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The optical lattice clock NICT-Sr1 regularly reports calibration measurements of the international timescale TAI. By comparing measurement results to the reports of eight primary frequency standards, we find the absolute frequency of the 87Sr clock transition to be f S r = 429 228 004 229 873.082 76 H z , with a fractional uncertainty of less than 1.8 × 10−16 approaching the systematic limits of the best realization of the SI second. Our result is consistent with other recent measurements and further supported by the loop closure over the absolute frequencies of 87Sr, 171Yb and direct optical measurements of their ratio.

The uncertainty evaluation of CSF2, the second caesium fountain primary frequency standard at PTB, is presented. The fountain uses optical molasses to cool atoms down to 0.6 µK. The atoms are launched vertically in a moving optical molasses, and state selected in the |F = 3, mF = 0 hyperfine ground state. During their ballistic flight, the atoms interact twice with a microwave field, thus completing the Ramsey interaction. With a launch height of 36.5 cm above the cavity centre, the central Ramsey fringe has a width of 0.9 Hz. About 3 × 104 atoms, 30% of the initial number in the |F = 3, mF = 0 state, are detected after their second interaction with the microwave field. Stabilizing the microwave frequency to the centre of the central Ramsey fringe, a typical relative frequency instability of 2.5 × 10−13(τ/s)−1/2 is obtained. The CSF2 systematic uncertainty for realizing the SI second is estimated as 0.80 × 10−15. First comparisons with the fountain CSF1 at the Physikalisch-Technische Bundesanstalt and other fountain frequency standards worldwide demonstrate agreement within the stated uncertainties.