Pharao: a Space Clock with Cold Cesium Atoms

We describe a cold atom clock designed for operating in micrograv@, the PHARAO project. Preliminary resulfs have a W y been obtained on earth and the prototJpe wil l be Wed in the reduced g r a v e of aircrajl parabolic flights in the beginning of 1997. The PHARAO prototype i s an extension of the work done at the BNM-LPTF on a cesium atomic fountain, which presents a resonance linewidth of 700 millihertz, afiequency stability of 1.5.10-'3s-'12 where r is the intepztion time in seeonds. The accuroeg of the fountain clack is presently Z.10-'5, more than three times better than previoudg achieved with uneoolul conventional devices. The expeeted rela!ive stahiliiy of the PHARAO cesium clock in space is about 3.10-" at one second or 10-16 wr day. Because the reduced gravity environmenf aLws a mode of operation of the clock di&ent $om &h fountains, the a m w q of PHARAO should surpass that of fountains m d be in the lo-" range. The PHARAO frequency standard could be a key element in future space missions in fundamental physics such as SORT (Solar Orbit RelotivitJ Test), dd&n of gmvitakid waves, or for the r e a l i b n of a global time scale and a new generation of positioning qstcm.