μPOP clock: A microcell atomic clock based on a double-resonance Ramsey scheme

We demonstrate and study a microcell microwave atomic clock based on optical-microwave Double Resonance (DR) interrogation operated in pulsed Ramsey scheme, called the POP clock, based on a micro-fabricated Rb vapor cell and a micro-loop-gap microwave resonator. For the mm-scale dimensions of this cell, the population and coherence relaxation rates of the Rb clock transition are on the order of 45 kHz, which puts constraints on the useful Ramsey times and overall pulse sequence in view of optimized clock performance. Our proof-of-principle demonstration of the POP clock shows that pulsed DR approach is nevertheless feasible and results in a short-term clock stability of 1×10-11 -1/2 and reaching the ≤ 2×10-12 level at timescales of 1 000 s to one day. The short-term instability budget established for the POP clock shows that the main limitation to the short-term stability arises from the detection noise. Thanks to the pulsed Ramsey scheme, light-shift effects are strongly reduced in the POP clock which opens perspectives for further improvements of long-term clock stability, in view of new generations of miniature vapor-cell clocks with enhanced performances based on the DR scheme.