Optical clocks represent the most advanced frontier in time measurement, providing ultraprecise frequency references that promise 100-times-greater precision and accuracy than the atomic clocks currently in use. This has potential implications for both fundamental physics and metrology.
In recent weeks, an international team of researchers completed the largest coordinated comparison of optical clocks ever conducted, involving 10 clocks across six different countries: Finland, France, Germany, Italy, the United Kingdom, and Japan.
INRiM also participated in the campaign, thanks to the contributions of researcher Marco Pizzocaro and researcher Cecilia Clivati, who are involved in the development of next-generation optical clocks and in optical fiber frequency transfer technologies.
The 45-day comparison network connected the clocks via satellite and ultra-stable optical fiber links, enabling centralized data collection and analysis to assess the level of agreement among the different systems.
In total, 38 frequency ratio comparisons were performed, including the first-ever direct measurement of four ratios. The results show an unprecedented level of consistency among the clocks, achieving uncertainties never reached before. This major achievement significantly contributes to the upcoming redefinition of the second in the International System of Units (SI), expected by 2030, and demonstrates the feasibility of a global network of optical clocks to support international time scales and enable new applications in fields that require extremely precise time and frequency signals.
However, the experiment also identified areas where further work is needed—for example, to confirm that all clocks operate as expected.