  We consider an ultracold gas of (non-condensed) bosons or fermions with two
internal states, and study the effect of a gradient of the transition frequency
between these states. When a $\pi/2$ RF pulse is applied to the sample,
exchange effects during collisions transfer the atoms into internal states
which depend on the direction of their velocity. This results, after a short
time, in a spatial separation between the two states. A kinetic equation is
solved analytically and numerically; the results agree well with the recent
observations of Lewandowski et al.
