The second is, from far, the unit of the SI system that is best realized, thanks to microwave (1010 Hz) atomic clocks under development since the 1960'. Though, in the last 5 years, a new generation of clocks, based on optical atomic transitions (1015 Hz), has considerably surpassed these traditional clocks. This opens a large field of possible tests, ranging from Fundamental Physics to Earth Science, like geodesy, that were so far impossible to achieve.
Are we ready for a new definition of the second?
I will present the basic principles of atomic clocks, discuss the limits and give examples of the state of the art machinery necessary to perform the spectroscopy of the atoms. I will, notably, show results of ongoing experiments using our European optical fiber link, operational since June 2015, and progressively connecting the 10 or more optical clocks developed in Europe. Finally I will discuss the infrastructure developed at SYRTE, Observatoire de Paris, in the perspective of the PHARAO-ACES mission (Atomic Clock Ensemble in Space), scheduled for launching in 2018.