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An oscillator that uses the quantized energy levels in atoms or molecules as the source of its resonance. The laws of quantum mechanics dictate that the energies of a bound system, such as an atom, have certain discrete values. An electromagnetic field at a particular frequency can boost an atom from one energy level to a higher one, or an atom at a high energy level can drop to a lower level by emitting energy. The resonance frequency, fo, of an atomic oscillator is the difference between the two energy levels divided by Planck’s constant, h.
The principle underlying the atomic oscillator is that since all atoms of a specific element are identical, they should produce exactly the same frequency when they absorb or release energy. In theory, the atom is a perfect “pendulum” whose oscillations are counted to measure a time interval. The national frequency standards developed by NIST and other laboratories derive their resonance frequency from the cesium atom and typically use cesium fountain technology. Rubidium oscillators are the lowest priced and most common atomic oscillators, but cesium beam and hydrogen maser atomic oscillators are also sold commercially in much smaller quantities.
NISTIR 8323 from NIST T&F Glossary