The feasibility of laser cooled fountain clocks has been demonstrated more than 15 years ago, just at the beginning of the atomic laser cooling era, at first using sodium atoms at Stanford and then by the SYRTE in collaboration with the LKB at ENS using caesium atoms. The first real caesium fountain frequency standard was made at SYRTE and works since the end of 1993. Since then many atomic fountains have been developed by most metrological institutes of Industrial countries.
The anticipated frequency stability performance of a few 10-14 for one second measurement time and accuracy of ~10-16 are, in our laboratory, almost obtained. The laboratory operates three different fountains: the first, the original caesium fountain has recently be improved; the second one derived from the prototype built for the space project PHARAO is transportable (the unique model in the world) and has been used for the measurement of the 1S-2S transition in atomic hydrogen in the Max Planck Institut fur Quantum in Garching.
The third one is now operated simultaneously with 87 rubidium and caesium. It has been used to demonstrate that the ratio of the hyperfine frequencies of 87Rb and caesium drifts by less than 5×10-16/year.
Comparisons with clocks in different laboratories using different atoms will allow a test of the stability of some fundamental interactions. Since a few years now, atomic fountains determine almost completely the accuracy of the SI second.