Physics and measurement of time

Hyperfine transition

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A hyperfine transition is a tiny energy jump between two levels of an atom, caused by the interaction between the nuclear spin and the electron spin. Its frequency is extremely constant.

For caesium-133 that frequency is exactly 9,192,631,770 Hz, the value that defines the SI second. Atomic clocks (caesium, rubidium, hydrogen maser) measure precisely this transition.

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