WASHINGTON — In a new study, researchers carried out the most extensive coordinated comparison of optical clocks to date by operating clocks and the links connecting them simultaneously across six countries. Spanning thousands of kilometers, the experiment represents a significant step toward redefining the second and ultimately establishing a global optical time scale.
"The accurate time and frequency signals provided by atomic clocks are essential for many everyday technologies — like GPS, managing power grids and keeping financial transactions in sync," said Helen Margolis, head of time and frequency at the National Physical Laboratory (NPL) in the United Kingdom. "Our findings could help to improve the performance of next-generation optical clocks, unleashing entirely new applications and advancing scientific endeavors that rely on time and frequency."
Optical clocks are a type of atomic clock that uses lasers to excite atoms in a controlled way that causes the atoms to shift between specific energy levels. These shifts happen at very precise frequencies, which serve as the "ticks" of the clock. Because these clocks come in various forms, each using different atoms to keep time, realizing the full potential of these precision timekeepers requires comparing them across long distances.
In Optica , Optica Publishing Group 's flagship journal for high-impact research, a multi-institutional group of researchers reports results from 38 comparisons — or frequency ratios — performed simultaneously with ten different optical clocks. Four of these comparisons were conducted directly for the first time, and many of the others were measured with much greater accuracy than before.
"These measurements provide critical information about what work is still needed for optical clocks to achieve the precision and reliability required for use in international timekeeping," said Marco Pizzocaro, senior researcher at the Instituto Nazionale Di Ricerca Metrologica (INRiM) in Italy. "Our experiment also showed how optical clocks across Europe can be linked to measure frequency ratios with state-of-the-art precision. This creates a distributed lab, which could also be used for carrying out tests of fundamental physics, such as searching for dark matter or testing the basic rules of physics."
Are optical clocks ready?
For decades, the global standard for keeping time has been based on an average of signals from cesium microwave atomic clocks around the world. However, as the precision and stability of optical clocks have steadily improved, there is growing momentum to redefine the International System of Units second to use optical clocks instead. Optical clocks are now about 100 times more accurate than the best cesium clocks and can measure time so accurately that they would lose or gain less than one second over billions of years.
However, using optical clocks for international timekeeping requires comparing data between various optical clocks to verify that they are performing as expected. To advance this work, the researchers carried out a highly coordinated comparison of optical clocks across six countries as part of a large collaborative EU-funded project.
"Comparing multiple clocks at the same time and using more than one type of link technology provides far more information than the mostly pairwise clock comparisons that have been carried out to date," said Thomas Lindvall, senior scientist at VTT MIKES in Finland. "With a coordinated set of measurements, it becomes possible to check consistency while also providing more trusted results. These results can help determine which optical clock(s) should be used in the new definition of the second."
Linking the clocks
To carry out the measurements, the researchers had to link the frequency outputs from the different optical clock systems. They did this using two methods: radio signals from satellites and laser light travelling through optical fibers.
The satellite method used GPS signals from the satellite navigation system, which was available to all the clocks included in the study. However, this linking technique has limited precision due to measurement uncertainties caused by factors like signal noise or instrument limits.
The researchers also used customized optical fiber links, which allowed measurements with 100 times greater precision than the satellite technique. However, these stable, high-precision connections could only be used to connect clocks in France, Germany and Italy during the international comparison. In addition, local comparisons within Germany and the UK — where multiple clocks were located at the same institute — were conducted with short optical fibers, which reduced uncertainty even more.
The researchers said that coordinating the simultaneous operation of ten high-performance clocks in various countries and all the links connecting the clocks required extensive planning that started well in advance of the measurements. The data analysis also brought some challenges.
"Not all the results confirmed what we expected, and we observed some inconsistencies in the measurements," said Rachel Godun, principal scientist at NPL. "However, comparing so many clocks at once and using more than one technique for linking the clocks made it easier to identify the source of the problem."
The experiment identified some areas where more work is needed. For example, to confirm that all clocks are performing as expected, measurement uncertainties must be reduced to match the precision of the clocks themselves. Repeated measurements will then be needed to confirm the reliable operation necessary to build confidence in both the clocks and the links. Beyond that, 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.
Paper: T. Lindvall, M. Pizzocaro, R. M. Godun, M. Abgrall, D. Akamatsu, A. Amy-Klein, E. Benkler, N. M. Bhatt, D. Calonico, E. Cantin, E. Cantoni, G. Cerretto, C. Chardonnet, M. A. Cifuentes Marin, C. Clivati, S. Condio, E. A. Curtis, H. Denker, S. Donadello, S. Dörscher, C. Feng, M. Filzinger, T. Fordell, I. Goti, K. Hanhijärvi, H. N. Hausser, I. R. Hill, K. Hosaka, N. Huntemann, M. Y. H. Johnson, J. Keller, J. Klose, T. Kobayashi, S. Koke, A. Kuhl, R. Le Targat, T. Legero, F. Levi, B. Lipphardt, C. Lisdat, H. Liu, J. Lodewyck, O. Lopez, M. Mazouth-Laurol, T. E. Mehlstäubler, A. Mura, A. Nishiyama, T. Nordmann, A. O. Parsons, G. Petit, B. Pointard, P.-E. Pottie, M. Risaro, B. I. Robertson, M. Schioppo, H. Shang, K. Stahl, M. Steinel, U. Sterr, A. Tofful, M. Tønnes, D.-B.-A. Tran, J. Tunesi, A. E. Wallin, H. S. Margolis, "Coordinated international comparisons between
optical clocks connected via fiber and satellite links," 12, (2025).
DOI: 10.1364/OPTICA.561754 .
About Optica Publishing Group
Optica Publishing Group is a division of the society, Optica , Advancing Optics and Photonics Worldwide. It publishes the largest collection of peer-reviewed and most-cited content in optics and photonics, including 18 prestigious journals, the society's flagship member magazine, and papers and videos from more than 835 conferences. With over 400,000 journal articles, conference papers and videos to search, discover and access, our publications portfolio represents the full range of research in the field from around the globe.
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