Testing the fundamental laws of physics such as Einstein's equivalence principle is one of the hottest topics in modern physics. Indeed, if a violation is discovered it would bring a revolution in our understanding of Nature—if, for example, the existence of a new type of force beyond the four known fundamental interactions was to be proven. Several recent theoretical models do predict violations of the equivalence principle, even if the predictions are not yet precise enough regarding the sensitivity required to observe these violations. This paper represents an important step in testing the stability of the fundamental constants because we use ultra-stable atomic fountain clocks with frequency stabilities which now approach one part in 10^{16}—in other words, no more than a single-second error over a duration of 300 million years.

The methodology had been proposed before using other types of clocks but with less sensitivity. With the recent developments in optical clocks, we anticipate major advances in this field in the coming years. This methodology is to be compared to astrophysical observations such as the analysis of quasar light emission and interstellar absorption. Controversial results exist today on these observations.

I became involved in this research with the development of laser cooling of atoms in the last 20 years and their applications to high precision measurements. Cold atoms are very slow and can be observed without perturbation for extended periods of time, approaching 1 second on the Earth (in a fountain) and, in the future, 100 seconds onboard satellites where microgravity conditions are available.