This paper describes advances in microwave frequency standards using
laser-cooled atoms at BNM-SYRTE. First, recent improvements of the 133Cs
and 87Rb atomic fountains are described. Thanks to the routine use of a
cryogenic sapphire oscillator as an ultra-stable local frequency reference, a
fountain frequency instability of 1.6×10−14τ−1/2 where τ
is the measurement time in seconds is measured. The second advance is a
powerful method to control the frequency shift due to cold collisions. These
two advances lead to a frequency stability of 2×10−16 at 50,000sforthefirsttimeforprimarystandards.Inaddition,theseclocksrealizetheSIsecondwithanaccuracyof7\times 10^{-16},oneorderofmagnitudebelowthatofuncooleddevices.Inasecondpart,wedescribetestsofpossiblevariationsoffundamentalconstantsusing^{87}Rband^{133}$Cs fountains.
Finally we give an update on the cold atom space clock PHARAO developed in
collaboration with CNES. This clock is one of the main instruments of the
ACES/ESA mission which is scheduled to fly on board the International Space
Station in 2008, enabling a new generation of relativity tests.Comment: 30 pages, 11 figure