COOLING AND TRAPPING OF NEUTRAL ATOMS

As early as 1917, Einstein had predicted that momentum is transferred in the absorption and emission of light, but it was not until the 1980's that such optical momentum transfer was used to cool and trap neutral atoms. By properly tuning laser light close to atomic transitions, atomic samples can be cooled to extremely low temperatures, the brightness of atomic beams can be enhanced to unprecedented values, and atoms can be manipulated with extraordinary precision. In this review several of the techniques for laser cooling and trapping of neutral atoms are described. We begin with a description of techniques used to decelerate atomic beams from thermal velocities. Such slowed beams were originally used to load optical molasses. A surprising result of these early experiments was the cooling of atoms below the Doppler temperature, which was the expected theoretical limit for laser cooling. To describe the experimentally discovered phenomenon of sub-Doppler laser cooling, several theories were proposed and tested in one dimensional experiments. Currently only this restricted case allows a detailed comparison between theory and experiment. Both the theory of sub-Doppler laser cooling and experiments in this field are described in the third section below. The first trapping of neutral atoms was accomplished with magnetostatic fields, described in the fourth section, and we discuss optical traps in the fifth section. The combination of such techniques have led to magneto-optical trapping and a wealth of new atomic physics experiments. Finally we discuss some applications of laser cooling, principally the subjects of atomic clocks, ultracold collisions, de Broglie wave optics, and non linear optics. We conclude with an outlook for the field of laser cooling.