Computer Simulation of Quantum Dynamics in a Classical Spin Environment

TL;DR

This paper reviews a formalism for simulating quantum systems interacting with classical spin environments, highlighting geometric phases and developing algorithms for weak coupling scenarios, with applications demonstrated on a two-level system.

Contribution

It introduces a novel formalism based on antisymmetric brackets for quantum-classical spin dynamics and derives integration algorithms for non-Markovian simulations.

Findings

Geometric phases are significant in quantum-classical Liouville dynamics.

New symmetric Trotter algorithms enable efficient simulation of weakly coupled systems.

Simulations of a two-level system reveal detailed quantum and classical spin dynamics.

Abstract

In this paper a formalism for studying the dynamics of quantum systems coupled to classical spin environments is reviewed. The theory is based on generalized antisymmetric brackets and naturally predicts open-path off-diagonal geometric phases in the evolution of the density matrix. It is shown that such geometric phases must also be considered in the quantum-classical Liouville equation for a classical bath with canonical phase space coordinates; this occurs whenever the adiabatics basis is complex (as in the case of a magnetic field coupled to the quantum subsystem). When the quantum subsystem is weakly coupled to the spin environment, non-adiabatic transitions can be neglected and one can construct an effective non-Markovian computer simulation scheme for open quantum system dynamics in classical spin environments. In order to tackle this case, integration algorithms based on the symmetric Trotter factorization of the classical-like spin propagator are derived. Such algorithms are applied to a model comprising a quantum two-level system coupled to a single classical spin in an external magnetic field. Starting from an excited state, the population difference and the coherences of this two-state model are simulated in time while the dynamics of the classical spin is monitored in detail. It is the author's opinion that the numerical evidence provided in this paper is a first step toward developing the simulation of quantum dynamics in classical spin environments into an effective tool. In turn, the ability to simulate such a dynamics can have a positive impact on various fields, among which, for example, nano-science.