Quantum Brownian motion induced by thermal noise in the presence of disorder

TL;DR

This paper demonstrates that a quantum particle on a lattice exhibits diffusive motion, akin to Brownian motion, due to thermal noise and disorder, with a positive finite diffusion constant under broad conditions.

Contribution

It introduces a Lindblad equation model combining random Schrödinger operators and thermal noise, proving the positivity and finiteness of the diffusion constant.

Findings

Diffusive quantum motion is proven under thermal noise and disorder.

The diffusion constant is strictly positive and finite.

In strong disorder, the diffusion constant approaches zero proportionally to the heat bath coupling.

Abstract

The motion of a quantum particle hopping on a simple cubic lattice under the influence of thermal noise and of a static random potential is expected to be diffusive, i.e., the particle is expected to exhibit `quantum Brownian motion', no matter how weak the thermal noise is. This is shown to be true in a model where the dynamics of the particle is governed by a Lindblad equation for a one-particle density matrix. The generator appearing in this equation is the sum of two terms: a Liouvillian corresponding to a random Schr\"odinger operator and a Lindbladian describing the effect of thermal noise in the kinetic limit. Under suitable but rather general assumptions on the Lindbladian, the diffusion constant characterizing the asymptotics of the motion of the particle is proven to be strictly positive and finite. If the disorder in the random potential is so large that transport is completely suppressed in the limit where the thermal noise is turned off, then the diffusion constant tends to $0$ proportional to the coupling of the particle to the heat bath.