaton gravity in the microcanonical ensemble. Minimizing the action yields extremizing the generalized entropy, consistent with the island formula.

Josiah Couch, Boston College - 29 Mar 2022

Title: Pants Decomposition as Circuit Complexity in 2D (T)QFT

Abstract: Motivated by the 'complexity = volume' and 'complexity = action' proposals of Susskind and collaborators, there have been many efforts over the past several years to study circuit complexity in the context of quantum field theory. I will present such an effort proposing that the pants decomposition of 2d manifolds gives a notion of complexity for 2D TQFTs, at least for states that can be prepared by Euclidean path integrals, along with related ideas. Based on https://arxiv.org/abs/2108.13427.

Antony Speranza, University of Illinois Urbana-Champaign - 15 Mar 2022

Title: Local gravitational charges and their algebra

Abstract: Charges associated with diffeomorphisms constitute an important set of observables in gravitational theories. In this talk, I will give an overview of the construction of gravitational charges for local subregions using covariant phase space techniques. For diffeomorphisms with a transverse component to the boundary, the charges are “nonintegrable,” in the sense that they fail to satisfy Hamilton’s equation due to the presence of fluxes, but can be defined via the Wald-Zoupas procedure. I will specifically emphasize that the ambiguities inherent in the Wald-Zoupas construction, as well as those present in covariant phase space constructions, can be fully resolved by a choice of action for the spacetime subregion. An algebra for the charges can be defined by their Poisson bracket on the subregion phase space, and I will show that this reproduces the bracket previously postulated by Barnich and Troessaert, thereby giving a novel derivation of this bracket from first principles. Finally, I will describe a related construction of gravitational charges using the extended phase space, in which additional edge mode degrees of freedom are included in the description. The extended phase space construction yields Hamiltonian (i.e. “integrable”) charges without employing the Wald-Zoupas procedure, and I further show that ambiguities in this construction are again resolved by appealing to the variational principle associated with the subregion action.

Jinzhao Wang, ETH Zurich - 7 Mar 2022

Title: The black hole information puzzle and the quantum de Finetti theorem

Abstract: The black hole information puzzle arises from a discrepancy between conclusions drawn from general relativity and quantum theory about the nature of the radiation emitted by a black hole. According to Hawking's original argument, the radiation is thermal and its entropy thus increases monotonically as the black hole evaporates. Conversely, due to the reversibility of time evolution according to quantum theory, the radiation entropy should start to decrease after a certain time, as predicted by the Page curve. This decrease has been confirmed by new calculations based on the replica trick, which also exhibit its geometrical origin: spacetime wormholes that form between the replicas. Here we analyse the discrepancy between these and Hawking's original conclusions from a quantum information theory viewpoint, using in particular the quantum de Finetti theorem. The theorem implies the existence of extra information, W, which is neither part of the black hole nor the radiation, but plays the role of a reference. The entropy obtained via the replica trick can then be identified to be the entropy S(R|W) of the radiation conditioned on the reference W, whereas Hawking's original result corresponds to the non-conditional entropy S(R). The entropy S(R|W), which mathematically is an ensemble average, gains an operational meaning in an experiment with N independently prepared black holes: For large N, it equals the regularized entropy of their joint radiation, S(R1⋯RN)/N. The discrepancy between this entropy and S(R) implies that the black holes are correlated, that is geometrically captured by the replica wormholes. I will also comment on the ensemble interpretation of the replica trick calculation. (Based on the joint work (arxiv.org/abs/2110.14653) with Renato Renner.)

Lucas Hackl, University of Melbourne - 28 Feb 2022

Title: Volume-law entanglement entropy of typical pure quantum states

Abstract: In this talk, I will discuss the statistical properties of entanglement entropy, which serves as a natural measure of quantum correlations between a subsystem and its complement. Entanglement is a defining feature of quantum theory and understanding its statistical properties has applications in many areas of physics (quantum information, statistical mechanics, condensed matter physics, black hole thermodynamics). First, I will introduce the physical model and explain its relevance for practical applications. Second, I will explain how the statistical ensemble of quantum states can naturally be described through the methods of random matrix theory. Third and finally, I review a number of new results describing the typical properties (e.g., average, variance) of the entanglement entropy for various ensembles of quantum states (general vs. Gaussian, arbitrary vs. fixed particle number). [based on arXiv:2112.06959 and arXiv:2103.05416]

Stefan Eccles, OIST - 7 Feb 2022

Title: Information spreading in chaotic quantum systems

Abstract: In chaotic quantum systems, information that is initially localized will generically spread and become inaccessible to local measurements. I discuss some tools for quantifying the rate at information spreads, and advocate a conceptual framework for viewing the process via an analogy with the Hayden-Preskill protocol of information recovery from black holes.

Phuc Nguyen, City University of New York and University of Haifa - 1 Feb 2022

Title: Scrambling and the black hole atmosphere

Abstract: 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.

Laurent Freidel, Perimeter Institute - 10 Dec 2021

Title: Local Holography: A quantum gravity paradigm to construct gravitational subsystems

Abstract: In this talk I will present a new perspective about the decomposition of gravitational systems into subsystems called local holography. I will emphasize the central role played by what we call the corner symmetry group in capturing all the necessary data need to glue back seamlessly quantum spacetime regions. I will explain some of the key results we have achieved in the construction of the representations of these groups. If time permits I will explain a new result about the canonical description of open gravitational systems and the relations of this approach with celestial holography.

Robert Oeckl, UNAM - 7 Dec 2021

Title: Hands on with the positive formalism

Abstract: This talk is meant as an application-oriented overview of the positive formalism. One emphasis is on its relation to frameworks from classical statistical mechanics, quantum foundations/information, quantum field theory and quantum gravity. The other emphasis is on simple and concrete examples from these different fields.

Slava Lysov, OIST - 15 Nov 2021

Title: Phase space on surface with the boundary via symplectic reduction

Abstract: In my talk I will briefly review the symplectic reduction construction for the phase space of the gauge theory using BF theory as an example. Then I will talk about possible problems and consistency conditions related to extension of the symplectic reduction construction for the Cauchy slices with boundary. I will perform the symplectic reduction in presence of boundary and show that results obey consistency conditions and match with symplectic spaces constructed by other methods.

Francesco Sartini, ENS Lyon - 8 Nov 2021

Title: Hidden symmetries in black holes

Abstract: The spacetime in the interior of a black hole can be described by an homogeneous line element, for which the Einstein–Hilbert action reduces to a one-dimensional mechanical model. We have shown that this model exhibits a symmetry under the (2+1)-dimensional Poincaré group. The existence of this symmetry unravels new aspects of symmetry for black holes, and opens the way towards a rigorous group quantization of the interior, which in turn provide a powerful tool to discriminate between different regularization schemes. Remarkably, the physical ISO(2,1) symmetry can be seen as a broken infinite-dimensional symmetry. This is done by reinterpreting the action for the model as a geometric action for the BMS3 group, where the configuration space variables are elements of the algebra bms3 and the equations of motion transform as coadjoint vectors.

Yasha Neiman, OIST - 27 Oct 2021

Title: A microscopic derivation of the quantum measurement postulates

Abstract: There is a disconnect between the "quantum foundations" community and practicing quantum physicists. Since the foundations community is incapable of agreeing on anything, the outside world is left with the impression that they accomplished nothing. Fortunately, this impression is false. In this talk, I will package some ideas from the foundations community into a neat derivation of the Copenhagen measurement postulates from a simple & explicit microscopic model. Because of the disconnect mentioned above, practising physicists tend to find this construction surprising and enlightening, while foundations people find it simultaneously false, trivial, misguided, naive, pointless and unoriginal. Come and find out which community you belong to!

Fabio Anza, UC Davis - 19 Oct 2021

Title: Quantum Physics of Information

Abstract: In this talk I will present a series of ideas and results coming out of my efforts to exploit differential-geometric tools to tackle many body quantum systems and, in particular, characterize the information theoretic properties of their dynamics. After a broad but brief look at the idea behind the physics of information, I will give an introduction to geometric quantum mechanics, discuss its relevance for thermodynamics how I intend to leverage it to address the out-of-equilibrium physics of many-body quantum systems.

Eduardo Testé, UCSB - 12 Oct 2021

Title: Mutual information superadditivity and unitarity bounds

Abstract: We derive the property of strong superadditivity of mutual information arising from the Markov property of the vacuum state in a conformal field theory and strong subadditivity of entanglement entropy. We show this inequality encodes unitarity bounds for different types of fields. These unitarity bounds are precisely the ones that saturate for free fields. This has a natural explanation in terms of the possibility of localizing algebras on null surfaces. A particular continuity property of mutual information characterizes free fields from the entropic point of view. We derive a general formula for the leading long distance term of the mutual information for regions of arbitrary shape which involves the modular flow of these regions. We obtain the general form of this leading term for two spheres with arbitrary orientations in spacetime, and for primary fields of any tensor representation. For free fields we further obtain the explicit form of the leading term for arbitrary regions with boundaries on null cones.

Djordje Racidevic, Brandeis - 4 Oct 2021

Title: An Introduction to the Lattice-Continuum Correspondence

Abstract: I will describe a simple and explicit way to construct continuum quantum field theories out of finite, nonperturbatively well defined lattice theories. This formalizes decades of intuition, leads to new definitions of various QFT concepts, and provides new insights into phase diagrams, dualities, and inherent limitations of well known QFTs. I will give a gentle formal introduction to this procedure, and will pick a few simple examples to illustrate it. Topics I may discuss (time permitting) include phases of clock models and Abelian gauge theories, the Noether theorem for discrete symmetries, lattice origins of contact terms and induced Chern-Simons actions, global anomalies in lattice QED, and new definitions of supersymmetric QFTs.

Juan Pedraza, University of Barcelona - 27 Sep 2021

Title: Lorentzian threads and holographic complexity

Abstract: The continuous min flow-max cut principle is used to reformulate the 'complexity=volume' conjecture using Lorentzian flows. Conceptually, discretized flows are interpreted in terms of `gatelines' that connect to a tensor network, such that the bulk calculation matches its information-theoretic definition. The bulk symplectic potential provides a 'canonical' configuration characterizing perturbations around arbitrary CFT states. Its consistency requires the bulk to obey linearized Einstein's equations, which are shown to be equivalent to the holographic first law of complexity, thereby advocating a notion of 'spacetime complexity'. Finally, we explain the need for a more general measure of complexity that captures the role of suboptimal flows or tensor network configurations. This talk is based on 2105.12735 and 2106.12585.

Etera Livine, ENS Lyon - 17 May 2021

Title: Bulk-boundary in loop quantum gravity

Abstract: I will discuss the notions of bulk spin network states and spin network boundary states and how they can be used to formulate the questions of holography and coarse-graining in the framework of loop quantum gravity.

Ana Alonso-Serrano, AEI Potsdam - 12 Apr 2021

Title: Quantum gravity phenomenology from thermodynamics of spacetime

Abstract: This work is based on the formalism developed in thermodynamics of spacetime to derive Einstein's equations from the proportionality of entropy with the area. When low energy quantum gravity effects are considered, an extra logarithmic term in the area is added to the entropy expression. Here, I will present the derivation of the quantum modified gravitational dynamics from this modified entropy and discuss its main features. Furthermore, I will outline the application of the modified dynamics to cosmology, suggesting the replacement of the Big Bang singularity by a regular bounce.

Christophe Goeller, ENS de Lyon - 22 Mar 2021

Title: A finite construction of asymptotic symmetries in 3D gravity

Abstract: In this talk, I will review the construction of the boundary symmetry algebra in first order 3D gravity in its most general framework, namely via the Mielke-Baekler Lagrangian, allowing for a source of curvature and torsion. I will show how the study of the current algebra and its associated Sugawara construction allows for two notions of quadratic charges: the usual diffeomorphism and its "dual". I will discuss their resulting algebra and its relation with the usual construction of the asymptotic boundary algebra. I will show that properties that are often associated to the asymptotic case, namely the construction of the double copy of Virasoro (or BMS in the flat limit) as symmetry algebra are in fact already present at finite distance.

Aldo Riello, UL Brussels - 1 Mar 2021, 8 Mar 2021

Title: Edge modes without edge modes: cutting and gluing in Yang-Mills gauge theory

Abstract: In this two-part talk, I will discuss gauge theories of the Yang-Mills kind in finite regions with boundaries.

In the first talk, I will address the “cutting problem”, namely the problem of giving a definition of the gauge-invariant degrees of freedom (dof) intrinsic to a finite and bounded region. By analyzing the symplectic reduction procedure in the presence of boundaries, and how boundaries interact with the Gauss constraint, I will show that the reduced phase space is not a symplectic space, but a symplectic foliation whose leaves correspond to electric-flux superselection sectors. In each superselection sector, the relevant dof are a regional generalization of the usual radiative dof (i.e. transverse photons). Key to obtaining this result is our indiscriminate treatment of bulk and boundary gauge transformations, which means no "edge modes" are ever introduced. The absence of edge modes, however, could raise the suspicion that some crucial information is missing, e.g. in relation to one’s ability to reconstruct the global radiative dof from the regional ones. This concern will be addressed in the second talk.

Indeed, in the second talk I will address the two-fold “gluing problem”. Whereas the first gluing problem concerns the reconstruction of the (usual) global radiative dof from the regional dof in each superselection sector, the second gluing problem is the well-known problem of explaining the non-factorizability of the gauge-invariant phase space over regions, namely the emergence, upon gluing, of new (gauge invariant) dof. Absent edge modes, only the regional radiative dof in each superselection sector are available in the discussion of the first and second gluing problems. But this raises an “edge-modes-without-edge-modes" puzzle: if all global radiative dof are reconstructible from their regional counterparts, how can new dof emerge upon gluing? My goal for this second talk will be to explain the resolution of the “edge-modes-without-edge-modes” puzzle. The resolution relies on the nonlocal nature of the regional radiative dof and on the relational nature of the new reconstructed dof. One of this new dof is the electric flux itself: understanding its reconstruction will shed further light on its superselection.

The main reference for this talk are: 2010.15894 for part 1, and section 6 of 2007.04013 (with H . Gomes) for pat 2. Note: whereas these papers strive for generality, in the talk I will for the most part focus on Maxwell theory, which, being Abelian, allows for a much simpler treatment. Only brief references will be made to the non-Abelian theory, aimed at emphasizing its extra features.

Josh Kirklin, OIST - 25 Feb 2021

Title: Uhlmann Phase, Black Hole Information and Holography

Abstract: In the 1980s, Armin Uhlmann described a natural generalisation of Berry phase to mixed states which has come to be known as Uhlmann phase. I will describe a series of recent results characterising the Uhlmann phase of subsystems in quantum gravity. First, assuming replica wormholes contribute to the path integral, I will show that measurements of the Uhlmann phase of Hawking radiation allows one to reconstruct the phase space of the interior of a black hole (past the Page time), providing a protocol for the recovery of all infalling classical information. This is a generalisation of a previous result concerning entanglement wedges in holography. Next, I describe a path integral formula for the Uhlmann phase of a generic highly entangled system, explaining how it appears to inevitably involve an extra dimension generated by modular flow, despite a lack of any holographic assumptions. Finally, I comment on the operational meaning of Uhlmann phase, and how it's significance may relate to decoherence.

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.