Cold atom clocks and applications

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^-14\^-1/2 where \ 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, 000s for the first time for primary standards. In addition, these clocks realize the SI second with an accuracy of 7\ 10^-16, one order of magnitude below that of uncooled devices. In a second part, we describe tests of possible variations of fundamental constants using ^87Rb and ^133Cs 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.