Researchers have conducted the most extensive coordinated comparison of optical atomic clocks across six countries, marking a significant step toward redefining the second and ultimately establishing a global optical time scale. This experiment, spanning thousands of kilometers, could help to improve the performance of optical atomic clocks, enabling new applications and advancing scientific endeavors that rely on time and frequency.
The study was published on 12 June 2025 in Optica, a journal dedicated to dissemination of peer-reviewed research across the entire spectrum of optics and photonics.
“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. 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 clocks should be used in the new definition of the second”, says Thomas Lindvall, Senior Scientist at VTT MIKES and one of the writers of the scientific article.
To carry out the measurements, the researchers linked the frequency outputs from the different optical clock systems by using two methods: radio signals from global navigation satellites and laser light travelling through optical fibres.
Read more: Optica's press release on 12 June 2025
Writers of the article: 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, 843-852 (2025).
DOI: 10.1364/OPTICA.561754.
Image: The experiment included optical clocks across six countries, including the VTT MIKES optical clock in Finland, shown here. The small spot between the electrode tips in the center of the picture is a single ion of strontium scattering light. Image credit: Anders Wallin
