Cold atom clocks

This paper reviews recent progress on microwave clocks using laser cooled n eutral atoms. With an ultra-stable cryogenic sapphire oscillator as interro gation oscillator, a cesium fountain operates at the quantum projection noi se limit. With 6.10(5) detected atoms, the relative frequency stability del ta nu/nu is 4(.)10(-14)tau (-1/2) where tau is the integration time in seco nds. This stability is comparable to that of hydrogen masers. At tau = 2(.) 10(4)s, the measured stability reaches 6(.)10(-16). Equally important is th e accuracy of the frequency standard since Cs-133 is the primary reference for the definition of the time unit, the second. The accuracy of our cesium fountain FO1 is presently 1.10(-15), currently the best reported value. A Rb-87 fountain has also been constructed and the Rb-87 ground-state hyper fine energy has been compared to the Cs primary standard with a relative ac curacy of 2(.)5(.)10(-15). Comparing the hyperfine energies of atoms with d ifferent atomic numbers Z, one can search for possible variations of the fi ne structure constant alpha = e(2)/hc with time. Measurements of the ratio nu(Rb-87)/nu(Cs-133) spread over an interval of 24 months indicate no chang e at a level of 3.110(-15)/year, placing a new upper limit for 1/alpha (d a lpha /dt). The second attractive feature of Rb-87 fountains is the smallnes s of the frequency shift induced by the mean field interaction between atom s. This shift is found to be at least similar to 50 times below that of ces ium. Finally, the interest of the microgravity of space for cold atom experiment s is outlined. A space mission, ACES, carrying ultra-stable clocks, is pres ented. ACES has been selected by the European Space Agency to fly on the In ternational Space Station in 2004.