Cold atom clocks: status, applications, and perspectives

Summary form only given. The ability to cool neutral atoms and trapped ions using laser light has brought a new generation of ultra-stable clocks to existence in the microwave and optical domains. The author introduces the basic concepts, and discuss the main factors contributing to the frequency stability and accuracy of these frequency standards. Some applications are presented of these ultra-stable clocks in fundamental physics such as the search for a possible drift of fundamental constants, tests of general relativity and comparison of distant clocks. The possibility of comparing clocks in various parts of the electromagnetic spectrum using mode-locked femtosecond lasers will lead to dramatic advances in the sensitivity of these tests. The interest in the space environment for cold atom experiments is outlined. Thanks to the reduced gravity in an Earth orbiting satellite, interaction times in the clock can significantly exceed that of fountain clocks on the Earth. Several space missions carrying laser cooled cesium and rubidium clocks as well as hydrogen masers are under development for flying onboard the International Space Station in 2006-2007.