Abstract
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 . For varying from to the Allan standard deviation of the frequency fluctuations is in excellent agreement with the law of atomic projection noise. With atoms, the relative frequency stability is , where 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.
- Received 29 October 1998
DOI:https://doi.org/10.1103/PhysRevLett.82.4619
©1999 American Physical Society