Optical clock

An optical clock is a type of atomic clock that uses an atomic transition in the optical range, such as the 728 nm transition in singly-ionized calcium. State-of-art optical clocks, which can measure atomic clock transition frequencies to better than one part in one million billion, represent the most precise measurements in the world. The precision of a clock is the smallest unit of time it can measure, and comes from counting oscillations of visible light, which oscillates at approximately 700 quadrillion times a second. These oscillations divide a second into 700 quadrillion intervals, with each of those intervals being roughly 10-18 seconds. By counting oscillations of laser light, one can measure time to within one such interval. The laser light is stabilized by the atomic transition; the trapped atom or atoms are excited when the laser light is resonant with the transition frequency. Oscillations of light in the optical range are counted using a frequency comb.

Optical clocks are a subset of atomic clocks, which measure electromagnetic radiation, typically microwaves. Optical light oscillates at frequencies near 500 THz, more than 50,000 times faster than the cesium microwave clock. For this reason, optical clocks are expected to replace microwave caesium clocks as the definition of the second. John L. Hall and Theodor W. Hansch shared the 2005 Nobel Prize in Physics for their contributions to optical clock development.

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