Networked light clocks lay foundation for redefining the second

Electronics
Technological Innovation Website Editorial Team - 06/17/2025

Accurate time and frequency signals are essential for technologies such as GPS, managing power grids and maintaining financial transactions. [Image: Lovisa Hakansson/Chalmers University of Technology]
Resetting the second
It seems clear that the future of time measurement belongs to optical atomic clocks , and the greatest interest lies in redefining our unit of time, the second , a constant of the International System of Units.
Optical atomic clocks represent a new generation of traditional atomic clocks , with the main difference being that in them the ticks are defined by the frequency of the oscillation of light from a laser, and not by an atom.
Today, optical clocks are already about 100 times more accurate than the best cesium clocks, measuring time so precisely that they would lose or gain less than a second over billions of years.
To redefine the second, however, it is necessary to compare data from several different optical clocks, to check whether they are working as expected and whether they agree with each other. To this end, researchers have now carried out a highly coordinated comparison of optical clocks in six countries, as part of a large collaborative project funded by the European Union.
"Comparing multiple clocks simultaneously and using more than one type of link technology provides much more information than the mostly pairwise clock comparisons performed to date," said Thomas Lindvall of the VTT Institute in Finland. "With a coordinated set of measurements, it is possible to check consistency and at the same time provide more reliable results. These results could help determine which optical clock(s) should be used in the new definition of the second."

Overview of the watch comparison campaign: 10 optical watches in six different countries were compared over 45 days. [Image: Thomas Lindvall et al. - 10.1364/OPTICA.561754]
Comparisons between watches
This was the most extensive coordinated comparison of optical clocks ever made, operating clocks and the links that connect them simultaneously in six countries, spanning thousands of kilometers.
There were 38 comparisons - or frequency ratios - performed simultaneously with ten different optical clocks. Four of these comparisons were performed directly for the first time, and many of the others were measured with much greater precision than before.
To perform the measurements, the researchers had to connect the frequency outputs of the different clocks, which they did using two methods: radio signals from satellites and laser light traveling through optical fibers. The satellite method used GPS signals from the navigation system, which were available to all the clocks included in the study. However, this technique has limited accuracy due to measurement uncertainties caused by factors such as signal noise or instrument limits.
Fiber optic connections enabled measurements to be made with 100 times greater accuracy than satellite technology. However, these stable, high-precision connections could only be used to connect clocks in France, Germany and Italy during the international comparison. Meanwhile, local comparisons in Germany and the UK – where several clocks were located at the same institute – were carried out using short fiber optics, which greatly reduced the uncertainty.
It needs to be improved
The experiment identified some areas where improvement is still needed.
For example, to confirm that all clocks are working as expected, measurement uncertainties must be reduced to match the accuracy of the clocks themselves. Repeated measurements will then be needed to confirm the reliable operation needed to build trust in both the clocks and the links. In addition, several other criteria must also be met before redefining the second, including proving that optical clocks can contribute regularly and consistently to international time scales.
"Not all the results matched what we expected, and we did see some inconsistencies in the measurements," said Rachel Godun of the UK's National Physical Laboratory (NPL). "However, comparing so many clocks at once and using more than one technique to connect them made it easier to identify the source of the problem."
"In short: The accuracy of the new optical clocks is being studied in extreme depth and detail, but this is what is needed to make the redefinition of the second a hundred times better than it is today. Scientists working in this area need to eliminate the smallest inaccuracies in their clocks so that we can all enjoy the immense advantages this will bring. And when it comes to eighteen decimal places, there is no excuse for escaping," summarized Professor José López-Urrutia from the Max Planck Institute for Nuclear Physics.
Article: Coordinated international comparisons between optical clocks connected via fiber and satellite links
Authors: Thomas Lindvall, Marco Pizzocaro, Rachel M. Godun, Michel Abgrall, Daisuke Akamatsu, Anne Amy-Klein, Erik Benkler, Nishant M. Bhatt, Davide Calonico, Etienne Cantin, Elena Cantoni, Giancarlo Cerretto, Christian Chardonnet, Miguel Angel Cifuentes Marin, Cecilia Clivati, Stefano Condio, E. Anne Curtis, Heiner Denker, Simone Donadello, Sören Dörscher, Chen-Hao Feng, Melina Filzinger, Thomas Fordell, Irene Goti, Kalle Hanhijärvi, H. Nimrod Hausser, Ian R. Hill, Kazumoto Hosaka, Nils Huntemann, Matthew YH Johnson, Jonas Keller, Joshua Klose, Takumi Kobayashi, Sebastian Koke, Alexander Kuhl, Rodolphe Le Targat, Thomas Legero, Filippo Levi, Burghard Lipphardt, Christian Lisdat, Hongli Liu, Jérôme Lodewyck, Olivier Lopez, Maxime Mazouth-Laurol, Tanja E. Mehlstäubler, Alberto Mura, Akiko Nishiyama, Tabea Nordmann, Adam O. Parsons, Gérard Petit, Benjamin Pointard, Paul-Eric Pottie, Matias Risaro, Billy I. Robertson, Marco Schioppo, Haosen Shang, Kilian Stahl, Martin Steinel, Uwe Sterr, Alexandra Tofful, Mads Tonnes, Dang-Bao-An Tran, Jacob Tunesi, Anders E. Wallin, Helen S. MargolisRevista: OpticaDOI: 10.1364/OPTICA.561754Other news about:
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