UNITARY EVOLUTION BETWEEN PURE AND MIXED STATES

We propose an extended quantum mechanical formalism that is based on a wave operator <(rho)over cap>, which is related to the ordinary densi ty matrix via rho = <(rho)over cap><(rho)over cap>(dagger). This forma lism allows a (generalized) unitary evolution between pure and mixed s tates. It also preserves much of the connection between symmetries and conservation laws. The new formalism is illustrated for the case of a two-level system.