Equilibration of quasi-isolated quantum systems

The evolution of a quasi-isolated finite quantum system from a nonequilibrium initial state is considered. The condition of quasi-isolation allows for the description of the system dynamics on the general basis, without specifying the system details and for arbitrary initial conditions. The influence of surrounding results in (at least partial) equilibration and decoherence. The resulting equilibrium state bears information on initial conditions and is characterized by a representative ensemble. It is shown that the system average information with time does not increase. The partial equilibration and non-increase of average information explain the irreversibility of time.