  An exact stochastic model for the thermalisation of quantum states is
proposed. The model has various physically appealing properties. The dynamics
are characterised by an underlying Schrodinger evolution, together with a
nonlinear term driving the system towards an asymptotic equilibrium state and a
stochastic term reflecting fluctuations. There are two free parameters, one of
which can be identified with the heat bath temperature, while the other
determines the characteristic time scale for thermalisation. Exact expressions
are derived for the evolutionary dynamics of the system energy, the system
entropy, and the associated density operator.
