LEAST-BIAS DESCRIPTION OF ATOMIC-BEAMS

We show how an unbiased quantal description of the center-of-mass prop erties of an atomic beam is achieved by employing an appropriate stati stical operator. A few examples with experimental relevance are treate d in order to illustrate the method. We find that the quantum correcti ons to the classical relation between the velocity distribution and th e time-of-flight signal are negligible in reasonable experimental situ ations. We apply the method to atoms traversing a longitudinal Stern-G erlach interferometer where they are exposed to a magnetic field gradi ent for a short period. The dependence of the fringe visibility upon t he strength and duration of the gradient is derived.