18SIB05 ROCIT – Robust Optical Clocks for International Timescales: Final

Optical atomic clocks have now reached levels of performance that clearly surpass the stability and accuracy achievable with caesium microwave primary frequency standards, with the result that a future optical redefinition of the SI second is anticipated. This project tackled key steps that must be taken prior to the redefinition. The robustness of optical clocks was improved so that they could run unattended for long periods, methods were developed for automatic data validation enabling prototype demonstrations of optically steered timescales, and the long-term reliability and reproducibility of the clocks were assessed through a coordinated programme of international comparisons. By the end of the project several European optical clocks were contributing on a regular basis to International Atomic Time (TAI) as secondary representations of the second, improving its stability and accuracy. These steps represent important progress towards the criteria set out in the international roadmap towards a redefinition of the second, and since international timescales underpin modern-day technology such as telecommunications and navigation systems, the work will benefit end users across a wide range of sectors in the longer term.

At VTT, very significant progress was made to the VTT MIKES 88Sr+ single-ion optical clocks. ARTIQ routines were developed for running the clock and for evaluating and minimizing systematic uncertainties. The clock was operated for the first time in the beginning of the project and significant improvements to the stability was made, for example three-layer magnetic shielding to allow operation in the magnetic field from the nearby metro train. In March 2022, the VTT clock participated in the international clock comparison campaign arranged within the project via a GPS link. The campaign involved 8 clocks from 6 different countries, with 4 Central-European clocks being connected via fibre link and the rest via GPS. VTT also carried out the analysis of the GPS comparison.