09:30  10:00

Vincenzo Alba
(SISSA Trieste)
Entanglement and thermodynamics after quantum quenches in integrable systems
Entanglement and entropy are key concepts standing at the foundations of quantum and statistical mechanics, respectively. In the last decade the study of quantum quenches revealed that these two concepts are intricately intertwined. Although the unitary time evolution ensuing from a pure initial state maintains the system globally at zero entropy, at long time after the quench local properties are captured by an appropriate statistical ensemble with non zero thermodynamic entropy, which can be interpreted as the entanglement accumulated during the dynamics. Therefore, understanding the postquench entanglement evolution unveils how thermodynamics emerges in isolated quantum systems. An exact computation of the entanglement dynamics has been provided only for noninteracting systems, and it was believed to be unfeasible for genuinely interacting models. Conversely, here we show that the standard quasiparticle picture of the entanglement evolution, complemented with integrabilitybased knowledge of the asymptotic state, leads to a complete analytical understanding of the entanglement dynamics in the spacetime scaling limit. Our framework requires only knowledge about the steady state, and the velocities of the lowlying excitations around it. We provide a thorough check of our result focusing on the spin1/2 Heisenberg XXZ chain, and considering quenches from several initial states.

10:00  10:30

Gergely Zarand
(Budapest University of Technology and Economics)
Semisemiclassical theory of onedimensional nonequilibrium systems
In my talk, I intend to review some of the applications of a recently developed semisemiclassical theory of nonequilibrium systems and quantum quenches [C.P Moca, M. Kormos, and G. Zaránd, Phys. Rev. Lett. 119, 100603 (2017)]. Our hybrid semiclassical method handles internal degrees of freedom completely quantum mechanically, and accounts efficiently for entanglement entropy generation by these. In nonequilibrium situations, we can follow time evolution up to timescales at which local thermalization occurs. As an application, we investigate the quench dynamics and phase fluctuations of a pair of tunnel coupled one dimensional Bose condensates described by the sineGordon model, where we have also obtained a full analytical semiclassical description of phase correlations in the socalled universal limit [M. Kormos and G. Zaránd, Phys. Rev. E 93, 062101 (2016)]. We validate our approach by nonAbelian TEBD calculations performed for the antiferromagnetic spin S=1 Heisenberg chain [M. Werner et al, unpublished].
We also apply our semiclassical approach to describe inhomogeneous charge (spin) relaxation and the formation of nonequilibrium steady states [M. Kormos, C.P Moca, and G. Zaránd, arXiv:1712.09466]. Again, in the universal limit, we obtain full analytical results, which we complete with Monte Carlo simulations for an antiferromagnetic spin 1 chain. Depending on the initial state, the spin transport is found to be ballistic or diffusive. In the ballistic case we identify a "second front" that moves more slowly than the maximal quasiparticle velocity, and spreads diffusively. We also observe local equilibration around the second front in terms of the densities of the particle species.

10:30  11:00

coffee break

11:00  11:30

Sergej Flach
(Institute of Basic Science, Daejeon)
Classical nonergodic metals
Classical many body interacting systems are typically chaotic and their microcanonical dynamics ensures that time averages and phase space averages are identical. In proximity to an integrable limit the properties of the network of nonintegrable action space perturbations will decide whether ergodicity will hold arbitrarily close to the limit (albeit with diverging relaxation times), or whether the system fragments into regular and chaotic parts and turns into a nonergodic conductor at a finite distance to the integrable limit.

11:30  12:00

Balazs Dora
(Budapest University of Technology and Economics)
Outoftimeordered density correlators in Luttinger liquids
Information scrambling and the butterfly effect in chaotic quantum systems can be diagnosed by outoftimeordered (OTO) commutators through an exponential growth and large late time value. We show that the latter feature shows up in a strongly correlated manybody system, a Luttinger liquid, whose density fluctuations we study at long and short wavelengths, both in equilibrium and after a quantum quench. We find rich behaviour combining robustly universal and nonuniversal features. The OTO commutators display temperature and initial state independent behaviour. For the short wavelength density operator, they reach a sizeable value after the light cone only in an interacting Luttinger liquid, where the bare excitations break up into collective modes. We benchmark our findings numerically on an interacting spinless fermion model in 1D, and find persistence of central features even in the nonintegrable case. As a nonuniversal feature, the short time growth exhibits a distance dependent power.

12:00  13:00

lunch

13:00  14:30

discussion  available rooms: seminar room 1/2, 1B1, 2A1, 2B1, open space 1D2

14:30  15:00

Achilleas Lazarides
(MaxPlanckInstitut für Physik komplexer Systeme)
FloquetLindblad manybody physics

15:00  15:30

Yevgeny Bar Lev
(Weizmann Institute of Science)
From slow to fast: information spreading in ergodic quantum systems
Information in local ergodic systems spreads behind a quasicausal lightcone with a constant velocity know as the LiebRobinson velocity. In this talk I will present two families of systems for which the LiebRobinson lightcones are deformed, and the causal regions are enhanced (suppressed), yielding asymptotically diverging (vanishing) LiebRobinson velocities. I will discuss the spatiotemporal shape of these anomalous lightcones.

15:30  15:40

group photo (to be published on the workshop web page)

15:40  16:30

coffee break and discussion

16:30  17:00

Michael Knap
(Technische Universität München)
Quantum thermalization dynamics: from information scrambling to emergent hydrodynamics
Generic, clean quantum manybody systems approach a thermal equilibrium after a long time evolution. In order to reach a global equilibrium, conserved quantities have to be transported across the whole system which is a rather slow process governed by diffusion. By contrast, the scrambling of quantum information is ballistic and hence can be characterized by a "butterfly" velocity. One way of describing the propagation of quantum information is to study outoftime ordered (OTO) correlation functions, which are unconventional correlation functions with time arguments that are not time ordered. Using matrixproductstate based numerical simulations, we compute such correlators at high temperatures in a onedimensional BoseHubbard model, where well defined quasiparticles cease to exist. Finally, we will discuss ways of experimentally characterizing these unconventional OTO correlation functions in synthetic quantum matter.

17:00  17:30

Victor Galitski
(University of Maryland)
Universal level statistics of the Lyapunovian  the outoftimeordered operator
The outoftimeordered correlator (OTOC) has been proposed as an indicator
of chaos in quantum systems due to its simple interpretation in the
semiclassical limit. In particular, its rate of possible exponential growth at
$\hbar \to 0$ is closely related to the classical Lyapunov exponent. Here we
explore how this approach to "quantum chaos" relates to the randommatrix
theoretical description. To do so, we introduce and study the level statistics
of the logarithm of the outoftimeordered operator, $\hat{\Lambda}(t) = \ln
\left(  \left[\hat{x}(t),\hat{p}(0) \right]^2 \right)/(2t)$, that we dub the
"Lyapunovian" or "Lyapunov operator" for brevity. The Lyapunovian's level
statistics is calculated explicitly for the quantum stadium billiard. It is
shown that in the bulk of the filtered spectrum, this statistics perfectly
aligns with the WignerDyson distribution. One of the advantages of looking at
the spectral statistics of this operator is that it has a well defined
semiclassical limit where it reduces to the matrix of uncorrelated classical
Lyapunov exponents in a partitioned phase space. We provide heuristic picture
interpolating these two limits using Moyal quantum mechanics. Our results show
that the Lyapunov operator may serve as a useful tool to characterize "quantum
chaos" and in particular quantumtoclassical correspondence in chaotic
systems, by connecting the semiclassical Lyapunov growth that we demonstrate at
early times, when the quantum effects are weak, to universal level repulsion
that hinges on strong quantum interference effects.

17:30  19:30

discussion  available rooms: seminar room 1/2, 1B1, 2A1, 2B1, open space 1D2

19:30  22:00

conference banquet
