Observation of atoms laser cooled below the Doppler limit.

Laser cooling is the use of laser radiation to reduce the relative motion of a collection of atoms. The most common type of laser cooling is "Doppler cooling" where the Doppler shift causes the radiation pressure force to be velocity dependent, thus damping atomic motion when the laser frequency is tuned below an atomic resonance. Atoms (or ions) cooled by this method have been used in applications as diverse as high-resolution spectroscopy and studies of collisions at low energies. Other current and future applications of laser cooling include velocity compression and focusing of atomic beams, loading and cooling of atoms in traps, cooling and storage of antimatter, study of condensed phases such as Coulomb crystals and Bose condensates, and study of low-energy atom-surface interactions. For a review of laser cooling and its applications, see Phillips et al. In most such applications, the colder the atoms, the better. Shortly after the idea for laser cooling of atoms ' appeared, it was realized4 that heating from random emissions and absorptions imposes a lower limit on the temperature. Several authors have worked out this theory. 7 For low laser intensity, in one dimension (1D) or 3D, the equilibrium temperature for free atoms is' t y 1 + (2~/y)'