Quantum projection noise in an atomic fountain: A high stability cesium frequency standard

We describe the operation of a laser cooled cesium fountain clock in the qu antum limited regime. An ultrastable cryogenic sapphire oscillator is used to measure the short-term frequency stability of the fountain as a function of the number of detected atoms N-at. For N-at varying from 4 X 10(4) to 6 X 10(5) the Allan standard deviation of the frequency fluctuations is in e xcellent agreement with the N-at(-1/2) law of atomic projection noise. With 6 X 10(5) atoms, the relative frequency stability is 4 x 10(-14)tau(-1/2) where tau is the integration time in seconds. This is the best short-term s tability ever reported for primary frequency standards, a factor of 5 impro vement over previous results. [S0031-9007(99)09299-6].