Coherent population trapping for the realization of a small, stable, atomic clock

The coherent population trapping phenomenon (CPT) has created interesting avenues in the practical realization of atomic frequency standards using laser optical pumping. In practice two different types of standards can be implemented: a passive clock based on the dark state created in the ensemble by the CPT phenomenon and a maser based on the ground state oscillating magnetization induced by the same phenomenon. The paper describes the basic physics involved in the approach and summarizes the progress made thus far in the practical implementation of such frequency standards.