A high stability atomic fountain clock using a cryogenic sapphire interrogation oscillator

We describe the operation of the cesium fountain clock in the quantum limited regime, yielding a frequency stability of 5 10/sup -14//spl tau/ /sup -1/2/, where /spl tau/ is the integration time in seconds. The measured frequency instability varies as 1//spl radic/N/sub at/ where N/sub at/ is the number of detected atoms, up to N/sub at/=6 10/sup 5/. Among the primary frequency standards, this is the best stability ever reported. The noise contribution of the fountain interrogation oscillator has been made negligible thanks to the ultra-low frequency noise in the 0.1-100 Hz Fourier frequency range of a cryogenic sapphire oscillator.