
chair: Lea Ferreira dos Santos

09:00  09:30

Xinhua Peng
(University of Science and Technology of China, Hefei)
Experimentally measuring outoftimeorder correlator using nuclear spins (virtual)
The idea of the outoftimeorder correlator (OTOC) has recently emerged in the study of both condensed matter systems and gravitational systems. It not only plays a key role in investigating the holographic duality between a strongly interacting quantum system and a gravitational system, it also diagnoses the chaotic behavior of manybody quantum systems and characterizes information scrambling. Here, we report the measurement of OTOCs of local operators for an Ising spin chain on a nuclear magnetic resonance (NMR) quantum simulator and observe that the OTOC behaves differently in the integrable and nonintegrable cases. We also observe the different behaviors of the OTOCs of local operators for different engineering Hamiltonians in solid NMR. Our experiment paves a way for experimentally studying quantum manybody complex dynamics, quantum chaos, holographic duality, and information scrambling in manybody quantum systems with quantum simulators.

09:30  10:00

Arul Lakshminarayan
(Indian Institute of Technology Madras)
OTOCs: from few qubits to weakly coupled semiclassical systems (virtual)
Low angular momentum kicked tops present solvable models wherein the OTOC
shows connections to the classical Lyapunov exponent. Reviewing these, we
also discuss the case of weakly coupled bipartite systems wherein there are
multiple regimes of scrambling with only the early phase being related to the classical Lyapunov exponent.

10:00  10:30

Michael Knap
(Technische Universität München)
Operator growth in constrained quantum matter (virtual)
The farfromequilibrium dynamics of generic interacting quantum systems is characterized by a handful of universal guiding principles, among them the ballistic spreading of initially local operators. Here, we show that in certain constrained manybody systems the structure of conservation laws can cause a drastic modification of this universal behavior. As an example, we study operator growth characterized by outoftimeorder correlations (OTOCs) in a dipoleconserving “fracton" chain. We identify a critical point with subballistically moving OTOC front, that separates a ballistic from a dynamically frozen phase. This critical point is tied to an underlying localization transition and we use its associated scaling properties to derive an effective description of the moving operator front via a biased random walk with long waiting times. We support our arguments numerically using classically simulable automaton circuits.
Johannes Feldmeier, Michael Knap, arXiv:2106.05292
Johannes Feldmeier, Pablo Sala, Giuseppe de Tomasi, Frank Pollmann, Michael Knap, Phys. Rev. Lett. 125, 245303 (2020).

10:30  11:00

coffee break and discussion via zoom

11:00  11:30

Martin Gärttner
(Universität Heidelberg)
Manybody dynamics of disordered Rydberg spin systems (onsite)
Disordered quantum spin systems host many intriguing phenomena such as localization and glasslike relaxation. With cold atoms excited to Rydberg states we can experimentally realize a broad class of disordered spin models. The combination of experiments and numerical simulations of such models allows us to gain a better understanding of their dynamics far from equilibrium. In this talk I will report on recent results on glassy relaxation behavior in Heisenberg spin models with power law interactions and positional disorder. I will discuss possible schemes for Hamiltonian engineering including timereversal of unitary dynamics allowing OTOC measurements.

11:30  12:00

Horacio Miguel Pastawski
(CONICET and Universidad Nacional de Córdoba)
Emergent decoherence induced by quantum chaos in a manybody system. A Loschmidt echo observation through NMR (onsite)
In the long quest to identify and compensate the sources of decoherence in manybody systems far from the ground state, the varied family of Loschmidt echoes (LEs) became an invaluable tool in many experimental techniques. A LE involves the implementation of a timereversal procedure to assess the effect of perturbations in a quantum excitation dynamics. However, when addressing microscopic systems one is repeatedly confronted with limitations that seem insurmountable. This led to the central hypothesis of irreversibility stating that the timescale of decoherence, T_3, is proportional to the timescale of the manybody interactions we reversed, T_2. In this work we evaluate the role of the perturbations in relation to T_2, quantifying the decaytime T_3. With this purpose, we implement two experimental schemes that modify the strength of the effective dipolar spinspin coupling, i.e.~1/2T_2, while keeping constant the timescale of the perturbations, T_Σ. This requires the application of solid pulses of offresonance radiofrequency irradiation, to create a Floquet effective Hamiltonian that describes a spinspin interaction reduced by a variable scale factor k. This factor, bearing positive and negative values, allows to vary the time scale of forward evolution, as well as that of the timereversed one. Strikingly, we observe the superposition of the normalized Loschmidt echoes for the bigger values of k. This manifests the intrinsic dynamic dominance over the perturbation factors, even when the Loschmidt echo is devised to reverse those intrinsic dynamics. Thus, in the limit where the reversible interactions dominate over perturbations, the LE decays within a timescale, T_3≈T_2/R with R= (0.15±0.01), confirming the emergence of a perturbation independent regime. Thus, the central hypothesis of irreversibility is solidly supported once more.

12:00  13:00

lunch break

13:00  15:30

poster session & discussion (via gather.town)

15:30  16:00

coffee break


chair: Henning Schomerus

16:00  16:30

Dario Poletti
(Singapore University of Technology and Design)
From the eigenstate thermalization hypothesis to algebraic relaxation of OTOCs in systems with conserved quantities (virtual)
The relaxation of outoftimeordered correlators (OTOCs) has been studied as a means to characterize the scrambling properties of a quantum system. We show that the presence of local conserved quantities typically results in, at the fastest, an algebraic relaxation of the OTOC provided (i) the dynamics is local and (ii) the system follows the eigenstate thermalization hypothesis. Our result relies on the algebraic scaling of the infinitetime value of OTOCs with system size, which is typical in thermalizing systems with local conserved quantities, and on the existence of finite speed of propagation of correlations for finiterangeinteraction systems. We show that time independence of the Hamiltonian is not necessary as the above conditions (i) and (ii) can occur in timedependent systems, both periodic or aperiodic. We also remark that our result can be extended to systems with powerlaw interactions.
Published in Physical Review B 104, 104306 (2021)

16:30  17:00

Steven L. Tomsovic
(Washington State University)
PostEhrenfest manybody quantum interferences in ultracold atoms (virtual)
Far outofequilibrium manybody quantum dynamics in isolated systems necessarily generate interferences beyond an Ehrenfest time scale, where quantum and classical expectation values diverge. Ultracold atomic gases provide a promising setting to explore these phenomena. Theoretically speaking, the heavilyreliedupon truncated Wigner approximation leaves out these interferences. We develop a semiclassical theory of coherent state propagation for manybody bosonic systems, which properly incorporates such missing quantum effects. For mesoscopically populated BoseHubbard systems, it is shown that this theory captures postEhrenfest quantum interference phenomena very accurately, and contains relevant phase information to perform manybody spectroscopy with high precision.

17:00  17:30

Wojciech Zurek
(Los Alamos National Laboratory)
Information scrambling, Loschmidt echo, and decoherence (virtual)
We demonstrate analytically and verify numerically that the outoftime order correlator is given by the thermal average of Loschmidt echo signals. This provides a direct link between the outof timeorder correlator – a recently suggested measure of information scrambling in quantum chaotic systems – and the Loschmidt echo, a wellappreciated familiar diagnostic that captures the dynam ical aspect of chaotic behavior in the time domain, and is accessible to experimental studies. (Bin Yan, Lukasz Cincio, Wojciech H. Zurek, Information Scrambling and Loschmidt Echo, Phys. Rev. Lett. 124, 160603 (2020), arXiv:1903.02651)

17:30  18:00

discussion via zoom

18:00  19:00

dinner

19:30  20:00

Mark Srednicki
(University of California, Santa Barbara)
Bounds on outoftimeorder correlators from the eigenstate thermalization hypothesis (virtual)
The bound on the growth rate of the outoftimeorder fourpoint correlator in chaotic quantum manybody quantum systems, conjectured by J. Maldacena, S. Shenker and D. Stanford, can be derived from the general structure of operator matrix elements that follows from the eigenstate thermalization hypothesis (ETH). This talk is based on joint work with C. Murthy.
