Dynamics of Collective Decoherence and Thermalization

TL;DR

This paper investigates how a collective environment affects the decoherence and thermalization of a quantum spin system, revealing how decay rates scale with the number of spins and interaction types without common approximations.

Contribution

It provides explicit decay rate formulas for collective spin systems coupled to a thermal bath, including the effects of energy conserving and exchange interactions, without relying on Markovian or weak coupling assumptions.

Findings

Off-diagonal decay rate scales as N^2 for energy conserving interaction.

Diagonal elements approach equilibrium at a rate independent of N.

Decay rates depend explicitly on system size and interaction details.

Abstract

We analyze the dynamics of N interacting spins (quantum register) collectively coupled to a thermal environment. Each spin experiences the same environment interaction, consisting of an energy conserving and an energy exchange part. We find the decay rates of the reduced density matrix elements in the energy basis. We show that if the spins do not interact among each other, then the fastest decay rates of off-diagonal matrix elements induced by the energy conserving interaction is of order N^2, while that one induced by the energy exchange interaction is of the order N only. Moreover, the diagonal matrix elements approach their limiting values at a rate independent of N. For a general spin system the decay rates depend in a rather complicated (but explicit) way on the size N and the interaction between the spins. Our method is based on a dynamical quantum resonance theory valid for small, fixed values of the couplings. We do not make Markov-, Born- or weak coupling (van Hove) approximations.