Accuracy evaluation of ITCsF2: a nitrogen cooled caesium fountain

Abstract

For almost two decades, caesium fountain primary frequency standards (PFS) have
\nrepresented the best realization of the definition of the second in the International System of
\nunits. Their accuracy has progressively improved with time, reaching a few parts in 10^-16.In this paper, we present the accuracy evaluation of ITCsF2, the new Cs fountain PFS developed
\nat the Italian National Metrological Institute, designed to be operated at cryogenic temperature
\nto reduce the blackbody radiation shift. The short-term stability of the ITCsF2 fountain is
\n2×10^−13τ^−1/2 when operated at high atomic density, and the relative inaccuracy reaches 2.3×10^−16. We also report four calibrations of International Atomic Time with a relative
\nfrequency agreement of
\n(−1.7±3.2)×10^−16, between ITCsF2 and the average of the other fountains operated in the world during the reference periods.