IntroductionIn our daily lives, we typically interact with a local time that is derived from Coordinated Universal Time (UTC) by applying a xed oset, such as +9 hours in the case of Japan Standard Time. But UTC itself, maintained by the International Bureau for Weights and Measures (BIPM), similarly relies on two underlying time scales [1]: At the lowest level, a weighted mean over more than 350 precision clocks at institutes throughout the world is used to realize the free atomic time scale EAL (Échelle Atomique Libre). While the result is highly stable, its accu racy is limited by the residual frequency errors of the contributing clocks.International Atomic Time (TAI) corrects for these errors to implement a scale interval that accurately repre-sents the SI second, before an integer number of leap seconds is added to create UTC, which maintains synchro-nization with Earth’s rotation.e continued calibration of TAI is performed by purpose-built frequency standards. For those directly im-ple menting the SI denition of the second based on the 9.192 GHz microwave transition in 133Cs, systematic uncer-tainties approach 1 part in 1016 [2]-[6]. In recent years op tical frequency standards have improved rapidly, and the laboratories that maintain Cs-based clocks now also oper-ate frequency standards that interrogate atomic tran si tions in the optical regime with signicantly lower uncer-tainties [7]-[13]. In a coordinated measurement per formed in December 2018, SYRTE and NICT became the rst institutes to contribute to the steering of TAI with such optical clocks. An accurately calibrated international time scale has direct scientic value for applications such as long-term studies of pulsar timing [14]. It is also a prerequisite for im proved agreement between local im ple men ta-tions [15]‑[18], where it aects a wide range of applications from satellite navigation to stock trading.Radio-frequency linkAs shown in Fig.1, the junction point of such a measure-ment is a hydrogen maser (HM) that is continuously com pared to TAI, while its radio frequency signals can simul taneously be evaluated by the strontium optical lattice clock NICT-Sr1. is maser is part of the Japan Standard Time System [19], as described in Section 3-1 of this publication. As such, it is compared to the locally generated UTC(NICT) signal by a dual-mixer time dierence (DMTD) system [20], which records data once per second. e time dierence of UTC(NICT) relative to UTC, as implemented by BIPM is continuously evaluated using a GPS-PPP link [21]. 12近い将来に実施が見込まれている秒の再定義を見据えて、光原子時計がローカルあるいはグローバルな時系生成へ寄与し始めている。NICTで開発しているストロンチウム光格子時計NICT-Sr1は、国際的な標準時系のひとつである国際原子時（TAI）の直近の歩度校正に初めて寄与した光原子時計の一つである。本稿では、NICT-Sr1による TAI校正の実施と、長距離遠隔評価における不確かさについて議論する。In anticipation of a future redefinition of the SI second, atomic clocks based on optical transitions are now contributing to the generation of local and global time scales. The strontium optical lattice clock NICT-Sr1 is one of two such clocks that first actively contributed to the steering of the inter-national time scale TAI. Here we discuss the implementation and uncertainty budget for such a long-distance remote evaluation of the time scale by a frequency standard in the optical domain.4-4 光 – マイクロ波リンクとTAI校正4-4TAI Calibration with an Optical StandardNils Nemitz 蜂須英和 中川史丸 伊東宏之 後藤忠広 井戸哲也Nils NEMITZ, Hidekazu HACHISU, Fumimaru NAKAGAWA, Hiroyuki ITO, Tadahiro GOTOH, and Tetsuya IDO914 原⼦周波数標準
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