Comparisons of single-ion Yb+ and Cs fountain clocks for searches for new physics

We employ two single-ion Yb⁺ optical clocks that use the ²S1_/2 ->² F7_/2 electric octupole (E3) transition as the reference. Because of their 3 ×10^-18 uncertainty and the strong sensitivity of the transition frequency on the fine structure constant α, comparisons with other atomic clocks enable improvements in searches for temporal variations of α. A particularly suitable transition for such a comparison is the ²S1_/2 ->² D3_/2 electric quadrupole transition of the same ion, that we regularly use to test frequency shifts of the E3 transition induced by residual fields on a magnified scale. Besides investigations for variations of α, long-term comparisons between the Yb⁺ and Cs fountain clocks, with their frequency being sensitive to the proton-to-electron mass ratio μ, allow us to improve present limits on the temporal variation of μ and use the data for searches for ultralight scalar dark matter.

This project has received funding under 15SIB03-OC18 from the EMPIR programme cofinanced by the Participating States and from the European Union's Horizon 2020 research and innovation programme.