A clock network for geodesy and fundamental science

TLDR

Optical clocks are essential tools for geoscience, astronomy, and tests of fundamental physics, and their utility depends on faithfully comparing distant clocks. This study reports a comparison of two strontium optical clocks over a 1,415‑km phase‑coherent fiber link between Paris and Braunschweig with an uncertainty of 5 × 10⁻¹⁷. The comparison uses a phase‑coherent frequency transfer over telecom fiber, with performance limited only by the clocks’ own instability and negligible transfer noise. The link achieves a fractional precision of 3 × 10⁻¹⁷ after 1,000 s, ten times better and over four orders of magnitude faster than previous long‑distance comparisons, enabling high‑resolution international clock comparisons that support redefining the SI second.

Abstract

Abstract Leveraging the unrivalled performance of optical clocks as key tools for geo-science, for astronomy and for fundamental physics beyond the standard model requires comparing the frequency of distant optical clocks faithfully. Here, we report on the comparison and agreement of two strontium optical clocks at an uncertainty of 5 × 10 −17 via a newly established phase-coherent frequency link connecting Paris and Braunschweig using 1,415 km of telecom fibre. The remote comparison is limited only by the instability and uncertainty of the strontium lattice clocks themselves, with negligible contributions from the optical frequency transfer. A fractional precision of 3 × 10 −17 is reached after only 1,000 s averaging time, which is already 10 times better and more than four orders of magnitude faster than any previous long-distance clock comparison. The capability of performing high resolution international clock comparisons paves the way for a redefinition of the unit of time and an all-optical dissemination of the SI-second.