Time scales steered by an optical clocks

Ido, Tetsuya; Hachisu, H.; Nakagawa, F.; Hanado, Y.

Although optical frequency standards made rapid progress these days, microwave standards are still employed for source oscillators of time scales because an oscillator free from phase jumps is a prerequisite. Currently, for high-end users such as national metrology laboratories, hydrogen masers (HM) have adequate balance of the stability and reliability. Optical clocks may play a role of the standards to which the time scales refer in order to adjust their scale intervals. The benefit of using optical frequency standards, in this case, would be the capability to evaluate the HM frequency more quickly. Then, we do not need to operate optical clocks continuously. One optical clock will be utilized not only for time keeping but also for other applications from time to time.

To investigate such possibilities of “optical” steering, we first evaluated the behavior of an HM over a few months with reference to a 87Sr lattice clock. The lattice clock was operated for 10000 s per day. Three sets of five-day measurements and a one-day measurement were distributed in two months with a separation of ~10 days. The HM is a part of the Japan Standard Time (JST) system and is linked to the International Atomic Time (TAI) via UTC(NICT). The evaluations referring to an optical clock clearly demonstrated a stable linear drift of HM frequency, indicating the capability of compensating the drift. This prospect was supported by a numerical simulation based on the record of HM-UTC(NICT)-UTC link. We assumed that the HM frequency and its linear drift rate are predicted relying on the optical calibration performed in past 45 days. Referring to the time differences of the HM from UTC(NICT), it was figured out that the difference between UTC and the “optically steered” time scale would be maintained below 3 ns, where the instability is 1.3×10^{-15} at five day

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