Connected optical atomic clocks searching for dark matter

Understanding dark matter, which makes up the major part of the matter in the universe, is one of the most important unsolved issues of physics today. It is assumed that dark matter is not evenly distributed in space, but that it concentrates in the form of lumps or layers through which the Earth can move. Researchers from France, the UK and from PTB have been investigating whether the transition frequencies of highly stable optical clocks change when they are immersed in such a cluster of dark matter consisting of light particles. A hypothetical coupling between dark matter and the fundamental constants that determine the clocks’ frequency could cause different frequency variations in different types of clocks.

Since it is impossible to measure a frequency change with only a single clock, optical clocks located in different places have been compared with each other via an optical fiber link. This simultaneously allowed the elimination of local disturbances from the data.

The frequency data of the clocks were first examined as to time-dependent changes which were then converted into a variation of the fine-structure constant α by means of the known sensitivities of the clocks.

Thanks to the excellent long-term stability of the optical clocks used, it was possible, for example, to restrict the time-dependent variation to |δα/α| < 5 × 10⁻¹⁷ for observation times of 1000 s.

The second step consisted in examining how strong the coupling between dark matter and α could be. Assuming that most of the dark matter in our galaxy is present in the form of spatially limited “clouds”, potential coupling processes could be considerably constrained, in particular for regions larger than 10⁴ km.