A chip-scale atomic clock based on 87Rb with improved frequency stability

We demonstrate a microfabricated atomic clock physics package based on coherent population trapping (CPT) on the D-1 line of Rb-87 atoms. The package occupies a volume of 12 mm(3) and requires 195 mW of power to operate at an ambient temperature of 200 degreesC. Compared to a previous microfabricated clock exciting the D-2 transition in Cs [1], this Rb-87 clock shows significantly improved short- and long-term stability. The instability at short times is 4 x 10(-11) / tau(1/2) and the improvement over the Cs device is due mainly to an increase in resonance amplitude. At longer times ( tau > 50 s), the improvement results from the reduction of a slow drift to - 5 x 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 D-1 line, spin-exchange collisions between Rb atoms and optical pumping appear to have increased importance compared to the D-2 line. (C) 2005 Optical Society of America.