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NICT REPORT 45Research Highlightscause it is still difficult to operate an optical clock continuously for one month or longer. Researchers at the NICT Space-Time Standards Laboratory including atomic physicists and time-composing experts have demonstrated a novel time scale called the “optical-microwave hybrid time scale” that combines an optical lattice clock with a hydrogen maser (HM). The 87Sr lattice clock, as a stan-dard for pace adjustment, is sparsely operated for three hours once a week. This operation calibrates the frequency of the HM, and the measurements over the latest 25 days allow them to predict how the HM ticking rate will change. Then, they can adjust the HM frequen-cy for the following week in advance to compensate for the predicted frequen-cy drift. “The method demonstrated here brings the benet of optical frequency stan-dards to time keeping.”The signal generated in this optical-microwave hybrid system continued for half a year without interruption. The resul-tant “one-second” was more accurate than that of UTC on that date, and the time deviated by 0.8 ns in half a year rela-tive to TT(BIPM), which is the most accu-rate time scale post-processed by the In-ternational Bureau of Weights and Measures (BIPM). This demonstration shows it is possible to keep time with re-spect to the future optical definition of the second, which may come into play in the next decade.Tetsuya Ido, director of NICT Space-Time Standards Laboratory, states “We serve the society by providing time endlessly without interruptions. The optical-microwave hybrid method demonstrated here brings the benefit of optical frequency standards to time keeping.” NICT, which generates Japan Stan-dard Time (JST), aims to apply this hy-brid method to the JST generation sys-tem step by step. The next step would be establishing a redundancy of opti-cal frequency references. Another opti-cal lattice clock or single-ion clock will work. They may utilize those in other laboratories by forming connections via optical fiber network or satellite-based frequency transfer. Dr. Ido has also said “Highly precise optical clocks are expected to be geo-detic sensors to detect the variation of gravitational environment. Such appli-cations demand a reference that re-mains unchanged. Highly accurate and stable national time scale may play this role that is available in 24h/7d as an infrastructure.”Fig.1 : 87Sr optical lattice clockReferenceHidekazu Hachisu, Fumimaru Nakagawa, Yuko Hanado, and Tetsuya Ido, “Months-long real-time generation of a time scale based on an optical clock,” Scientic Reports 8:4243 (2018). DOI: 10.1038/s41598-018-22423-5URL: http://www.nature.com/articles/s41598-018-22423-5