Quantum Regression Theory and Efficient Computation of Response Functions for Non-Markovian Open Systems
Xiantao Li, Chunhao Wang
TL;DR
This work extends linear-response theory to non-Markovian open quantum systems by deriving a memoryless, system-only generalization of the quantum regression theorem (QRT) that remains time-local. The authors express the two-time response function $\chi(t_1,t_2)$ as a decomposition involving time-local generators $\mathcal{L}_0$, $\mathcal{L}_A(t)$, $\mathcal{L}_C(t)$, $\mathcal{L}_B(t)$, and Lamb shifts $H_B(t)$, with bath spectral properties entering through two-point bath correlation functions $C_{jk}(t)$. They show how to propagate commutators and anti-commutators efficiently and provide quantum algorithms that estimate two-time correlations with cost scaling polylogarithmically in the system dimension and as $\epsilon^{-1.25}$, thereby removing the Born–Markov separability constraint. The framework offers a scalable path to compute nonequilibrium properties of open quantum systems using gate-based quantum computation, potentially enabling simulations beyond traditional non-Markovian techniques.
Abstract
Linear response functions are a cornerstone concept in physics as they enable efficient estimation of many dynamical properties. In addition to predicting dynamics of observables under perturbations without resimulating the system, these response functions lead to electric conductivity, magnetic susceptibility, dielectric constants, etc. Estimating two-time correlation functions is a key ingredient of measuring linear response functions. However, for open quantum systems, simulating the reduced density operator with a quantum master equation only yields \emph{one-point} observables and is insufficient for this task. In this paper, we develop a memoryless, system-only formulation of two-point correlations for open quantum systems that extends the standard quantum regression theorem (QRT) beyond the Markov limit. We further incorporate the spectral property of the bath and express the time propagators in the response function as the memoryless generators in Lindblad-type forms. The resulting expressions recast the total response function into evolutions generated by time-dependent Hamiltonian and Lindblad primitives together with the more challenging propagation of commutators and anti-commutators. In addition to the derivation of the new QRT, we present quantum algorithms for these primitives and obtain an estimator for two-time correlations whose cost scales poly-logarithmically in the system dimension and $1/ε^{1.25}$ in the target accuracy $ε$. The framework removes the separability (Born-Markov) assumption and offers a pathway to efficient computation of nonequilibrium properties from open quantum systems.