An optical atomic clock

An optical atomic clock, one based on radiation coming from a single mercury ion in a trap, has been devised by physicists at NIST Boulder. In the best atomic clock used currently in defining the universal unit of time, the emission of light from a cesium atom at a characteristic microwave frequency of about 9 GHz is observed-the cycles in the light wave are counted by an electronic circuit. The difficulty with using higher frequency light is that it has been harder to count the cycles.

In recent years, however, this counting problem has been solved by mixing high frequency waves with lesser-frequency waves (see Update 351). The new NIST device monitors visible/UV light from a mercury ion at a frequency of about 1 PHz (10¹⁵ Hz). With some further work optical atomic clocks should be able to surpass in precision the best efforts of microwave atomic clocks, about one part in 10¹⁵. (Diddams et al., Science, 3 August 2001.)