On the use of intensity optical pumping and coherent population trapping techniques in the implementation of atomic frequency standards

The paper summarizes the relative advantages and disadvantages of coherent population trapping (CPT) or intensity optical pumping (IOP) for the implementation of a passive atomic frequency standard using the isotope ⁸⁷Rb. This paper outlines the basic principles common to both CPT and IOP when using laser optical pumping, and makes explicit their similarities and their differences. This paper describes experimental results obtained in the same cell on the characteristics of the CPT and IOP ⁸⁷Rb-hyperfine-resonance line. The measurements showed that the signal contrast is larger in CPT than in IOP for the same resulting line width; the light shift is smaller in CPT than in IOP, and is easier to control; in principle, a passive frequency standard based on CPT has a smaller size than that based on IOP, due to the absence of a microwave cavity. Conclusions on overall expectations for the future of such frequency standards are drawn.