CONCLUSION

We have realised a prototype frequency standard based on an atomic fountain of cold cæsium atoms and have obtained Ramsey patterns with linewidth below 2 Hz. A large part of the observed noise is due to effects of fluctuations of time of arrival ($\pm$ 1 %) and the normalization technique used; this will be reduced by the use of a second probe beam to provide improved normalisation of the signal. We have also shown that the ratio of atoms present in the $\left\vert F = 3, M_{F} =0 \right>$ level can be increased without significant heating. Although no systematic investigations have been done, it has been observed that the S/N is also increased in the same proportion.

This research is sponsored by the National Physical Laboratory. KPZ and AMS are research fellows at Christ Church and Merton College respectively. We acknowledge valuable discussions with A. Clairon, D. Knight, P. Whibberly and C. Salomon