Stimulated Raman Clock Transition Without a Differential ac Stark Shift

We considered a stimulated Raman clock transition between the ground hyperfine states of an alkali atom by using the second-order perturbation theory and the spherical tensor formalism. When the light fields are circularly polarized and properly detuned, the differential ac Stark shift of the clock transition vanishes with non-vanishing transition amplitude. With a two-zone Raman Ramsey method and a slow atomic beam, the proposed scheme should result in a clock with a systematic shift on the order of a few mHz for the case of cesium. We also considered the possibility of using the differential ac Stark shift of the clock transition for an optical frequency standard. The precision of such a standard is still limited by the linewidth of the nearest excited state. We found, however, that use of a microwave transition provides significant advantages as well.