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 quantum 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 Nat. For Nat varying from 4 × 104 to 6 × 105 the Allan standard deviation of the frequency fluctuations is in excellent agreement with the Nat-1/2 law of atomic projection noise. With 6 × 105 atoms, the relative frequency stability is 4 × 10-14τ-1/2where Τ is the integration time in seconds. This is the best short-term stability ever reported for primary frequency standards, a factor of 5 improvement over previous results. © 1999 The American Physical Society.G. Santarelli, Ph. Laurent, P. Lemonde, and A. Clairon, A. G. Mann, S. Chang, and A. N. Luiten, C. Salomo