T. Kobayashi, National Metrology Institute of Japan; D. Akamatsu, National Metrology Institute of Japan & Yokohama National University; K. Hosaka, Yokohama National University; Y. Hisai}, Yokohama National University; A. Nishiyama, A. Kawasaki, M. Wada, H. Inaba, T. Tanabe, National Metrology Institute of Japan; F.-L. Hong, Yokohama National University; M. Yasuda, National Metrology Institute of Japan

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Optical clocks (optical lattice clocks and single ion clocks) have achieved fractional uncertainties of 10^{-19} to 10^{-18} levels. These achievements have stimulated discussion regarding a redefinition of the SI second. In particular, Consultative Committee for Time and Frequency (CCTF) has determined several milestones towards the redefinition, one of which is regular contribution to International Atomic Time (TAI) by optical clocks. For this purpose, several Sr and Yb optical lattice clocks have started to contribute to TAI. TAI is a global timescale computed by the Bureau International des Poids et Measures (BIPM). The calibration of TAI is performed by measuring the frequency difference between TAI and primary and secondary frequency standards averaged over a month. While the uptimes of Cs fountain clocks are typically more than 80 % for one month, optical clocks are typically operated for much shorter periods, which usually limits the calibration accuracies of TAI by optical clocks. To make use of the advantage of low systematic uncertainties of optical clocks, it is desirable that the robustness of optical clocks reaches a level comparable to that of Cs fountain clocks. At National Metrology Institute of Japan (NMIJ), we have developed an Yb optical lattice clock (NMIJ-Yb1) which can be operated nearly continuously for a long period. At first, we have demonstrated the operation of NMIJ-Yb1 with an uptime of 80.3 % for half a year from October 2019 to March 2020. After this first demonstration, we have reduced some of the most common interruptions that stop the clock operation, and achieved even higher uptimes, e.g., 94.5 % for 30 days (August 2021), and 97.0 % for 20 days (March 2022). The operation of NMIJ-Yb1 is supported by a laser system based on a home-made fiber comb, automatic laser relock scheme, and remote monitoring and controlling systems. During the operation, we remotely monitor several experimental parameters including the number of atoms trapped in an optical lattice, the excitation ratio of the clock transition, the beat frequencies between the lasers and the comb. When one of the experimental parameters is outside the normal range, a data acquisition computer automatically sends an email alert, allowing us to immediately fix the problems. Significant reductions in the uptimes (dead time of > 1 day) are mostly caused by rare events such as a large earthquake, a typhoon, and the shutdown of the electricity for facility maintenance. In 2020, NMIJ-Yb1 has been adopted as a secondary frequency standard that can calibrate the frequency of TAI. From August 2021, we have submitted 11 reports on the TAI evaluations carried out within the month of the TAI computation. The nearly continuous operation enables us to reduce the uncertainty due to the dead time of the optical clock to less than a few parts in 10^{-16}, although the frequency stability of our hydrogen maser used as a local flywheel is limited by a flicker floor of 2×10^{-15}. The calibration uncertainty is typically limited by the uncertainty of the satellite link and the uncertainty of the recommended frequency of Yb. In this presentation, we will report the details of the clock apparatus for realizing the nearly continuous operation and future perspectives.