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.

Time scales consistently provide precise time stamps and time intervals by combining atomic frequency standards with a reliable local oscillator. Optical frequency standards, however, have not been applied to the generation of time scales, although they provide superb accuracy and stability these days. Here, by steering an oscillator frequency based on the intermittent operation of a 87Sr optical lattice clock, we realized an “optically steered” time scale TA(Sr) that was continuously generated for half a year. The resultant time scale was as stable as International Atomic Time (TAI) with its accuracy at the 10−16 level. We also compared the time scale with TT(BIPM16). TT(BIPM) is computed in deferred time each January based on a weighted average of the evaluations of the frequency of TAI using primary and secondary frequency standards. The variation of the time difference TA(Sr) – TT(BIPM16) was 0.79 ns after 5 months, suggesting the compatibility of using optical clocks for time scale generation. The steady signal also demonstrated the capability to evaluate one-month mean scale intervals of TAI over all six months with comparable uncertainties to those of primary frequency standards (PFSs).

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.

Wireless two-way interferometry (Wi-Wi) is the simplified version of "carrier phase based two-way satellite time and frequency transfer," wherein a wireless communication technology is used instead of a satellite communication technology. We used the carrier phase of a 2.4 GHz ZigBee module to measure the variation of two rubidium clocks at remote sites. Since clocks in the ZigBee module are much less precise than rubidium clocks, the carrier phase of the ZigBee signal cannot be used to compare two rubidium clocks in a simple manner. Using a technique to cancel the clock error of transmitters, we demonstrated picosecond-level precision measurement of the time variation of clocks between two remote systems. This synchronization technique at picosecond-level precision opens the door to low-cost wireless positioning at millimeter accuracy.

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