Seminars

Title: “Holography of traversable wormholes: non-maximal chaos and teleportation”

Abstract:
I will discuss the time-ordered and time-disordered correlation functions in the background of the Gao-Jafferis-Wall (GJW) traversable wormhole. The time-disordered correlators exhibit a spectrum of Lyapunov exponents with some or all exponents below the maximal chaos bound. This is rooted in the absence of the black hole horizon and the non-local coupling necessary to produce the wormhole in the first place. The question is now: what is the field theory dual of the GJW wormhole? In principle, we may interpret the wormhole as an EPR pair of entangled systems allowing teleportation, or as a single system in contact with a heat bath. In the former case the Lyapunov exponents are related to the fidelity of the teleportation and in the latter to the quantum chaos in the system. To understand this dichotomy I consider the scalar field on the wormhole background and find a large freedom in defining the boundary action on the two-sided boundary, corresponding to the symmetry of thermofield dynamics (TFD) Hamiltonians. The details of the field theory dual thus depend on the choice of the boundary terms in the action.

Title: “Detecting chaos in hydrodynamics”

Abstract:
Hydrodynamics assumes local equilibration and equilibration assumes ergodic mixing driven by chaos — at least in semi-classical systems. For a generic such system the timescales of macroscopic thermalization and onset of microscopic chaos are very different. Nevertheless, it is a pillar of Boltzmann transport that long-time equilibration can be computed from microscopic dynamics. We show how in these systems the two timescales are in fact related. Moreover, we show that a similar connection between both scales surprisingly also exists in extremely strongly coupled systems through a phenomenon christened pole-skipping in hydrodynamic response.

Title: “Entanglement entropies of equilibrated pure states and the origin of replica wormholes”

Abstract:
I will explain an approximation method that gives a simple universal expression for the Renyi entropies of an equilibrated pure state in a chaotic quantum many-body system. This expression is independent of the details of the initial state and hence reflects thermalization, while also being manifestly consistent with unitarity. I will discuss how applying this method to models of evaporating black holes leads to a derivation of replica wormholes, which have recently been used to address the information loss paradox in these models. More generally, this approach elucidates the role played by Euclidean path integrals in calculations of time-evolved Renyi entropies. It also helps address the question of whether an average over theories is needed in order to explain results from replica wormholes. 

Slides:

Title: “Chiral hydrodynamics of plasma in strong magnetic fields”

Abstract:
In this presentation I will motivate and construct the hydrodynamic description of a chiral fermion fluid subject to a strong magnetic field. Such a description can be, for example, applied to the quark-gluon-plasma phase generated in heavy-ion-collisions, or to Weyl semimetals. Kubo formulae are computed which relate 22 transport coefficients to particular correlation functions. Among those transport coefficients, 8 are novel. Known transport coefficients, such as the Hall viscosity and Hall conductivity, are now splitting into two each, one longitudinal and one transverse to the magnetic field. We provide a successful validity check by computing all transport coefficients in a specific holographic model.

Slides:

Title: “Towards chaos: scattering with a highly excited string”

Title: “Slow scrambling in extremal BTZ and microstate geometries”

Abstract:
Out-of-time-order correlators (OTOCs) that capture maximally chaotic properties of a black hole are determined by scattering processes near the horizon. This prompts the question to what extent OTOCs display chaotic behaviour in horizonless microstate geometries. This question is complicated by the fact that Lyapunov growth of OTOCs requires nonzero temperature, whereas constructions of microstate geometries have been mostly restricted to extremal black holes. 

We compute OTOCs for a class of extremal black holes, namely maximally rotating BTZ black holes, and show that on average they display “slow scrambling”, characterized by cubic (rather than exponential) growth. Superposed on this average power-law growth is a sawtooth pattern, whose steep parts correspond to brief periods of Lyapunov growth associated to the nonzero temperature of the right-moving degrees of freedom in a dual conformal field theory. 

Next we study the extent to which these OTOCs are modified in certain “superstrata”, horizonless microstate geometries corresponding to these black holes. Rather than an infinite throat ending on a horizon, these geometries have a very deep but finite throat ending in a cap. We find that the superstrata display the same slow scrambling as maximally rotating BTZ black holes, except that for large enough time intervals the growth of the OTOC is cut off by effects related to the cap region, some of which we evaluate explicitly.

Title: “Hydrodynamic Fluctuations in Spin Chains and QFTs”

Abstract:
Hydrodynamics – broadly understood as the late-time dynamics of slow excitations – is distinguished by its sparing assumption: thermalization. Its applications are therefore widespread. This talk will focus on consequences of stochastic fluctuations of these slow excitations. When such fluctuations are irrelevant, hydrodynamics serves as a controlled EFT of thermalizing systems: I will show how this EFT can be used to resolve a recent puzzle in the thermalization of SU(2) spin chains, and how it leads to a surprising prediction of a “diffuson cascade”. When fluctuations are strong, diffusion breaks down and is typically replaced by dissipation in the KPZ universality class. I will discuss how this applies to 2d QFTs and the edges of quantum Hall systems.

Slides:

Title: “On systems of maximal chaos”

Title: “Hydrodynamic diffusion and its breakdown near AdS2 fixed points”

Slides:

Time: 15:00 instead of the usual 13:00

Title: “Pole skipping and related probes away from maximal chaos”

Abstract:
Quantum chaotic dynamics is associated to diverse physical phenomena and signatures. In this talk, we focus on the pole skipping phenomenon in non-maximally chaotic theories. Our discussion is informed by results from AdS/CFT and from models of SYK type. We study the interplay of the pole skipping phenomenon with the quantum butterfly effect and thermalization through the lens of entanglement entropy.

Slides: