Improvement of the frequency stability below the Dick limit with a continuous atomic fountain clock.

The frequency instability of a shot-noise limited atomic fountain clock is inversely proportional to its signal-tonoise ratio. Therefore, increasing the atomic flux is a direct way to improve the stability. Nevertheless, in pulsed operation, the local oscillator noise limits the performance via the Dick effect. We experimentally demonstrate here that a continuous atomic fountain allows one to overcome this limitation. In this work, we take advantage of two-laser optical pumping on a cold cesium beam to increase the useful fountain flux and, thus, to reduce the frequency instability below the Dick limit. A stability of 6 × 10(-14)τ(-1/2) has been measured with the continuous cesium fountain FOCS-2.