Optical clock

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Optical clocks are a subset of atomic clocks and are based on transitions that emit electromagnetic radiation in the range of 200 nm to 1000 nm. Like other atomic clocks, they are based on the measurement of the resonant frequency of atoms. However, the first atomic clocks mostly operated at microwave frequencies, and therefore were much slower than optical-frequency clocks could be. Optical light oscillates at frequencies near 500 THz, more than 50,000 times faster than the cesium microwave atomic clock. Because of their speed, optical clocks have been demonstrated to keep time with less error than microwave caesium clocks for 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. Optical clocks have been based on a number of atoms, including magnesium, aluminum, potassium, calcium, rubidium, strontium, indium, ytterbium, mercury, and radium. These atoms emit electromagnetic radiation when stimulated by coherent, intense light, for example the 728 nm transition in singly-ionized calcium or the 467 nm and 436 nm transitions in singly-ionized ytterbium. State-of-art optical clocks, which can measure atomic clock transition frequencies to better than one part in 1018, 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...