Time evolution of the wave packet of a continuously observed free quantum particle

Recently, the stochastic nonlinear wave equation describing the dynamics of an observed quantum system has been solved for a free particle [Phys. Rev. A 45, 1347 (1992)]. In this paper, transient effects that occur during a c ontinuous observation of the particle position are discussed. It is shown t hat the dispersion of the Gaussian wave packet of the particle always decre ases in the beginning of the observation, then takes the form of oscillatio ns which, after a short time, decay rapidly to the asymptotic value.