Atomic Clock

An atomic clock is a timepiece that keeps time by measuring the resonant frequency at which atoms transition between quantized energy states, a frequency fixed by physics rather than by any mechanical part. The SI second is itself defined this way, as the duration of 9,192,631,770 cycles of the radiation from the unperturbed ground-state transition of the caesium-133 atom, and networks of atomic clocks maintain International Atomic Time, from which Coordinated Universal Time is derived. Their precision underlies satellite navigation systems such as GPS and Galileo, where a timing error of a few nanoseconds can shift a position fix by about thirty centimeters. The first atomic clock was built by Louis Essen with Jack Parry at the National Physical Laboratory in the United Kingdom in 1955, using the caesium resonance later adopted as the definition of the second.