For the first time, optical clocks have exceeded 50% of the total weighting used in the steering of International Atomic Time (TAI). Achieved in the September 2026 evaluation, this result marks an important milestone in international timekeeping and in the path towards the redefinition of the second.
Among the contributing frequency standards were three optical clocks from the ARCsecond consortium: the ytterbium-ion optical clocks at the Physikalisch-Technische Bundesanstalt (PTB) in Germany and the National Physical Laboratory (NPL) in the United Kingdom, and the strontium-ion optical clock at VTT in Finland.
Crossing this threshold means that, for the first time, the majority of the weighted contribution to the realisation of the SI second originated from optical rather than microwave frequency standards. This achievement reflects the progress made across the international metrology community in the performance, robustness and availability of optical clocks.
These instruments are proving increasingly capable of sustaining a central role in international timekeeping. To ensure that TAI performance is maintained after the redefinition of the second, optical clocks must not only outperform existing primary standards but also contribute to TAI routinely and reliably.
Although the weighting assigned to individual standards varies from month to month, surpassing the 50% threshold for the first time demonstrates the growing readiness of optical clocks to support a future optical redefinition of the second.