At the National Institute of Standards and Technology in Boulder, Colo., researchers have developed a 12-mm3 atomic clock based on the D1 line in rubidium-87 that may find applications in mobile communications and navigation systems. The instrument, which they describe in the Feb. 21 issue of Optics Express, displays superior short- and long-term stability compared with their first, recently reported clock, which was based on the D2 transition in cesium. The clock features a 1-mm3 cell filled with rubidium vapor, into which heating elements, an optics assembly, a near-IR vertical-cavity surface-emitting laser and a photodetector are integrated. At an ambient temperature of 22 °C, the device consumes 195 mW, and the researchers predict that this may be lowered to less than 50 mW. Over periods of less than a minute, the atomic clock displays a frequency instability in the range of 10211, but this increases to 1029 for periods as long as a day, which the scientists attribute to contamination of the vapor cell with nitrogen. A potential limitation of the device involves the existence of a dark trapping state that requires the cell to be heated to 120 °C, consuming power.