The Quantum Physics of Atomic Frequency Standards

Volumes 1 and 2 of The Quantum Physics of Atomic Frequency Standards, henceforth referred to as QPAFS (1989), were written in the 1980s and were published in 1989. They covered, in some detail, work done up to 1987 on the development of atomic frequencystandards. The text included a description of their development at that time, as well as a description of the research on the physics supporting that development. Since that time, the field has remained a very active part of the research program of many national laboratories and institutes. Work has remained intensive in many sectors connected to the refinement of classical frequency standards based on atoms such as rubidium (Rb), caesium (Cs), hydrogen (H), and selected ions in the microwave range, while new projects were started in connection to the realization of stable and accurate frequency standards in the optical range. For example, intensive studies were made on the use of lasers in the optical pumping and cooling of Rb and Cs as well as on the development of a new type of standard based on the quantum-mechanical phenomenon called coherent population trapping (CPT). Regarding Cs and Rb, laser cooling of atoms has made possible the realization of an old dream in which a small blob of atoms, cooledin the microkelvin range, is projected upward at a slow speed in the gravitational field of the earth and the atoms fall back like water droplets in a fountain--