Design of CSF1
The use of cold atoms in a caesium atomic clock is practical only if one avoids perturbations of the excitation of the microwave reference transition through the light which cools the atoms. It is therefore useful to spatially separate the optical and microwave interaction zones. In order to obtain a small linewidth of the atomic resonance signal, the unperturbed interaction time between the atoms and the microwave field should be as large as possible.
The fountain clock scheme satisfies these requirements. A construction sketch of the vacuum system of CSF1 ist shown in the figure. A cloud of up to 107 Cs atoms is collected in a magneto-optical trap (MOT) where the atoms are laser cooled by means of "optical molasses". Here the mean atomic velocity is in the range of a few cm/s. The cloud is thrown upward by suitably detuned laser fields. The interaction time with the microwave field corresponds to the time between the two passages through the microwave cavity and is approximately 0.5 s. At the end of each measurement cycle, the population of the atomic energy levels is determined through an optical detection scheme.
