ac Stark shift measurements of the clock transition in cold Cs atoms: Scalar and tensor light shifts of theD2transition

The ac Stark shift, or light shift, is a physical phenomenon that plays a fundamental role in many applications ranging from basic atomic physics to applied quantum electronics. Here, we discuss experiments testing light-shift theory in a cold-atom cesium fountain clock for the Cs D2 transition (i.e., 62S1/2  62P3/2 at 852 nm). Cold-atom fountains represent a nearly ideal system for the study of light shifts: 1) the atoms can be perturbed by a field of arbitrary character (e.g., coherent field or non-classical field); 2) there are no trapping fields to complicate data interpretation; 3) the probed atoms are essentially motionless in their center-of-mass reference frame, T ~ 1 K, and 4) the atoms are in an essentially collisionless environment. Moreover, in the present work the resolution of the Cs excited-state hyperfine splittings implies that the D2 ac Stark shift contains a non-zero tensor polarizability contribution, which does not appear in vapor phase experiments due to Doppler-broadening. Here, we test the linearity of the ac Stark shift with field intensity, and measure the light shift as a function of field frequency, generating a “light-shift curve.” We have improved on the previous best test of theory by a factor of two, and after subtracting the theoretical scalar light shift from the experimental light-shift curves, we have isolated and tested the tensor light shift for an alkali D2 transition.