Compression of Atomic Phase Space Using an Asymmetric One-Way Barrier
  We show how to construct asymmetric optical barriers for atoms. These barriers can be used to compress phase space of a sample by creating a confined region in space where atoms can accumulate with heating at the single photon recoil level. We illustrate our method with a simple two-level model and then show how it can be applied to more realistic multi-level atoms. 
M. G. Raizen, A. M. Dudarev, Qian Niu, and N. J. Fisch