We demonstrate a microfabricated atomic clock physics package based on coherent population trapping (CPT) on the D1 line of 87Rb atoms. The package occupies a volume of 12 mm3 and requires 195 mW of power to operate at an ambient temperature of 200 °C. Compared to a previous microfabricated clock exciting the D2 transition in Cs , this 87Rb clock shows significantly improved short- and long-term stability. The instability at short times is 4×10-11/τ1/2 and the improvement over the Cs device is due mainly to an increase in resonance amplitude. At longer times (τ>50 s), the improvement results from the reduction of a slow drift to -5×10-9/day. The drift is most likely caused by a chemical reaction of nitrogen and barium inside the cell. When probing the atoms on the D1 line, spin-exchange collisions between Rb atoms and optical pumping appear to have increased importance compared to the D2 line.
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