Timekeepers of the future: Optical atomic clocks

Optical atomic clocks based on laser-cooled atoms or single trapped ions have made rapid progress over the past few years, with the most advanced now having reached levels of stability and uncertainty that significantly surpass the performance of caesium primary frequency standards. These new clocks are already being applied for tests of fundamental physics and the possibility of a redefinition of the SI secord in terms of an optical transition frequency is being considered by the international metrology community.

At NPL we are developing optical atomic clocks based on single trapped ytterbium and strontium ions, and a strontium optical lattice clock. To validate the uncertainty budgets of this new generation of optical clocks it is essential that we compare their frequencies to those of optical clocks being developed by independent research groups in other laboratories. These comparisons need to be performed at a level commensurate with the performance of the optical clocks. However, until recently almost all information about the reproducibility of optical clocks came from independent absolute frequency measurements made in different institutes, and was therefore limited by the uncertainty of the local caesium primary standards used to realise the SI second.

This situation is now changing, with a variety of techniques having recently been used to compare optical clocks directly. Optical clocks developed at NPL and in other European laboratories have now been compared not only by means of independent absolute frequency measurements, but also via GPS satellite links and by using a broadband two-way time and frequency transfer technique. Further information about clock reproducibility can be obtained by comparing optical frequency ratio measurements performed locally within different laboratories and in future by using the recently established optical fibre link to LNE-SYRTE and PTB. Within the framework of the "International Timescales with Optical Clocks (ITOC)" project, new gravity surveys have also been carried out at and around NPL and other European National Measurement Institutes and used to compute an improved European geoid model. This has significantly improved our knowledge of the gravitational redshift corrections that must be applied when comparing optical atomic clocks in the different institutes.