Nondemolition observation of a free quantum particle
  A stochastic model of a continuous nondemolition observation of a free quantum Brownian motion is presented. The nonlinear stochastic wave equation describing the posterior dynamics of the observed quantum system is solved in a Gaussian case for a free particle of mass m. It is shown that the dispersion of the wave packet does not increase to infinity like for the free unobserved particle but tends to the finite limit. 
V. P. Belavkin and P. Staszewski