Caesium standard
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In telecommunication, a caesium standard is a primary frequency standard in which electronic transitions between the two hyperfine ground states of caesium-133 atoms is used to control the output frequency.
Note: By the definition of the SI second, the frequency of the radiation in a transition 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