Poster Details

Poster Session, Tuesday Mar 12, 4:00 PM

Lea Santos (for Mohamad Niknam) / Sensitivity of quantum information to environment perturbations measured with the out-of-time-order correlation function / In a quantum system coupled with a non-Markovian environment, quantum information may flow out or flow into the system. Measuring quantum information flow and its sensitivity to perturbations is important for a better understanding of open quantum systems and for the implementation of quantum technologies. Information gets shared between the system and the environment by means of system-environment correlations that grow during their interaction. We design a nuclear magnetic resonance experiment to directly observe the evolution of the system-environment correlations and use the second moment of their distribution as a natural metric for quantifying the flow of information. In a second experiment, where the dynamics in the environment is taken into account, we study the sensitivity of the shared quantum information to environment perturbations. The metric used in this case is the out-of-time-order correlation function (OTOC). By analyzing the decay of the OTOC as a function of the system-environment correlations spread instead of the evolution time, we are able to demonstrate its exponential behavior.
Felix Izrailev / Onset of many-body chaos and thermalization in finite systems of interacting Bose and Fermi particles /
Masaki Tezuka / Characterization of many-body quantum chaos by quantum Lyapunov spectrum and two-point correlators /
Efim Rozenbaum / Quantum Chaos in Classically Non-Chaotic Systems / One of the general goals in the field of quantum chaos is to establish a correspondence between the dynamics of classical chaotic systems and their quantum counterparts. For isolated systems in the absence of decoherence, this correspondence in dynamics usually persists over an extremely short time -- the Ehrenfest time, logarithmic in $\hbar$ -- because quantum-mechanical interference washes out classical chaos. We demonstrate that a new kind of drastic disagreement can occur between quantum and classical descriptions of the same model even within this early-time window. Remarkably, quantum mechanics appears capable of playing the opposite to its usual role and brings chaos to classically non-chaotic systems. Our calculations employ the out-of-time-ordered correlator (OTOC) -- a diagnostic that has a clear classical meaning but is still applicable to general quantum systems. Specifically, we show that certain non-convex polygonal billiards, whose classical Lyapunov exponents are always zero, demonstrate a Lyapunov-like exponential growth of OTOC at early times with an $\hbar$-dependent Lyapunov rate. This behavior is sharply contrasted with the oscillatory early-time behavior of OTOC in convex polygonal billiards, which are also classically non-chaotic. These results suggest that in general, classical-to-quantum correspondence in dynamics may be violated even at early stages of quantum evolution before quantum interference comes into play.
Meenu Kumari / Untangling entanglement and chaos / The effect of classical chaos on the generation of quantum entanglement in spin systems has puzzled physicists for a couple of decades. We explain it in spin systems by analytically establishing a connection between entanglement generation and a measure of delocalization of a quantum state in such systems. While delocalization is a generic feature of quantum chaotic systems, it is more nuanced in regular systems. We establish that entanglement is a signature of chaos only in a semiclassical regime in periodically driven spin systems. Our work provides a new approach to analyzing quantum chaos and designing systems that can efficiently generate entanglement.
Masanori Hanada / Ant trail/black hole correspondence / In 1998, Witten conjectured that the Schwarzschild black hole, which has negative specific heat, is described by certain gauge theory. We explain how such an exotic object can appear from gauge theory. We argue that the confined and deconfined phases in gauge theories are connected by a partially deconfined phase (i.e. SU(M) in SU(N), where M<N, is deconfined), which can be stable or unstable depending on the details of the theory. When this phase is unstable, it is the gauge theory counterpart of the small black hole phase in the dual string theory. Partial deconfinement is closely related to the Gross-Witten-Wadia transition, and is likely to be relevant to the QCD phase transition. The mechanism of partial deconfinement is related to a generic property of a class of systems. As an instructive example, we demonstrate the similarity between the Yang-Mills theory/string theory and a mathematical model of the collective behavior of ants [Beekman et al., Proceedings of the National Academy of Sciences, 2001]. By identifying the D-brane, open string and black hole with the ant, pheromone and ant trail, the dynamics of two systems closely resemble with each other, and qualitatively the same phase structures are obtained.
Horacio Pastawski / Central Hypothesis of Irreversibility: a quest for emergent intrinsic decoherence and thermalization in many-body quantum chaos through the Loschmidt Echoes and OTOCs / If a magnetic polarization excess is locally injected in a crystal of interacting spins in thermal equilibrium, this ‘excitation’ would scramble as consequence of spin–spin interactions. Such an apparently irreversible process is known as spin diffusion and it can lead the system back to ‘equilibrium’. Even so, a unitary quantum dynamics would ensure a precise memory of the non-equilibrium initial condition. Then, if at a certain time, say t/2, an experimental protocol reverses the many-body dynamics by changing the sign of the effective Hamiltonian, it would drive the system back to the initial non-equilibrium state at time t. As a matter of fact, the reversal is always perturbed by small experimental imperfections and/or uncontrolled internal or environmental degrees of freedom. This limits the amount of signal M(t) recovered locally at time t. The degradation of M(t) accounts for these perturbations, which can also be seen as the sources of decoherence. This general idea defines the Loschmidt echo (LE), [1] which embodies the various time-reversal procedures implemented in nuclear magnetic resonance. Here, we overview our two decades quest by revisiting selected phenomena that underlie the LE dynamics including chaos, decoherence, localization and equilibration. This guiding thread ultimately leads us to the discussion of decoherence and irreversibility as an emergent phenomenon. [2,3] In a collaboration with P. Cappellaro’s MIT group, [4] we study nuclear spins in an Adamantane crystal to quantify the recoverability of information encoded in a quantum many-body scrambled state in presence of perturbations. In an unbounded solid-state nuclear spin system whose interactions we control, the out-of-time-ordered correlators (OTOCs) reveals a diffusive scrambling dynamics up among 10100 states of the Hilbert space, while Loschmidt Echoes (LEs) asses the recovery of local information. We find that when the coherent many-body dynamics dominates over the perturbation, the LE decay rate only depends on the interactions themselves confirming our Central Hypothesis of Irreversibility. [2] This striking perturbation-independent regime sheds new light over many-body thermalization and sets a new challenge in the quest to build ever complex quantum devices which our combined techniques should help to elucidate.
Jorge Hirsch / Quantum and Classical Lyapunov Exponents in Atom-Field Interaction Systems /
Andrew Guo / Lieb-Robinson bounds for strongly long-ranged interactions and their applications to scrambling /
Xiao Chen / Entanglement transition in quantum circuits /
Nathan Lysne /
Jiecheng Zhang /
Bryce Kobrin /
Kevin Kuper / Finite Bandwidth Effects in Optimal Control / Accurate control of complex quantum systems is a key part of digital quantum computation and analog quantum simulation. As our toolbox for quantum control improves, new limitations inherent to a specific approach will often surface. Here we focus on a model system consisting of individual Cs atoms driven by phase-modulated radio-frequency and microwave magnetic fields. In the standard approach to optimal control, one typically performs a numerical search for phase modulation waveforms that are piecewise constant in time. This helps keep the computational overhead at manageable levels for Hilbert space dimensions equivalent to at least a handful of qubits. On the downside, discontinuous phase steps in the control waveforms cannot be reproduced in the laboratory and this will necessarily limit the accuracy of control. We present results from an attempt to characterize the transfer function of our control chain and compensate for amplitude and phase deviations in a bandwidth that captures most of the waveform power spectral density. While this measurably improves the fidelity of the applied control fields, we observe no discernable difference in the accuracy with which we can implement unitary quantum maps.
Mauro Schiulaz / Self-averaging Properties of the Dynamics of Quantum Many-Body Chaotic Systems /
Quntao Zhang /
Wen Wei Ho / Quantum many-body scarring /
Greg Bentsen /