Local time scale upgrade with a transportable optical clock

Precise time scales are the universal base for all measurements. Although the laboratory optical clocks offer better uncertainty and instability at the level of 10−18 or even lower, their reliability and availability have greatly limited the global application to improve the precision of time scale. Here we report the deployment of a compact and transportable optical clock (TOC) to a timekeeping institution and steering an active hydrogen maser (HM) to generate an optical time scale (OTS), realizing the upgrade of the local time scale. The optical clock was transported over 1200 km by express delivery and resumed work as normal promptly, and its extremely high uptime of 93.6% in the half-year enabled us to precisely correct the frequency drift of HM, ultimately achieving an unprecedented monthly instability of 4 × 10−17. This steering experiment with a deployable optical clock marks a significant advancement, demonstrating that a timing accuracy below 100 ps per month can be achieved feasibly in various timekeeping institutions where HMs are typically employed as the primary contributor to timekeeping. In the future, mobile OTSs based on such TOCs can be deployed flexibly and rapidly, which is particularly important in scenarios lacking International Atomic Time reference.