Out-of-time-ordered correlation functions in open systems: A Feynman-Vernon influence functional approach

Abstract

Recent theoretical and experimental studies have shown significance of quantum information scrambling (i.e. a spread of quantum information over a system degrees of freedom) for problems encountered in high-energy physics, quantum information, and condensed matter. Due to complexity of quantum many-body systems it is plausible that new developments in this field will be achieved by experimental explorations. Since noise effects are inevitably present in experimental implementations, a better theoretical understanding of quantum information scrambling in systems affected by noise is needed. To address this problem we study indicators of quantum scramblingout-of-time-ordered correlation functions (OTOCs) in open quantum systems. As most experimental protocols for measuring OTOCs are based on backward time evolution we consider two possible scenarios of joint system-environment dynamics reversal: In the first one the evolution of the environment is reversed, whereas in the second it is not. We derive general formulas for OTOCs in those cases as well as study in detail the model of a spin chain coupled to the environment of harmonic oscillators. In the latter case we derive expressions for open systems OTOCs in terms of Feynman-Vernon influence functional. Subsequently, assuming that dephasing dominates over dissipation, we provide bounds on open system OTOCs and illustrate them for a spectral density known from the spin-boson problem. In addition to being significant for quantum information scrambling, our results also advance understating of decoherence in processes involving backward time evolution.

The Generalized OTOC from Supersymmetric Quantum Mechanics—Study of Random Fluctuations from Eigenstate Representation of Correlation Functions

“…We have already calculated the numerator C (2) (t 1 , t 2 ) as given in Equation (73). So, here our only job is compute the two sets of equal time thermal correlators in the eigenstate representation.…”

Complexity from the reduced density matrix: a new diagnostic for chaos

“…This might open up possibilities to explore complexity as an extension of entropy [74,75] and other measures of correlations (for eg. OTOC) [76][77][78] for open systems.…”

Classical and Quantum Chaos in Chirally-Driven, Dissipative Bose-Hubbard Systems

“…either t or t + τ ); (ii), we apply the operators associated with t 1 on both the evolved ket and bra or, where this is not possible, we define four helper states following Mølmer et al [19]; next, (iii), we apply either forward or backward evolution in time from t 1 to t 2 , as necessary (i.e., forward evolution from t 1 = t to t 2 = t + τ or backward evolution from t 1 = t + τ to t 2 = t). The latter is done according to Ĥ → − Ĥ but with the dissipative part of the Lindblad evolution remaining the same [29]; finally, (iv), we find the expectation value of the operator or product of operators associated with time t 2 .…”