Phys. Rev. Lett. 76, 1192–1195 (1996)Unitary Evolution between Pure and Mixed StatesReceived 3 May 1995; revised 18 December 1995; published in the issue dated 19 February 1996 We propose an extended quantum mechanical formalism that is based on a wave operator ϱ̂, which is related to the ordinary density matrix via ρ = ϱ̂ϱ̂†. This formalism allows a (generalized) unitary evolution between pure and mixed states. 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. © 1996 The American Physical Society URL:
http://link.aps.org/doi/10.1103/PhysRevLett.76.1192
DOI:
10.1103/PhysRevLett.76.1192
PACS:
03.65.Bz
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