Cold Atom Clocks

This paper reviews recent progress on microwave clocks using laser cooled neutral atoms. With an ultra-stable cryogenic sapphire oscillator as interrogation oscillator, a cesium fountain operates at the quantum projection noise limit. With 6 · 10 5 detected atoms, the relative frequency stability δ v / v is 4 · 10 -14 τ -1/2 where τ is the integration time in seconds. This stability is comparable to that of hydrogen masers. At τ = 2 · 10 4 s, the measured stability reaches 6 · 10 -16 . Equally important is the accuracy of the frequency standard since 133 Cs 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 87 Rb fountain has also been constructed and the 87 Rb ground-state hyperfine energy has been compared to the Cs primary standard with a relative accuracy of 2.5 · 10 -15 . Comparing the hyperfine energies of atoms with different atomic numbers Z , one can search for possible variations of the fine structure constant α = e 2 / ℏc with time. Measurements of the ratio v ( 87 Rb)/ v ( 133 Cs) spread over an interval of 24 months indicate no change at a level of 3.110 -15 /year, placing a new upper limit for 1/α(dα/d t ). The second attractive feature of 87 Rb fountains is the smallness of the frequency shift induced by the mean field interaction between atoms. This shift is found to be at least ∼ 50 times below that of cesium. Finally, the interest of the microgravity of space for cold atom experiments is outlined. A space mission, ACES, can ultra-stable clocks, is presented. ACES has been selected by the European Space Agency to fly on the International Space Station in 2004.