PHARAO is a space program developed by French laboratories and the French space agency dealing with the construction and the utilization of a cold atomic clock. In space, an atomic clock using cold atoms could have better performances than on Earth. We have constructed a compact and reliable clock prototype which can operate on Earth or in a microgravity environment. The prototype has worked since November 1996. On Earth, the microwave resonance linewidth is 13 Hz, limited by gravity. Recently, we have successfully tested the prototype operation on board the ZERO-G/CNES aircraft. During the parabolic flights, we have recorded a 7 Hz microwave resonance linewidth. We have demonstrated for the first time that a reduced gravity environment improves atomic spectroscopy. The prototype will now serve as a reference for the development of a space qualified clock through industrial contracts. On earth, the prototype will be used as a transportable frequency standard. It will be an ideal tool for the comparison of distant ground based clock at a level which cannot be attained with existing time transfer technique. For this application, the prototype will be transformed into an atomic fountain to optimize its performances.
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