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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?

The caesium atomic clock as described below was developed in the 1950s and is still produced along the same lines. The most accurate versions, however, employ new features as will be dealt with in the answer to the following question.
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 sorted by using a magnetic field so that only atoms in one particular state reach the cavity resonator. There the atoms are irradiated by a microwave field and stimulated to change their state. Dependent on the microwave frequency the probability for a state change is larger or smaller. After the- atoms have left the cavity resonator, the apparatus again sorts the atoms, this time collecting in a special chamber only those that have changed their state. At a particular frequency of the microwave field, the probability for a state change is maximum and the maximum number of atoms per time unit is caught. The respective frequency is kept fixed and counted. Per definition, after 9 192 631 770 periods of the microwave field one second has elapsed.