Laser Cooling of Atoms and Ions

As generators of intense, spectrally pure radiation at optical frequencies, lasers serve not only as stabilized sources for frequency standards in that region of the spectrum, but equally important, they have changed the entire embodiment of the microwave cesium standard. They provide a means of slowing down the thermal motion of atomic particles. This is critically important when the resonance frequency of a transition in an atom or ion is used as a reference, since it is essential that the Doppler frequency shifts due to the particle motion be eliminated as far as possible. This can be accomplished by what is now called laser cooling, a truly remarkable technique, which we take up in this chapter. It is a technique that has made it possible to reach particle velocities corresponding to temperatures only a small fraction of a degree above absolute zero, where all thermal motion ceases.