Session P4b: Timekeeping and Quantum Networking

Coordinating GNSS-Based Comparisons to Monitor Optical Clocks and Timescales with Reduced Latency
Nils Nemitz, Mads Tønnes, Hidekazu Hachisu, Tadahiro Gotoh, Nozomi Ohtsubo, Tetsuya Ido, National Institute of Information and Communications Technology (NICT); Takehiko Tanabe, Takumi Kobayashi, Akiko Nishiyama, Akio Kawasaki, Masami Yasuda, National Metrology Institute of Japan (NMIJ), National Institute of Advanced Industrial Science and Technology (AIST); Shilpa Manandhar, In Cheol Seo, Yan Ying Liu, Yung Chuen Tan, Yusong Meng, National Metrology Centre [NMC); Joon Hyo Rhee, Gyeong Won Choi, Young Kyu Lee, Huidong Kim, Chang Yong Park, Myoung-Sun Heo, Dai-Hyuk Yu, and Won-Kyu Lee, Korea Research Institute of Standards and Science (KRISS)
Location: Seaview A/B
Date/Time: Thursday, Jan. 30, 10:40 a.m.

As optical clocks now regularly contribute to International Atomic Time and are beginning to find application in steering national time scales, it is more and more important to consistently monitor their performance. Most metrological institutes are not yet connected by high-performance fiber frequency links, which leaves the space-borne international time links as the most practical option for clock comparisons between them. The evaluation of these links that the International Bureau of Weights and Measures (BIPM) provides in its Circular T is as convenient as it is reliable, but the monthly publication schedule introduces a latency that can delay the detection of abnormal behavior in an institute’s reference clocks, and thus increase the accumulation of a time error.
We are working to realize an ongoing exchange of clock and time scale data between the institutes of the “OptAsia” collaboration for timekeeping with optical clocks, consisting of NICT, NMIJ, NMC and KRISS. By establishing consistent data formats and automated evaluation methods, we seek to make the comparison results available to all members of the collaboration as quickly and conveniently as possible.
We are using the Canadian Spatial Reference System Precise Point Positioning (CSRS-PPP) service offered by Natural Resources Canada (NRCan) to obtain the evolution of differences between the local reference time scales with a delay of typically two weeks or less. The service implements an integer ambiguity resolution (PPP-AR), which provides a significant stability improvement [1] over the conventional float-ambiguity PPP. Our own statistical evaluations promise an uncertainty of less than 5×10^(-17) within a 5-day interval, in the absence of dead time from intermittent operation of the participating clocks. Where such dead time uncertainties are unavoidable, we characterize them based on a noise model for the hydrogen masers serving as the flywheels that provide frequency hold-over during interruptions. As the operating time is often limited by human resources, we will discuss strategies to optimize its allocation, aiming to reach a target uncertainty in the minimum amount of time.
Our presentation will summarize the outcomes of early clock comparisons between NICT, NMIJ and KRISS. By the time of the meeting, we hope to additionally show initial demonstrations of improved data collection and processing, as well as visualization and distribution of results. We are already working on a software solution to automatically merge the raw data submitted by each institute. By collecting the frequency differences between the optical clocks, flywheel hydrogen masers and local reference time scales, and then combining them with the PPP-AR evaluation, a comparison can be calculated for each pair of participating clocks. The results of this calculation will be stored in a database accessible to all members of the OptAsia collaboration and visualized in a virtual dashboard display.
We hope that future improvements to CSRS-PPP or a similar service will further reduce the evaluation delay and help reveal frequency differences of less than 10^(-16) within a week of the measurement. Day-boundary discontinuities currently make it hard to exploit the full long-term stability of PPP-AR, but future support for overlapping multi-day evaluations promises to mitigate this, to enable confirmation of frequency agreement to 10^(-17) within a month-long measurement campaign.