• Accepted Paper

Examination of classical simulations for Heisenberg-Langevin equations for spin-1/2

Scott D. Linz and Jochen Gemmer

APS Open Sci. - Accepted 19 October, 2026

DOI: https://doi.org/10.1103/sx3r-jrcq

Abstract

A system of spins coupled to a bath is a traditional setup in open quantum systems. Through Heisenberg’s equation, the spin dynamics can be modeled by a set of first-order differential equations. Interpreting the terms as colored noise and non-Markovian damping, one can write them as quantum-mechanical Heisenberg-Langevin (HL) equations. These are notoriously difficult to solve because of the high dimensionality of the Hilbert space. Classical generalized Langevin equations, involving non-Markovian damping and colored noise, are well understood and can be treated numerically with relative ease. Thus, a classical ansatz can be made by substituting quantum expectation values with classical functions. This allows the application of standard methods developed for classical stochastic dynamical systems to tackle spin dynamics. However, this approach is uncontrolled and should be benchmarked against known quantum dynamics. In this investigation, a Hamiltonian for spin dynamics is modified to obtain a setup analogous to the Weisskopf-Wigner (WW) theory of spontaneous emission, enabling a comparison of the results. This will be compared for T=0 and with a slight adaptation, also for the high-temperature regime. In the zero-temperature limit, we observe nearly identical decay initially. However, the steady-state differs significantly for the different approaches. In the high-temperature regime, this discrepancy with respect to the steady-state becomes a lot less pronounced.

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