Light shifts in a pulsed cold-atom coherent-population-trapping clock

Abstract

Field-grade atomic clocks capable of primary standard performance in compact physics packages would be of significant value in applications ranging from network synchronization to inertial navigation. A coherent-population-trapping clock featuring laser-cooled $^{87}\mathrm{Rb}$ atoms and pulsed Ramsey interrogation is a strong candidate for this technology if the frequency biases can be minimized and controlled. Here we characterize the light shift in a cold-atom coherent-population-trapping clock, explaining observed shifts in terms of phase shifts that arise during the formation of dark-state coherences combined with optical-pumping effects caused by unwanted incoherent light in the interrogation spectrum. Measurements are compared with existing and new theoretical treatments, and a laser configuration is identified that would reduce clock frequency uncertainty from light shifts to a fractional frequency level of $\ensuremath{\Delta}\ensuremath{\nu}/\ensuremath{\nu}=4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}14}$ per 100 kHz of laser frequency uncertainty.