Quantum state diffusion, density matrix diagonalization, and decoherent histories: A model

We analyze the quantum evolution of a particle moving in a potential in interaction with an environment of harmonic oscillators in a thermal state, using the quantum diffusion (QSD) picture of Gisin and Percival. The QSD picture exploits a mathematical connection between the usual Makrovian master equation for the evolution of the density operator and a class of stochastic nonlinear Schrodinger equations (Ito equation) for a pure state \psi], and appears to supply a good description of individual systems and processes. We find approximate stationary solutions to the Ito equation (exact, for the case of quadratic potentials). The solutions are Gaussians, localized around a point in phase space undergoing classical Brownian motion. We show, for quadratic potentials, that every initial state approaches these stationary solutions in the long time limit. We recover the density operator corresponding to these solutions, and thus show, for this particular model, that the QSD picture effectively supplies a prescription for approximately diagonalizing the density operator in a basis of phase space localized states. We show that the rate of localization is related to the decoherence time, and also to the time scale on which thermal and quantum fluctuations become comparable. We use these results to exemplify the general connection between the QSD picture and the decoherent histories approach to quantum mechanics, discussed previously by Diosi, Gisin, Halliwell, and Percival.