The atomic resonance signal of CSF1
In both thermal-beam and fountain clocks, the caesium atoms undergo two successive interactions with a microwave field. The linewidth W of the atomic resonance signal is determined by the temporal separation T between the microwave excitations (and thus by the velocity of the atoms) according to W = 1/(2T).
In the figure the atomic resonance signal observed with CSF1 is shown in green colour. Each data point corresponds to the result of a single measurement cycle. The separation of adjacent signal maxima corresponds to 2W = 1/T. During operation as an atomic clock, the frequency of the microwave excitation is controlled in order to coincide with the central maximum. The observed small linewidth (W ~ 1 Hz) necessitates the use of laser-cooled atoms. For comparison, the black curve shows the atomic resonance signal of the clock CS1 which uses a thermal atomic beam.
