Atomic-velocity class selection using quantum interference

A method enabling atomic velocity class selection by means of quantum interference in the two-photon ionization of an atom through two quasiresonant intermediate levels is studied. This method is compatible with the Doppler cooling in optical molasses, and it is able to attain temperatures colder than the Doppler limit. The advantages and limitations of this method are discussed. We study the effect of the competition with the Doppler cooling for temperatures lower than the Doppler limit, when the usual Doppler process heats the atoms rather than cools them. The method is shown to be limited essentially by the loss of ground-state atoms due to ionization. We also propose and study a ‘‘source’’ scheme in which new atoms are continuously injected into the system, leading to a nonvanishing stationary number of cold atoms. Finally, we propose generalizations of the method that allows us to combine it with Sisyphus-type mechanisms. © 1996 The American Physical Society.