Optical clocks and fiber links

Optical clocks are now the most stable and accurate frequency standards, with performances in the low 10 −18 , outperforming by two orders of magnitude the best microwave primary standards based on cesium. This opens the question of a possible redefinition of the SI (Systeme International) second with an optical transition. However, several operational steps have to be demonstrated before such a redefinition can be effective: reliable and interconnected clocks are necessary to realize the definition. Here, we report on several experimental advents in this direction, including the remote comparison of optical lattice clocks via phase compensated optical fiber links between different metrology institutes across Europe.