Workshop on Non-Equilibrium Physics and Holography

Programme

Tuesday 12 July Wednesday 13 July Thursday 14 July Friday 15 July
09.00 - 10.00 09.00 - 09.45 Registration
09.45 - 10.00 Introductory Remarks
10.00 - 11.30 Hong Liu Subir Sachdev Pavel Kovtun Amos Yarom
11.30 - 11.45 Coffee Break
11.45 - 13.15 Joerg Schmiedmayer John Thomas Paul Romatschke Joe Bhaseen
13.15 - 14.30 Lunch Break
14.30 - 16.00 Juergen Berges Julian Sonner Sean Hartnoll Departure
16.00 - 16.30 Coffee Break
16.30 - 18.00 Ulrich Schneider Stefan Kehrein Philip Phillips

Speakers and Talks (slides and audio)

Juergen Berges (U. Heidelberg): Universality far from equilibrium

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Abstract: New universality classes of many-body systems far from equilibrium provide unexpected links between seemingly disparate physical systems on vastly different energy scales. I will present evidence that characteristic dynamical properties of non-Abelian plasmas in the ultra-relativistic limit can also be found in superfluid Bose gases. As a consequence, compact table-top experiments with ultracold atoms may help to understand aspects of the thermalization process of heavy-ion collisions. I will describe how the nonequilibrium time evolution in the universal regime is described in terms of dynamical scaling exponents and scaling functions associated with nonthermal fixed points, which is similar in spirit to the description of critical phenomena in thermal equilibrium. The values of the nonthermal scaling exponents differ in general from their thermal counterparts, and there is no fine-tuning involved in contrast to the required tuning to some critical temperature or other parameter for the equilibrium case.

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Joe Bhaseen (King's College London): Non-Equilibrium Energy Flow: From Shocks to Rarefaction Waves

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Abstract: We review advances in understanding non-equilibrium energy flow in arbitrary dimensions [1,2]. We discuss the fertile links between condensed matter, gauge-gravity duality and hydrodynamics.
[1] Bhaseen, Doyon, Lucas and Schalm, "Energy flow in quantum critical systems far from equilibrium", Nature Physics 11, 509 (2015).
[2] Lucas, Schalm, Doyon, and Bhaseen, "Shock waves, rarefaction waves, and nonequilibrium steady states in quantum critical systems", Phys. Rev. D 94, 025004 (2016).

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Sean Hartnoll (Stanford): Hydrodynamic theory of phase-fluctuating superconductivity

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Abstract: Superconductors can support dissipationless electric current due to macroscopic phase coherence. Vortices are the enemy of superconductivity: as vortices move around the sample, they can disorder the coherent phase and destroy superconductivity. It turns out that several deep and controversial issues in condensed matter physics have to do with whether `phase-disordered’ superconductors can exist at zero temperature, as I will review. I will describe an effective field theory approach to this problem that clarifies some of the issues and leads to an elegant rederivation of some known results.

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Stefan Kehrein (U. Göttingen): Reversibility and Irreversibility in Quantum Many-Body Systems

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Abstract: The question of thermalisation in closed quantum many-body systems has received a lot of attention in recent years. A closely related question is whether a closed quantum system shows irreversible dynamics and what me mean by this. While in classical systems the notion of irreversibility is well understood, a useful definition of irreversibility for quantum many-body systems is not obvious. In this talk I will look at possible definitions and present results for integrable and non-integrable quantum many-body systems.

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Pavel Kovtun (U. Victoria): Hydrodynamics of polarized relativistic matter

Abstract: I will describe the thermodynamics of isotropic relativistic matter subject to external electromagnetic and gravitational fields, taking into account the effects of electric and magnetic polarization. The polarization vectors are intrinsically ambiguous, but the physical effects are not. Thermodynamics can be extended to hydrodynamics in which electric and magnetic fields are dynamical and couple to thermal and mechanical degrees of freedom.

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Hong Liu (MIT): Thoughts on theoretical approaches to non-equilibrium physics

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Abstract: In the first half of my talk I am planning to summarize various insights from holography into non-equilibrium physics.
In the second half, I am planning to discuss the fluid effective field theory we developed recently focusing mainly on the main physical picture and what we hope it can help us accomplish.

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Philip Phillips (U. Illinois): Non-Local Boundary Actions and Anomalous Dimensions: Application to the Strange Metal

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Abstract: I will critically examine the bulk boundary correspondence in gauge-gravity duality with an eye for identifying the boundary operator dual to bulk fields. For the simplest case of a free bulk scalar field, I will show that at large $N$, the boundary operator is the fractional Laplacian. Such an operator is non-local in the sense that computing the fractional Laplacian of a function requires knowledge of the function over all space. Boundary locality is shown to arise if the metric is geodesically incomplete. I generalize this result to a gauge field and show how boundary non-local actions arise for gauge theories. In such non-local actions, the gauge field acquires an anomalous dimension. I will then review recent proposals for the strange metal in the cuprates in which not only the current but the vector potential as well has an anomalous dimension. I will show that the fractional Aharonov-Bohm effect can be used to detect the existence of anomalous dimensions for the gauge field. I will close by showing how such non-local theories can explain the f-sum rule violation in the cuprates.

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Paul Romatschke (U. Colorado): String-theory Inspired Predictions for Novel Collective modes in Cold Atom Experiments

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Abstract: Very different strongly interacting quantum systems such as Fermi gases, quark-gluon plasmas and black holes are known to exhibit quantitatively similar damping of hydrodynamic modes. It is not known if such similarities extend beyond the hydrodynamic limit. Do non-hydrodynamic collective modes in Fermi gases with strong interactions also match those from string theory calculations? In order to answer this question, predictions for novel types of modes outside the hydrodynamic regime in trapped Fermi gases based on Lifshitz black holes are made. These predictions are amenable to direct testing with current state-of-the-art cold atom experiments.

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Subir Sachdev (Harvard U.): Quantum matter without quasiparticles: random fermion models, black holes, and graphene

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Abstract: Strongly interacting states of matter are characterized by a shortest possible “thermalization” or “phase coherence” or “Lyapunov” or “scrambling” time of order hbar/( k_B T). I will describe solvable random fermion models which realize such a state of quantum matter. Their low energy description matches closely the properties of quantum gravity near AdS2 black hole horizons. These connections inspired a theory of thermoelectric transport in a non-quasiparticle regime of graphene near its charge-neutrality point. I will describe the results of recent experiments, which are largely compatible with the theoretical predictions.

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Joerg Schmiedmayer (Vienna Center for Quantum Science and Technology (VCQ), TU Vienna): Scrambling, Relaxation and the emergence of thermalization in an isolated many body quantum systems

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Abstract: The evolution of an isolated quantum system is unitary. This is simple to probe for small systems consisting of few non-interacting particles. But what happens if the system becomes large and its constituents interact? In general, one will not be able to follow the evolution of the complex many body eigenstates. Ultra cold quantum gases are an ideal system to probe these aspects of many body quantum physics and the related quantum fields. Our pet systems are one-dimensional Bose-gases. Interfering two systems allows studying coherence between the two quantum fields and the full distribution functions and correlation functions give detailed insight into the many body states and their non-equilibrium evolution. In our experiments we study how the coherence created between the two isolated one-dimensional quantum gases by coherent splitting slowly degrades by coupling to the many internal degrees of freedom available [1]. We find that a one-dimensional quantum system relaxes to a pre-thermalisatized quasi steady state [2] which emerges through a light cone like spreading of ’de-coherence’ [3]. The pre-thermalized state is described by a generalized Gibbs ensemble [4]. Finally, we investigate the further evolution away from the pre-thermalized state. On one hand we show that by engineering the Quasiparticles we can create many body quantum revivals. On the other hand, we point to two distinct ways for further relaxation towards a final state that appears indistinguishable from a thermally relaxed state. The system looks like two classically separated objects. This illustrates how classical physics can emerge from unitary evolution of a complex enough quantum system. In a further experiment we study the quantum Sine-Gordon model realized by two tunnel coupled 1d super fluids. Measuring high order correlation functions and evaluation how they factorize allows to characterize the essential features of the SG model: the relevant quasi-particles, their interactions and the different topologically distinct vacuum-states the quasi-particles live in [5]. The experiment thus provides comprehensive insight into the components needed to solve a non-trivial quantum field theory and gives further insight into the steady states, and if they are thermal. We conjecture that our experiments points to a universal way through which relaxation in isolated many body quantum systems proceeds if the low energy dynamics is dominated by scrambling of the eigenmodes of long lived excitations (quasi particles) [6].
Supported by the Wittgenstein Prize, the Austrian Science Foundation (FWF) SFB FoQuS: F40-P10 and the EU through the ERC-AdG QuantumRelax
[1] S. Hofferberth et al. Nature, 449, 324 (2007).
[2] M. Gring et al., Science, 337, 1318 (2012); D. Adu Smith et al. NJP, 15, 075011 (2013).
[3] T. Langen et al., Nature Physics, 9, 640–643 (2013).
[4] T. Langen et al., Science 348 207-211 (2015).
[5] T. Schweigler et al., arXiv:1505.03126.
[6] T. Langen, T. Gasenzer, J. Schmiedmayer, J. Stat. Mech. 064009 (2016)

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Ulrich Schneider (U. Cambridge): Non-equilibrium dynamics of quantum gases in optical lattices

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Abstract: The out-of-equilibrium dynamics of interacting many-body systems presents one of the most challenging problems in modern many-body physics with implications ranging from thermalization dynamics over transport properties to novel transient effects and the formation of order. During the last years, ultracold atoms in optical lattices have emerged as a very versatile platform to study quantum many-body physics in a clean and well-controlled environment. Compared to electrons in a solid, the much longer timescales of atoms in optical lattices render them especially well suited to study out-of-equilibrium dynamics.
I will first discuss sudden expansion experiments in 1D and 2D Hubbard models, where we could for the first time observe ballistic transport in a strongly interacting system. Furthermore, I will show how the unique control possibilities available for ultracold atoms have allowed us to experimentally observe the real-time dynamics of a Quantum Phase Transition by measuring the associated emergence of coherence.

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Julian Sonner (U. Geneva): Quantum Quenches and Black-Hole Collapse

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Abstract: Holography allows us to formulate questions about quantum gravity in terms of more ordinary quantum field theories without gravity. A natural and long-standing goal has been to understand the physics of black holes using holographic duality. I will report on some recent progress on this question formulating the spherical collapse of an in-falling shell of null matter in three dimensions in terms of a first-principles CFT calculation. I will argue that the apparent loss of information in the CFT can be traced back to late-time non-perturbative effects in an expansion in large central charge.

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John Thomas (North Carolina State U.): Scale Invariance and Quantum Hydrodynamics in Expanding Strongly Interacting Fermi Gases

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Abstract: Optically-trapped, ultra-cold gases of spin 1/2-up and spin 1/2-down 6Li atoms model exotic, strongly interacting systems in nature. A bias magnetic field tunes the gas to a collisional (Feshbach) resonance, where the dilute atomic cloud becomes a scale-invariant, strongly interacting fluid: Shock waves are produced when two clouds collide. I will describe our recent observations of scale-invariant expansion and conformal symmetry breaking, as well as measurements of shear viscosity eta and limits on the bulk viscosity in such clouds. Using the previously measured entropy density s, the eta/s ratio obtained in the experiments is close to that of a quark-gluon plasma, comparable to the minimum conjectured for a "perfect fluid" using conformal field theory methods.

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Amos Yarom (Technion): One fish, two fish, red fish, blue fish

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Abstract: I will consider properties of a steady state generated by placing two semi infinite heat baths in contact with each other. The dynamics of the system under consideration are governed by a conformal field theory. When the number of space-time dimensions is very large the equations of motion for the system simplify. The ``phase diagram'' associated with the steady state, the dual, dynamical, black hole description of this problem, and its relation to the fluid/gravity correspondence will be discussed.