A novel scheme for coherent population trapping clock

A new scheme of constructive polarization modulation for coherent population trapping (CPT) clock is proposed. In this scheme, the polarization of a bichromatic laser beam is modulated between two opposite circular polarizations, to avoid trapping the population in the extreme Zeeman sublevels. We show that if an appropriate phase modulation between the two optical components of the bichromatic laser is applied synchronously, the two CPT dark states which are produced successively by the alternate polarizations add constructively. The experimental results show that the observed high contrast CPT signal is favorable for CPT clock application.