Thomann, Pierre

2002, Joyet, Alain, Mileti, Gaetano, Thomann, Pierre, Dudle, G.

We describe a primary fountain frequency standard operating with a continuous beam of laser-cooled Cs atoms. In such a device, aliasing effects, which may degrade the short-term stability in pulsed fountains, are removed and atomic-noise limited stability can be achieved with a state-of-the art, but commercially available, local oscillator. The present experimental short-term stability is measured to be 2.5 · 10-13τ-1/2. Another feature of the continuous fountain is the reduced atomic density and higher beam temperature which reduces the collisional shift of the atomic frequency below the 10-15 level. The light-shift is an undesirable characteristic of the continuous operation. Without a light-trap, a light-shift of the order of 10-12 has been measured. The shift is stable enough not to affect the frequency stability to 104 seconds (2.5 · 10-15). A rotating light-trap has been constructed and tested to bring the light-shift and the corresponding uncertainty to a negligible level. Various contributions to the accuracy are studied.

2000, Dudle, G., Mileti, Gaetano, Joyet, Alain, Fretel, Emmanuel, Berthoud, P., Thomann, Pierre

We report on the primary frequency standard now under construction at the Observatoire de Neuchatel (ON). The design is based on a continuous fountain of laser-cooled cesium atoms, which combines two advantages: the negligible contribution of collisions to the inaccuracy and the absence of stability degradation caused by aliasing effects encountered in pulsed operation. The design is reviewed with special emphasis on the specific features of a continuous fountain, namely the source, the microwave cavity (TE₀₂₁ mode), and the microwave modulation scheme. The possible sources of frequency biases and their expected contributions to the error budget are discussed. Based on present data, an accuracy in the low 10^-15 range and a short-term stability of 7•10^-14 are attainable simultaneously under the same operating conditions.