Accurate time underpins many everyday technologies, particularly in the areas of communications and navigation. A redefinition of the second in terms of an optical frequency will help improve accuracy further, but a number of challenges remain before this can be achieved. Improved methods for comparing the frequencies of optical clocks constructed in different laboratories are needed to help build confidence. Scientists will also need to improve evaluations of the effects of Einstein’s theory of general relativity, and in particular the local effects that gravity has on each clock due to the very high levels of stability and accuracy demonstrated by the clocks. This project made significant advances in addressing the key steps that must be taken prior to the redefinition. An extensive programme of comparisons between high-accuracy European optical atomic clocks was performed, and relativistic effects influencing clock comparisons were evaluated at an improved level of accuracy.
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