Absolute frequency of ⁸⁷Sr at 1.8 × 10⁻¹⁶ uncertainty by reference to remote primary frequency standards

Abstract 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.