The Time and Frequency Standard Laboratory has been creating increasingly stable atomic clocks that are required for the GPS and Galileo and for the tests of General Relativity. It has commissioned a clock using a fountain of caesium atoms cooled to a temperature only just above absolute zero (-273 degrees Celsius), which leads to a stability of the order of 10-15. Consequently, they will not wander by more than one-thousandth of a second in 30,000 years. This precision was improved still further recently, and further progress is expected in zero gravity, when a clock is placed in the international space station (Project PHARAO).

caption : Optical bench of the cesium fountain - credits : Observatoire de Paris