SI-traceable measurement of an optical frequency at the low 10^-16 level without a local primary standard.

SI-traceable measurements of optical frequencies using International Atomic Time (TAI) do not require a local primary frequency reference, but suffer from an uncertainty in tracing to the SI second. For the measurement of the ⁸⁷Sr lattice clock transition, we have reduced this uncertainty to the low 10^-16 level by averaging three sets of ten-day intermittent measurements, in which we operated the lattice clock for 10⁴ s on each day. Moreover, a combined oscillator of two hydrogen masers was employed as a local flywheel oscillator (LFO) in order to mitigate the impact of sporadic excursion of LFO frequency. The resultant absolute frequency with fractional uncertainty of 4.3 × 10^-16 agrees with other measurements based on local state-of-the-art cesium fountains.