The PHARAO project: Towards a space clock using cold Cs atoms

For several years, the "BNM-Laboratoire Primaire du Temps et des Frequences" has worked on a cold atom frequency standard. With a cesium atomic fountain a resonance line width of 700 mHz has been obtained leading 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 the 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 environment. The "Centre National d'Etudes Spatiales" (the French space agency), the "BNM-Laboratoire Primaire du Temps et des Frequences", the "Laboratoire de l'Horloge Atomique" and the "Laboratoire Kastler Brossel" have set up a collaboration to investigate a space frequency standard using cold atoms: the PHARAO project. A microgravity prototype has been constructed and operated first in the reduced gravity of aircraft parabolic flights in May 1997. It is designed as a transportable frequency standard. The PHARAO frequency standard could be a key element in future space missions in fundamental physics such as SORT (solar orbit relativity test), detection of gravitational waves, or for the realization of a global time scale and a new generation of positioning system. (C) 1998 Elsevier Science Ltd. All rights reserved.