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

Which clock is the real PTB master clock today?

In the second half of the 1990s, PTB developed its first "caesium fountain clock" which proved to be much more accurate than the older atomic clocks of "classical" design. This is due to the use of laser-cooled caesium atoms which are catapulted 1 m upwards like droplets in a water fountain. On a ballistic trajectory the rising and falling atoms cross a cavity resonator in which they are irradiated. As for the traditional  atomic clocks, the relative number of atoms in the two  states of lowest energy changes depending on the frequency of the microwave field. As before, this change is observed, and the microwave frequency is stabilized to obtain maximum transition probability.
Which is the key difference new - old? Unlike the situation in an atomic beam, the atomic velocity is much lower, and all atoms practically have the same velocity. Therefore, the duration of the microwave irradiation is about 50 times longer compared to the former clocks, which results in a more precise determination of the atomic resonance frequency. This principle worked quite fine, so PTB got a second fountain clock later. And what about the accuracy? Continue with the next question!