Towards a Portable Two-Photon Cesium Optical Clock*

Optical clocks have the prospect to greatly improve the precision and portability of timekeeping devices. With this goal in mind, we are developing a clock stabilized to a two-photon optical transition in cesium (Cs). Two-photon transitions can be insensitive to first-order Doppler shifts and are thus good candidates for portable clocks. Cs atoms in a hot vapor cell are driven on the 6S to 7D transition by a retroreflected laser stabilized to the atomic fluorescence peak. Systematic errors and fluorescence detection requirements for this Cs transition are favorable compared to similar rubidium-based vapor clocks. We directly compare the optical reference to a GPS steered microwave clock via an optical frequency comb to extract an upper bound on clock performance. We also exploit photonic integrated circuit technology to develop tantalum pentoxide microresonators than can produce Kerr soliton frequency combs. In the future, the stability of our optical reference will be transferred through these chip-based combs to the microwave domain allowing for direct interfacing with conventional electronics and devices.