Cesium standard

In telecommunication, a cesium standard is a primary frequency standard in which electronic transitions between the two hyperfine ground states of cesium-133 atoms is used to control the output frequency.

Note : The energy level between the two hyperfine ground states corresponds, in the absence of external influences (e.g. , the Earth's magnetic field), to a frequency of 9,192,631,770 Hz.

Source: from Federal Standard 1037C and from MIL-STD-188