Researchers across the UK, France, Germany, Italy, Finland and Japan have completed a highly coordinated comparison of optical clocks – a significant step towards redefining the second. This EU-funded project involved 38 frequency ratio comparisons across ten different optical clocks, advancing the precision of time measurement.
Atomic clocks are fundamental to modern technologies such as satellite navigation and telecommunications, as well as to fundamental scientific research. The current standard clock is based on caesium-133 atoms, which use microwave radiation to excite electrons between two specific energy levels.
This microwave frequency defines the second as the SI unit of time.
Optical clocks, however, operate using light frequencies rather than microwaves, enabling much greater accuracy due to their higher operating frequencies. Despite their potential, optical clocks have yet to be adopted internationally as the new standard for the second.
A critical requirement before this can happen is the ability to compare different types of optical clocks over long distances to ensure consistent performance. To achieve this, researchers linked the frequency outputs of the various clocks using two main methods – satellite radio signals and laser light transmitted through optical fibres. The method used GPS signals accessible to all participating clocks, while customised optical fibre links provided measurements with 100 times greater precision.
Fibre links were used for international connections between clocks in France, Germany and Italy, as well as for shorter connections within institutes in the UK and Germany.
The data set generated by this campaign significantly enhances the body of knowledge on international clock comparisons. The uncertainties and consistency of these measurements will influence which optical transitions are selected for the future definition of the second. However, even lower uncertainties will be needed before the redefinition can take place, alongside other criteria such as demonstrating that optical clocks can reliably contribute to the international atomic time scale. All of this lays important groundwork for the forthcoming redefinition of the SI second and marks a key milestone in the evolution of timekeeping.
