Over the past fifty years, advances in high-resolution microwave and optical spectroscopy have been correlated with those in time metrology. Clocks have become 9 to 10 orders of magnitude more accurate, thanks in particular to the development of direct measurement of optical frequencies, made possible by the invention of frequency combs some fifteen years ago. In this fifth lesson, we look at these spectacular advances and try to sketch out a few avenues for the future of this extremely precise physics.
Hydrogen spectroscopy, the progress of which is recalled in the first part of the lesson, has served as a testing ground for developments in optical frequency measurements, while continually improving the precision of our knowledge of the Rydberg constant and Lamb shift. The saturated absorption method, followed by the two-photon spectroscopy method, which eliminated the Doppler effect, were described. In the second part, a brief history of time measurement is sketched out, highlighting the fact that the progress made in the six centuries of classical physics that preceded the advent of quanta - while considerable - has been far outstripped by that made possible by the development of atomic clocks since the 1950s. The general principle of clocks remains the same: they count the periods of an oscillator by means of an escapement mechanism. An energy source compensates for the oscillator's damping by minimally affecting its frequency. In classical clocks, the oscillator has been a foliot undergoing torsional oscillation (cathedral clocks), then a mechanical pendulum or spring, and finally a vibrating quartz crystal. In quantum clocks, the oscillator was initially a microwave field locked onto a hyperfine atomic transition (the time standard defined by the cesium clock since the 1960s). The principle of the first Caesium clocks measuring the atomic transition using Ramsey interferometry was recalled. The improvements made to these clocks in the 1990s through the use of laser-cooled atoms were then described, as well as the PHARAO cold atom clock project due to go on board the International Space Station in 2016.
