Abstract
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).
Highlights
Time scales form a foundation of modern society, enabling technologies such as global navigation satellite systems, radio-wave interferometer arrays, and the synchronization of grids for telecommunication networks and electric power systems
Advances in optical frequency standards (OFSs) have accelerated in the last twenty years, resulting in greatly improved stability and accuracy over microwave frequency standards[9]. This situation has led the community of time and frequency standards to seriously consider the future redefinition of the SI second, where the SI second is realized by OFSs and the accuracy of the scale interval in TAI is maintained by OFSs10
The instabilities of hydrogen maser (HM) are normally lowest at around 104 – 105 s in terms of the Allan deviation. This is due to the existence of a linear frequency drift, and such characteristics are found in Fig. 1 from the instabilities of HM1 and HM2 that we used in this work
Summary
Time scales form a foundation of modern society, enabling technologies such as global navigation satellite systems, radio-wave interferometer arrays, and the synchronization of grids for telecommunication networks and electric power systems. A more realistic way being currently discussed is steering a local oscillator with reference to an OFS instead of a microwave frequency standard The feasibility of this method with an OFS was investigated by post-processing using data of OFS operations over 10 − 80 days[11,12]. Evaluations by using an optical clock with an up-time ratio below 2% make it much easier for OFSs to contribute to TAI, suggesting a novel approach to maintain the accuracy of the TAI scale interval For this goal, we discuss the requirements of the OFS operation rate and HM instability, which should be useful for applying the proposed scheme to other combinations of OFSs and flywheels
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