Generalized uncertainty relations and long-time limits for quantum Brownian motion models.

ABSTRACT: We study the time evolution of the reduced density operator for a class of quantum Brownian motion models consisting of a particle moving in a potential V (x) and coupled to an environment of harmonic oscillators in a thermal state. Our principle tool is the Wigner function of the reduced density operator, and for linear systems we derive an explicit expression for the Wigner function propagator. We use it to derive two generalized uncertainty relations. The first consists of a sharp lower bound on the uncertainty function, U = (∆p)2(∆q)2, after evolution for time t in the presence of an environment. The second, a stronger and simpler result, consists of a lower bound at time t on the quantity, A2 = U−C2 pq, where Cpq = 1 2〈∆p̂∆q̂+∆q̂∆p̂〉. (A is essentially the area enclosed by the 1−σ contour of the Wigner function). In both cases the minimizing initial state is a correlated coherent state (a non-minimal Gaussian pure state), and in the first case the lower bound is only an envelope. These generalized uncertainty relations supply a measure of the comparative size of quantum and thermal fluctuations. We prove two simple inequalites, relating uncertainty to von Neumann entropy, −Tr(ρ ln ρ), and the von Neumann entropy to linear entropy, 1−Trρ2. We also prove some results on the long-time limit of the Wigner function for arbitrary initial states. For the harmonic oscillator the Wigner function for all initial states becomes a Gaussian at large times (often, but not always, a thermal state). We derive the explicit forms of the long-time limit for the free particle (which does not in general go to a Gaussian), and also for more general potentials in the approximation of high temperature. We discuss connections with previous work by Hu and Zhang and by Paz and Zurek.