Least-bias description of atomic beams

We show how an unbiased quantal description of the center-of-mass properties of an atomic beam is achieved by employing an appropriate statistical operator. A few examples with experimental relevance are treated in order to illustrate the method. We find that the quantum corrections to the classical relation between the velocity distribution and the time-of-flight signal are negligible in reasonable experimental situations. We apply the method to atoms traversing a longitudinal Stern-Gerlach interferometer where they are exposed to a magnetic field gradient for a short period. The dependence of the fringe visibility upon the strength and duration of the gradient is derived.