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Panoramic view of the clock hall at PTB with the four caesium clocks CS1, CS2, CSF1 and CSF2.

How does an atomic clock work?

In the atomic clock the unit of time is realized in accordance with its definition. Caesium atoms are evaporated in a furnace. Each atom can be in one of the two states of lowest energy that caesium atoms can assume. After evaporation the atoms are magnetically sorted so that only atoms in one particular state reach the cavity resonator. There the atoms are irradiated by a magnetic microwave field and stimulated to change their state. After they have left the cavity resonator, the apparatus again sorts the atoms, this time collecting those that have changed their state in a special chamber. At a particular frequency of the microwave field, the maximum number is caught; this frequency is kept fixed and counted. After 9 192 631 770 periods one second has elapsed.