The LPTF preliminary accuracy evaluation of cesium fountain frequency standard

Most of the frequency shifts which limit the accuracy of cesium atomic frequency standards are considerably reduced when using ultra-cold atoms and long interrogation times in a Ramsey geometry. These shifts are also more easily evaluated than in traditional cesium beam standards. Most of these shifts are reduced to 10^-16 or less and can be calculated accurately such as relativistic shifts, cavity pulling, Rabi pulling. Other effects have to be directly measured, for example cold collisions, residual first order Doppler shift, quadratic Zeeman shift. At present, the accuracy of the fountain is estimated at 3 10^-15. A second fountain, using velocity selective Raman transitions in two dimensions, is being built leading to a potential improvement of a factor 10 of the fountain accuracy evaluation