Statistical mechanics of an optical phase space compressor

We describe the statistical mechanics of a new method to produce very cold atoms or molecules. The method results from trapping a gas in a potential well, and sweeping through the well a semi-permeable barrier, one that allows particles to leave but not to return. If the sweep is sufficiently slow, all the particles trapped in the well compress into a very cold gas. We derive analytical expressions for the velocity distribution of particles in the cold gas, compare these results with numerical simulations, and discuss limitations on reduction in energy due to imperfections of the barrier.