  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.
