Title: First international comparison of fountain primary frequency standards via a long distance optical fiber link,
Abstract: We report on the first comparison of distant caesium fountain primary
frequency standards (PFSs) via an optical fiber link. The 1415 km long optical
link connects two PFSs at LNE-SYRTE (Laboratoire National de métrologie et
d'Essais - SYstème de Références Temps-Espace) in Paris (France)
with two at PTB (Physikalisch-Technische Bundesanstalt) in Braunschweig
(Germany). For a long time, these PFSs have been major contributors to accuracy
of the International Atomic Time (TAI), with stated accuracies of around
$3\times 10^{-16}$. They have also been the references for a number of absolute
measurements of clock transition frequencies in various optical frequency
standards in view of a future redefinition of the second. The phase coherent
optical frequency transfer via a stabilized telecom fiber link enables far
better resolution than any other means of frequency transfer based on satellite
links. The agreement for each pair of distant fountains compared is well within
the combined uncertainty of a few 10$^{-16}$ for all the comparisons, which
fully supports the stated PFSs' uncertainties. The comparison also includes a
rubidium fountain frequency standard participating in the steering of TAI and
enables a new absolute determination of the $^{87}$Rb ground state hyperfine
transition frequency with an uncertainty of $3.1\times 10^{-16}$.
This paper is dedicated to the memory of André Clairon, who passed away
on the 24$^{th}$ of December 2015, for his pioneering and long-lasting efforts
in atomic fountains. He also pioneered optical links from as early as 1997.