Thermalisation by a boson bath in a pure state

TL;DR

This paper demonstrates that a quantum system can thermalize through interaction with a boson bath starting from a pure state, without assuming initial thermal equilibrium, and the system's final state depends on the bath's initial energy.

Contribution

It shows that thermalization occurs for almost all pure initial bath states with well-defined energy, without requiring the bath to be initially at thermal equilibrium.

Findings

The system relaxes into a canonical equilibrium state determined by the initial bath energy.

The system's evolution matches that of a bath initially at the corresponding temperature.

Thermalization conditions are identified for other types of baths.

Abstract

We consider a quantum system weakly coupled to a large heat bath of harmonic oscillators. It is well known that such a boson bath initially at thermal equilibrium thermalises the system. We show that assuming a priori an equilibrium state is not necessary to obtain the thermalisation of the system. We determine the complete Schr\"odinger time evolution of the subsystem of interest for an initial pure product state of the composite system consisting of the considered system and the bath. We find that the system relaxes into canonical equilibrium for almost all initial pure bath states of macroscopically well-defined energy. The temperature of the system asymptotic thermal state is determined by the energy of the initial bath state as the corresponding microcanonical temperature. Moreover, the time evolution of the system is identical to the one obtained assuming that the boson bath is initially at thermal equilibrium with this temperature. A significant part of our approach is applicable to other baths and we identify the bath features which are requisite for the thermalisation studied.