Interaction with a pre and post selected environment and recoherence

The interaction of an open system S with a preand post-selected environment is studied. In general, under such circumstances S can not be described in terms of a density matrix, even when S in not post-selected. However, a simple description in terms of a two-state (TS) is always available. The two-state of S evolves in ti me from an initially ‘pure’ TS to a ‘mixed’ TS and back to a final ‘pure’ TS. This generic process is governed by a modified Liouville equation, which is derived. For a sub-class of observables, which can still be described by an ordinary density matrix, this evolution generates recoherence to a final pure state. In some cases post-selection can even suppress any decoherence. e-mail: [email protected]