Project B02: Optical clocks testing fundamental physics

Results

Towards this goal, we have tested local position invariance (LPI) by repeatedly comparing the output of atomic clocks realizing different clock transition frequencies. The different sensitivities of the involved atomic states to fundamental constants, such as the fine-structure constant α or the proton-to-electron mass ratio μ, allows us to either identify the corresponding violating constant or to constrain its instability. Within DQ-mat, we have recently established the most stringent bounds on temporal variations and a potential coupling to gravity for α and μ. These investigations profited in particular from the high sensitivity of the clock transitions of the Yb⁺ ion to variations of α.

To perform reliable tests and to further push the limits of our searches for new-physics effects, we have improved the understanding of frequency shifts resulting from blackbody radiation and the lattice laser light for the Sr clock, which aided to resolve a discrepancy between experimentally and theoretically determined atomic parameters and lead to a better understanding of the atomic structure of Sr. We have started to operate a Yb⁺ ion clock setup in which ⁸⁸Sr⁺ can be employed as ancillary ion, e.g., to measure the perturbing thermal radiation. The latter investigation has also helped to resolve a serious tension in recent measurements of the ⁸⁸Sr⁺ clock transition frequency.

Both systems, the ⁸⁷Sr lattice clock and the ¹⁷¹Yb⁺ ion clock, have also been employed in long-term frequency comparisons and to search for another new-physics curiosity: dark matter.