Wuhan–Shanghai optical-clock comparison hits 2.18×10^-18, meeting SI-second roadmap threshold

A China-based research team says it has carried out the first intercity optical-clock frequency-ratio measurement over long distance to meet a key uncertainty target on the roadmap for a future redefinition of the SI second, reporting a 2.18 × 10^-18 total uncertainty over a 1,350-kilometer link between Wuhan and Shanghai.

The result, described in an arXiv preprint posted Oct. 25 and not yet peer-reviewed, would place the work below the 5 × 10^-18 threshold set by the BIPM and its Consultative Committee for Time and Frequency for one class of validation experiments needed before any redefinition of the second. The authors were careful to frame the claim narrowly. “To our knowledge, this is the first intercity long distance frequency-ratio measurement, whose total uncertainty reaches the level required by the redefinition roadmap,” the authors wrote in the abstract of arXiv:2610.34250. Even so, the broader standards community is still trying to resolve inconsistencies among some ultra-precise optical-clock and frequency-ratio results.

The paper, “Yb/Sr optical clock comparison at 2.2E-18 uncertainty over a 1350-km fiber link,” is by Lei Hou and colleagues at the Innovation Academy for Precision Measurement Science and Technology in Wuhan, the University of Science and Technology of China and other collaborators. The experiment compared a ytterbium optical lattice clock in Wuhan with a strontium optical lattice clock in Shanghai using a phase-stabilized 1,350-kilometer fiber link carried on a nonexclusive commercial telecom route rather than a dedicated private line.

Over a 58-day campaign in the summer of 2026, the team reported 274 effective hours of synchronous operation, assembled from 16 jump-free segments. It measured a Yb/Sr frequency ratio of 1.2075070393433377160, with a Bayesian statistical uncertainty of 1.14 × 10^-18 and a total uncertainty of 2.18 × 10^-18.

Why that number matters is straightforward, even if the clocks are not. Optical clocks use ultrastable atomic transitions as time references and are widely seen as candidates for a future replacement of today’s cesium-based definition of the second. Under the international roadmap, one mandatory criterion is independent frequency-ratio measurements between different laboratories and different atomic species with overall uncertainty below 5 × 10^-18. According to the context laid out in the paper, results at that level had previously been achieved only a few times for co-located clocks, while the highest-precision remote comparisons generally relied on local or kilometer-scale links rather than city-to-city spans on working telecom infrastructure.

The same Wuhan-Shanghai link also produced the paper’s second notable result: evidence that a long-distance clock network can act as a gravity sensor. Because clock frequencies shift slightly with gravitational potential — an effect known as gravitational redshift — clocks sensitive at the 10^-18 level can, in principle, detect subtle changes in Earth’s gravity field. “The measured ratio exhibits a significant correlation with Earth tides,” the authors wrote in the abstract. The paper says that response matched geophysical models within statistical uncertainty, suggesting the network could support both standards-grade clock comparisons and intercity geodetic measurements.

That dual-use point may be as important as the headline number. The authors describe the work as evidence that a practical long-distance optical-clock network on commercial fiber can simultaneously compare clocks at the level relevant to SI-second work and sense gravitational-potential changes over large distances.

Still, the result should be read as a significant step, not a settled standards breakthrough. The work is a preprint, and 2026 draft materials for the General Conference on Weights and Measures noted “noticeable inconsistencies” remain in some optical-frequency-standard comparisons and in the global set of frequency-ratio measurements. Peer review and further independent comparisons will matter as much as the record-low uncertainty itself.