Historically deeply rooted in the progresses of astronomy and navigation, time and frequency metrology has now evolved to be centered on atomic physics experiments. Frequency measurements and atomic-based time-scales are among the highest accurately measured quantities to date, and many technological and scientific applications alike are nowadays grounded in highly accurate time and frequency atomic-based measurements. The SI second is, as paramount example, currently defined with reference to a microwave frequency atomic transition in cesium atoms.
I will begin with a presentation of the various concepts and tools pertaining to the science of frequency metrology and time keeping. The concepts of time scale, frequency standard, oscillator, noise, stability and accuracy will, in particular be introduced and explained.
I will pursue with showing the current trend in frequency metrology. Whereas the current definition of the second is based on a microwave frequency reference, it is widely accepted that the future definition will be based on an optical frequency atomic reference, as the accuracy of optical frequency measurements has now largely overcome that of microwave frequency measurements. The constant progress of optical frequency measurements over the last decades has, indeed, led to accuracy in the 10-18 range, almost 2 orders of magnitude better than the best microwave frequency standards (atomic fountain clocks). Several hard technological points are however identified that need to be solved in order to further the progress of optical clocks. One such effect is the uncertainty in the black-body radiation shift that impacts frequency standards. Another one is the quality of the oscillator (ultra-stable laser) used to probe the atomic transition, as it currently substantially degrades the optical clock performance, as well as the technique used to compared and transfer the information between various clocks and oscillators, either in the the frequency domain (clocks operating at different frequencies) or space (remote locations).
I will finish by showing how the recent progress is now allowing new scientific and technological applications for optical clocks, in particular chronometric geodesy, tests of fundamental theories and record-low noise oscillators.