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

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 Sr-87 clock transition to be f(Sr) = 429 228 004 229 873.082(76) Hz, with a fractional uncertainty of less than 1.8 x 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 Sr-87, Yb-171 and direct optical measurements of their ratio.

Automated summary

The optical lattice clock NICT-Sr1 calibrates the international timescale TAI and determines the absolute frequency of the Sr-87 clock transition to be 429 228 004 229 873.082 Hz, with a high level of precision approaching the systematic limits of the best realization of the SI second. This result is consistent with other recent measurements and is further supported by loop closure over the absolute frequencies of Sr-87, Yb-171, and direct optical measurements of their ratio.