Three-Dimensional Laser Cooling of Helium Beyond the Single-Photon Recoil Limit.

We report the first observation of subrecoil laser cooling in three dimensions, where we use the method of velocity-selective coherent population trapping. We have cooled a cloud of trapped metastable helium atoms to 180 nK, 22 times below the single-photon recoil temperature, with an enhancement in the momentum-space density of 16. We find that the efficiency with which the velocity-selective trap is populated can be augmented by Sisyphus-type frictional forces, while the cooling process is accompanied by the diffusion of atoms to states of very high velocity. {copyright} {ital 1995} {ital The} {ital American} {ital Physical} {ital Society}.