Stark shift of the Cs clock transition frequency: A new experimental approach

The blackbody radiation shift, a manifestation of the Stark effect due to blackbody radiation, contributes to the systematic uncertainty in Cs-based frequency standards at a level of 1 times 10^-15. Few measurements of the third order scalar electric polarizability of the Cs ground states, mainly responsible of the AC Stark shift in atomic clocks, have given better than 10% accuracy. We report progress in the development of a fully optical Ramsey pump-probe measurement in a thermal atomic beam, based on coherent population trapping (CPT), for the measurement of the third order scalar and tensor polarizabilities of the Cs ground states. We give details of the apparatus and measurement techniques as well as our first 500 Hz half-period Ramsey fringes.