Microfabricated atomic frequency references

We report on the fabrication of a physics package for a chip-scale atomic clock with a volume of 9.5 mm/sup 3/, consuming 75 mW of power. The design is described in detail and is strongly motivated by the goal of wafer-level fabrication and assembly. A fractional frequency instability of 2.4 /spl times/ 10/sup -10/ at one second of integration is demonstrated; the long-term drift is -2 /spl times/ 10/sup -8//day. We discuss the performance of the clock at short and long integration times as well as its power consumption, and suggest possible improvements.