Comparison with an uncertainty of 2/spl times/10/sup -16/ between two primary frequency standards

We present a comparison between SYRTE's Cs fountain FO1 and Cs-Rb fountain FO2 using a cryogenic sapphire oscillator (CSO) to generate an ultra-stable interrogation signal. A description of the experimental setup is given and the clock accuracies are discussed. For the first time, a frequency resolution in the low 10/sup -16/ is achieved in the comparison between two primary standards. The paper also summarizes recent contributions of these fountains to research activities such as test of the stability of fundamental constants, timekeeping and test of sub-systems of the PHARAO space clock.