Paper detail

Performance of a prototype atomic clock based on lin||lin coherent population trapping resonances in Rb atomic vapor

We report on the performance of the first table-top prototype atomic clock based on coherent population trapping (CPT) resonances with parallel linearly polarized optical fields (lin||lin configuration). Our apparatus uses a vertical cavity surface emitting laser (VCSEL) tuned to the D1 line of 87Rb with current modulation at the 87Rb hyperfine frequency. We demonstrate cancellation of the first-order light shift by proper choice of rf modulation power, and further improve our prototype clock stability by optimizing the parameters of the microwave lock loop. Operating in these optimal conditions, we measured a short-term fractional frequency stability (Allan deviation) 2*10^{-11} tau^{-1/2} for observation times 1s<tau< 20s. This value is limited by large VCSEL phase noise and environmental temperature fluctuation. Further improvements in frequency stability should be possible with an apparatus designed as a dedicated lin||lin CPT resonance clock with environmental impacts minimized.