THE PHARAO PROJECT - TOWARDS A SPACE CLOCK USING COLD CS ATOMS

For several years, the ''BNM-Laboratoire Primaire du Temps et des Freq uences'' has worked on a cold atom frequency standard. With a cesium a tomic fountain a resonance line width of 700 mHz has been obtained lea ding to a short-term stability of 2 x 10(-13) tau(-1/2) down to 2 x 10 (-15) at 10(4) s. A first evaluation of the fountain accuracy has been performed resulting in an accuracy of 3 x 10(-15), three times better than previously achieved with thermal beams frequency standards. In t he atomic fountain, gravity limits the interaction time to similar to 1 s, hence the resonance line width to similar to 0.5 Hz. A factor of 10 re duction in the line width could be obtained in a micro-gravity e nvironment. The ''Centre National d'Etudes Spatiales'' (the French spa ce agency), the ''BNM-Laboratoire Primaire du Temps et des Frequences' ', the ''Laboratoire de l'Horloge Atomique'' and the ''Laboratoire Kas tler Brossel'' have set up a collaboration to investigate a space freq uency standard using cold atoms: the PHARAO project. A microgravity pr ototype has been constructed and operated first in the reduced gravity of aircraft parabolic flights in May 1997. It is designed as a transp ortable frequency standard. The PHARAO frequency standard could be a k ey element in future space missions in fundamental physics such as SOR T (solar orbit relativity test), detection of gravitational waves, or for the realization of a global time scale and a new generation of pos itioning system. (C) 1998 Elsevier Science Ltd. All rights reserved.