On the Noisy Road to Open Quantum Dynamics: The Place of Stochastic Hamiltonians

Abstract: Stochastic evolution underpins several approaches to the dynamics of open quantum systems, such as random modulation of Hamiltonian parameters, the stochastic Schr\"odinger equation (SSE), and the stochastic Liouville equation (SLE). In a stochastic formulation, the open-system problem is reduced from a coupled system-environment dynamics to an effective system-only description, with dissipative evolution recovered by ensemble averaging. In this work, we aim at a self-contained and accessible presentation of these approaches to further elaborate on their common roots in essential concepts of stochastic calculus and to delineate the conditions under which they are equivalent. We also discuss how different formulations naturally lead to different numerical time-integration schemes, better suited for either classical simulation platforms, based on finite-difference approximations, or quantum algorithms, that employ random unitary maps. Our analysis supplies a unified perspective and actionable recipes for classical and quantum implementations of stochastic evolution in the simulation of open quantum systems.