A clock network for geodesy and fundamental science

TL;DR

This paper demonstrates the first high-precision comparison of independent optical clocks separated by 700 km using a 1415 km fiber link, achieving unprecedented accuracy and speed, which advances fundamental science and time measurement standards.

Contribution

It introduces a phase-coherent optical frequency transfer method over a long fiber link, enabling highly accurate and rapid comparisons of distant optical clocks for the first time.

Findings

Fractional precision of 3×10^{-17} achieved

Comparison time reduced to 1000 seconds

Method surpasses classical frequency transfer techniques

Abstract

Leveraging the unrivaled performance of optical clocks in applications in fundamental physics beyond the standard model, in geo-sciences, and in astronomy requires comparing the frequency of distant optical clocks truthfully. Meeting this requirement, we report on the first comparison and agreement of fully independent optical clocks separated by 700 km being only limited by the uncertainties of the clocks themselves. This is achieved by a phase-coherent optical frequency transfer via a 1415 km long telecom fiber link that enables substantially better precision than classical means of frequency transfer. The fractional precision in comparing the optical clocks of three parts in $10^{17}$ was reached after only 1000 s averaging time, which is already 10 times better and more than four orders of magnitude faster than with any other existing frequency transfer method. 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.