First International Comparison ofFountain Primary Frequency StandardsVia a Long Distance Optical Fiber Link

International audienceWe report on the first comparison of distant caesiumfountain primary frequency standards (PFSs) via an optical fiberlink. The fountains are FO1 and FO2 (Cs and Rb), at LNESYRTEin Paris and, CSF1 and CSF2 at PTB in Braunschweig.These fountains regularly contribute to the calibration of InternationalAtomic Time (TAI) and have been utilized for a numberof absolute measurements of optical clock transition frequencies.Benefiting from the phase coherent optical frequency transfervia a stabilized telecom fiber link, they are compared, for thefirst time, without uncertainty from the link. All fountains agreewell within the combined comparison uncertainties of a few10􀀀16, which fully supports their stated uncertainties. Besides,the comparisons yield a new precise absolute determination ofthe 87Rb ground state hyperfine transition frequency recognizedas a secondary representation of the second