Spotlight on optics: Performance and uses of the Refimeve metrological signal, 1,000 km from the source

09 décembre 2025 par Véronique Avy
This article, written by Mathieu Collombon in collaboration with several researchers from the CIML team, is one of two selected each month to appear in the Optica Publishing Group's journals.

Sending an atomic clock signal via a 1,000 km long telecommunications fiber: what happens at the other end? Over the past decade, French metrologists have built a national fiber optic network, Refimeve, capable of distributing atomic time and frequency signals with surprisingly low residual errors of less than 10⁻1⁹. Refimeve’s performance has been verified by looping the frequency signals from the clock source in Paris through the Refimeve network and sending them back to Paris. Although technically reliable, this verification leaves a crucial question unanswered: What is the actual quality of the Refimeve signal at the other end, a thousand kilometers from Paris?

In this research article, the developers of Refimeve teamed up with researchers from Aix-Marseille University to answer this question directly. By comparing the Refimeve signal to an independent reference signal generated in Marseille using ultra-stable lasers and a frequency comb laser, the authors were able to validate the Refimeve signal 1,000 km from the source.

Why is Refimeve important if Marseille already has its own ultra-stable reference frequency? Because having a very stable frequency signal is not the same as having a traceable reference frequency, i.e., an absolute frequency expressed in Hertz (which is the inverse of the SI unit of time, the second). Ensuring traceability is a legal responsibility of national metrology institutes such as the Paris Observatory, which co-authored this article. The ability to disseminate traceable time and frequency data via networks such as Refimeve, as the authors brilliantly demonstrate here, will enable new research in the fields of precision measurement, geodesy, and fundamental physics, and will contribute directly to the ongoing international effort to redefine the SI second using optical clocks.

Link to the Spotlight on Optics page

Link to the article

 

Article Reference :

 

  • Performance and uses of the Refimeve metrological signal, 1000 km from the source
  • Mathieu Collombon, Etienne Cantin, Gaëtan Hagel, Paul-Eric Pottie, Marie Houssin, and Caroline Champenois
  • J. Opt. Soc. Am. B 42(11) 2429-2435 (2025)

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