T. Ido, H. Hachisu, N. Nemitz, H. Ito, N. Ohtsubo, Y. Miyauchi, M. Morikawa and K. Matsubara, National Institute of Information and Communications Technology

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The redefinition of the SI second is being discussed in the community of time and frequency metrology, as optical clocks have made strong progress in the past two decades and now clearly outperform the best microwave atomic clocks in terms of stability and accuracy. Accurate frequency standards are extremely useful for generating timescales, but even a highly reliable single frequency standard is typically not sufficient to generate a continuous timescale. This is an obstacle that needs to be overcome if optical clocks are to take over the responsibilities of the current cesium fountains. In 2018, NICT combined an intermittently operating optical lattice clock with a hydrogen maser to generate a high-precision real-time timescale signal that deviated from TT-BIPM by less than one nanosecond over five months. Since then, the operation of the system has been improved particularly in terms of the optical-to-microwave comparison and evaluation of the hydrogen maser microwave signal with respect to the standard frequency of the optical lattice clock. Since August 2021, the optical clock has been operated once or more per week on a regular basis, and adjustments to the frequency of the hydrogen maser signal create a real-time “optical” timescale TA(Sr). Based on these regular measurements, evaluations of the TAI scale interval have also been reported to BIPM throughout the year. NICT has maintained an ensemble timescale based on about 18 commercial cesium clocks since 2006, and a hydrogen maser signal has been steered to this ensemble timescale to generate UTC(NICT). After TA(Sr) has become available in August 2021, UTC(NICT) is further adjusted twice per week by referring to the optical timescale TA(Sr). Adding the optical steering to the conventional microwave-based system, confers the benefit of the optical clock without losing the robustness of the system. As a result, the deviation of UTC(NICT) with respect to UTC, which previously often reached more than 15ns, has been suppressed to less than 5ns over one year.