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A recently-published article in  Nature describes the work of EMRP project SIB55 'International timescales with optical clocks', which uses a broadband two-way satellite time and frequency transfer (TWSTFT) link to compare optical clocks at INRIM (Italy), LNE-SYRTE (France), NPL (UK) and PTB (Germany). Nature is the leading weekly, international scientific journal with over 6 million visitors per month.The SI second is currently defined in terms of a transition frequency in the caesium atom. However the most advanced optical atomic clocks have now reached levels of stability and uncertainty that significantly surpass the performance of caesium primary standards.

Comparison exercises such as this are needed to build confidence in the new generation of optical clocks by validating their performance levels, to anchor their frequencies to the present definition of the second with the lowest possible uncertainty, and to establish the leading contenders for a new definition of the second.

SIB55 coordinator Helen Margolis, NPL, and partner Fritz Riehle, PTB, are quoted in the article. Margolis spoke of future plans to conduct an even more accurate optical clock comparison using fibre-optics, with Riehle adding: “Europe is in a unique position as it has a high density of the best clocks in the world.”

The article can be found here