Linear response and relaxation in quantum lattice systems

Kenichiro Aoki, Dimitri Kusnezov

Masashi Ban, Sachiko Kitajima, Toshihico Arimitsu +1 more

A linear response theory for open quantum systems is formulated by means of the time-local and time-nonlocal quantum master equations, where a relevant quantum system interacts with a thermal reservoir as well as with an external classical field. A linear response function that characterizes how a relaxation process deviates from its intrinsic process by a weak external field is obtained by extracting the linear terms with respect to the external field from the quantum master equation. It consists of four parts. One represents the linear response of a quantum system when system-reservoir correlation at an initial time and correlation between reservoir states at different times are neglected. The others are correction terms due to these effects. The linear response function is compared with the Kubo formula in the usual linear response theory. To investigate the properties of the linear response of an open quantum system, an exactly solvable model for a stochastic dephasing of a two-level system is examined. Furthermore, the method for deriving the linear response function is applied for calculating two-time correlation functions of open quantum systems. It is shown that the quantum regression theorem is not valid for open quantum systems unless their reduced time evolution is Markovian.

Yu. G. Kondrat’ev

Angelo Russomanno, Alessandro Silva, Giuseppe E. Santoro

We address the issue of the validity of linear response theory for a closed quantum system subject to a periodic external driving. Linear response theory (LRT) predicts energy absorption at frequencies of the external driving where the imaginary part of the appropriate response function is different from zero. Here we show that, for a fairly general nonlinear many-body system on a lattice subject to an extensive perturbation, this approximation should be expected to be valid only up to a time t * depending on the strength of the driving, beyond which the true coherent Schrödinger evolution departs from the linear response prediction and the system stops absorbing energy from the driving. We exemplify this phenomenon in detail with the example of a quantum Ising chain subject to a time-periodic modulation of the transverse field, by comparing an exact Floquet analysis with the standard results of LRT. In this context, we also show that if the perturbation is just local, the system is expected in the thermodynamic limit to keep absorbing energy, and LRT works at all times. We finally argue more generally the validity of the scenario presented for closed quantum many-body lattice systems with a bound on the energy-per-site spectrum, discussing the experimental relevance of our findings in the context of cold atoms in optical lattices and ultra-fast spectroscopy experiments.

T.R. KOEHLER

Andrey R. Kolovsky

This paper studies a simple model of conductivity—a quantum particle (an atom) in a lattice, subject to a static field and interacting with the bath of Bose atoms. Results from direct numerical simulations of the system dynamics and statistical analysis of the eigenstates are compared with predictions from linear response theory, analytical solutions of the master equation, and the theory of quantum chaos. It is shown, in particular, that the behavior of the current–current correlation function, entering Kubo’s equation, is entirely defined by the chaotic properties of the system and that the phenomenological relaxation constant, entering the master equation, is actually encoded in this correlation function.