Final Report Summary - ADOC (Advance Optical Clocks)
The project sought extreme precisions in optical atomic spectroscopy and atomic clocks and explored novel possibilities that stem from these extreme precisions. One part of the project focused on studying neutral mercury (Hg) has a promising candidate to realize to achieve precisions better than 1 part in 1E18. A clock with such precision would make an error not larger than half a second over the age of the Universe (about 14 billion years). Hg was chosen for this investigation because it has great properties to realize the optical lattice clock scheme and because it is comparatively insensitive to thermal radiation, also called blackbody radiation. Hg also comes with challenges, in particular because it is requiring unusual laser sources in the ultraviolet wavelength range. During the project, we developed a Hg optical lattice clock experimental setup, we studied its physics and more generally atomic properties of Hg. For this, we developed novel technologies and methods, notably along the line of producing, manipulating and detecting samples of laser-cooled Hg. All this together yields improving the Hg clock precision by 100 and starting using it both for metrology and for fundamental physics. We made a series of accurate comparisons of Hg with other transitions which advanced the international effort toward a redefinition of the unit of time. We used the Hg clock, connected to other clocks via a European network of coherent optical fiber links to search for certain types of dark matter. All together our investigations generally confirmed the potential of Hg for an extremely precise clock. They helped identifying next key axes of development and anticipating possible ultimate limits. A second part of the project explored emerging applications of optical clocks with extreme precision to Earth science. Remote comparisons of identical clocks measure Einstein’s gravitational redshift, an effect of the modification of space-time in the vicinity of masses. At the Earth surface, a precision of 1E-18 yields gravitational potential differences to a level equivalent to 1 cm. An array of such measurements can enhance our novel of Earth gravity field. The method is called chronometric geodesy. During the project, we develop novel gravity field reconstruction methods that can clock data in conjunction with other types of measurements. Then, we developed a fully synthetic framework that numerically models the entire reconstruction process and to estimate the quality of this reconstruction. Using this framework, we started simulating cases where clocks data are expected to be most valuable. Our work gave the first quantitative study of that kind, showing the benefit of clocks, for example in the case of hilly areas (French Alps being taken as the representative test area). The synthetic framework as general capabilities and we are continuing to exploit it other several other case of interest with the goal to define future pilot programs of chronometric geodesy. Efforts were also made to raise awareness and interest within the geoscience community and others, by publishing review articles and by helping bringing out scientific committees dealing with the matter.