Atomic clocks based on coherent population trapping: basic theoretical models and frequency stability

The paper gives an overview of our present theoretical understanding of coherent population trapping in alkali metal atoms as used in the implementation of an atomic frequency standard. Two approaches are examined: The passive approach in which the microwave hyperfine resonance is detected directly on the transmitted radiation and the maser approach in which the same resonance is observed by means of stimulated emission in a microwave cavity. The theoretical analysis reviewed is supported by experimental results.