= Atomic clock
{tag=Atomic physics}
{wiki}
An <atomic clock> uses an atomic transition as a <frequency> reference for an <oscillator>; counting its calibrated cycles realizes a time interval. In a passive beam implementation, the <chemical atoms> are repeatedly prepared, interrogated and detected, while <feedback control> corrects the continuously running <oscillator>. The <Ramsey separated oscillatory fields experiment> explains how internal <quantum phase> becomes a measurable population signal.
The <SI> <second> fixes the unperturbed <ground state> <hyperfine structure> transition <frequency> of <caesium>-133 to exactly $9\,192\,631\,770\,\mathrm{Hz}$. This is a defining value for the ideal transition, rather than a guarantee that an operating apparatus has no shifts.<SI base unit: second (s) by Bureau International des Poids et Mesures>[SI base unit: second (s), defining paragraph and equivalent duration in periods of the unperturbed caesium-133 ground-state hyperfine transition]{ref}
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