Work package leader: Rachel Godun, NPL

Other participants: INRIM, LNE, OBSPARIS, PTB, VTT, CNRS, LUH, SRC PAS, UMK

The aim of this work package is to check the long-term international consistency of optical atomic clocks through a coordinated programme of frequency comparisons. Extensive and repeated clock comparisons of this sort are the only way to determine the level at which it will be possible to redefine the SI second.

Using optical fibre links, our target is to reduce the comparison uncertainties between optical clocks across Europe by an order of magnitude for the most advanced clocks. The European fibre network that currently links NPL, OBSPARIS and PTB will also be extended to include INRIM. Satellite links will be used to include more optical clocks in the comparisons, opening up the possibility of involving state-of-the-art clocks from organisations outside Europe.

Local optical frequency ratio measurements will be performed, with our target again being to improve the uncertainties by an order of magnitude or more for the most advanced optical clocks. Such measurements will enable European optical clocks to be compared with those in other continents, even though no high accuracy links are available over such long distances.

Finally, traceability to the present definition of the second will be ensured by including caesium primary frequency standards in the comparison programme. Here the target uncertainties will be at the low parts in 10¹⁶ level, limited by the performance of the local caesium fountains. Nevertheless, such measurements will allow the frequencies of all optical clocks involved in the comparison programme to be determined with the lowest possible uncertainty relative to the current definition of the second. This is essential in order to maximise the impact of including them in international timescales and to avoid any discernible discontinuity occurring at the point of an anticipated future redefinition of the second.

If you operate an optical atomic clock and are interested in participating in our clock comparison campaigns, please contact the coordinator, Helen Margolis at NPL, for more details.