High contrast dark resonances in a cold-atom clock probed with counterpropagating circularly polarized beams

A cold-atom coherent population trapping clock based on σ+-σ- interrogation realized by counter-propagating optical fields of opposite circular polarization is presented. The simultaneous use of σ+ and σ- polarizations prevents atoms from being trapped in the end magnetic sublevels, significantly enhancing the contrast over interrogation with a single circular polarization. Because the system is based on cold atoms and there is very little relaxation, nearly complete dark states are created, and coherent population trapping resonances with maximum contrast are observed. A frequency stability of 1.3×10-11/√τ is achieved, which averages down to 2×10-13 after a 40 000 s integration period.