Progress Towards a Two-photon Cesium Clock

We are developing a compact clock stabilized to a two-photon optical transition in cesium (Cs). 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. The optical reference is portable and fully contained within 7U of rack space. In collaboration with Canada's NRC, we have characterized our optical reference stability by comparing to an active Hydrogen maser via an optical frequency comb. Our optical reference has an Allan deviation of 5e-13 at one second and a minimum Allan deviation of 5e-14 near 1000 seconds. Long-term performance is primarily limited by drifts in vapour cell temperature and laser intensity. Work is continuing on an improved vapour cell oven and an improved laser-intensity stabilization scheme to reduce our Allan deviation by an order of magnitude at both short and long times. To transfer the stability of the optical reference to conventional electronics, we are developing chip-scale optical frequency combs using photonic integrated circuit ring resonators.