Optical clocks, including the optical lattice clocks developed at SYRTE, have now demonstrated accuracies beyond those of the best cesium microwave clocks used to realise the second, the unit of time of the International System of Units. For this reason, the scientific community is preparing for a redefinition of the second based on optical clock transitions in the coming decade. However, optical clocks using various atoms and trapping technologies are currently under study, and no candidate is surpassing the others as a substitute for cesium. In view of this situation, it can be expected that the second will no longer be defined from a single atomic species, but from a set of optical transitions of several atomic species.

At first glance, it may seem that using several reference transitions at once necessarily leads to a contradiction. However, this is not the case: in an article published in Metrologia, Jérôme Lodewyck, a CNRS researcher at LNE-SYRTE (CNRS, Observatoire de Paris, PSL Université Paris, Sorbonne Université), proposes a definition of the second based on several transitions. It shows that it is free of contradiction and feasible according to all the criteria in use at the Comité International des Poids et Mesures (CIPM).

The second would be defined by the weighted geometric mean of the frequency of several optical transitions. Thanks to a set of global high-precision frequency ratio measurements, it will be possible to realise it with clocks based on any transition of the set, without significantly degrading the uncertainty of the realization. Finally, it will be possible to regularly update the weights of each transition according to the future developments of clocks, without compromising the continuity of the unit.

Although conceptually very different from the current definition, the unit proposed here is a continuation of the work carried out by the Comité Consultatif Temps Fréquences of the Comité International des Poids et Mesures on the establishment of a list of secondary representations of the second, since it is a generalization of this concept. In this way, it is compatible with the methods currently used by the community, in particular for the steering of the International Atomic Time with microwave and optical clocks.

The adoption of this unit would make it possible to redefine the second without waiting for a given transition to surpass the others, and will be resilient to emerging technologies. Finally, it promotes the diversity of atomic species studied in metrology laboratories, which is the basis for many fundamental physics tests.

Reference

On a definition of the SI second with a set of optical clock transitions, Jérôme Lodewyck, Metrologia (2019)