  We investigate the evolution of the quantum state for a free particle placed
into a random external potential of white-noise type. The master equation for
the density matrix is derived by means of path integral method. We propose an
equivalent stochastic process where the wavefunction satisfies a nonlinear
Schr\"odinger equation except for random moments at which it shows orthogonal
jumps. The relation of our work to the usual theory of quantum noise and
damping is briefly discussed.
