Entanglement and irreversibility in the approach to thermal equilibrium

When a physical system is put in contact with a very large thermal bath, it undergoes a dissipative (i.e., an apparently irreversible) process that leads to thermal equilibrium. This dynamical process can be described fully within quantum physics, involving only unitary, therefore reversible, maps. The information, initially present in the system, is not erased, but is diluted in the bath because of entanglement. Irreversibility may arise if, after quantum information has been thus diluted, some classical information is lost. This paper reviews a model for thermalization that displays these features. Two new analytical results are provided for the zero-temperature channels: a new quantitative measure of entanglement, and a study of irreversibility in the case where the lost classical information is the label of the particles in the bath.