Standard atomic clocks operate at microwave frequencies, based on the natural vibrations of the cesium atom – the primary definition of the second. Optical atomic clocks, running at higher frequencies, offer greater precision because they divide time into smaller units and have a high "quality factor," which reflects how long the atoms can tick on their own, without outside help. Optical clocks are expected to be the basis for a future redefinition of the second.
In the original chip-scale atomic clock from NIST, the atoms were probed with a microwave frequency. Commercial versions of this clock have become an industry standard for portable applications requiring high timing stability. However, they require initial calibration and their frequency can drift over time, resulting in significant timing errors.
Compact optical clocks are a possible step up. Until now, optical clocks have been bulky and complex, operated only as experiments by metrological institutions and universities.
Optical ticks in rubidium have been studied extensively for use as frequency standards and are accurate enough to be used as length standards. The rubidium vapor cell and the two frequency combs are microfabricated in the same way as computer chips. This means they could support further integration of electronics and optics and could be mass produced – a path toward commercially viable, compact optical clocks.