Searches for new physics with Yb⁺ optical clocks

Besides the use of optical clocks in metrology, comparisons of these clocks enable searches for violations of the Einstein equivalence principle. The ¹⁷¹Yb⁺ optical clock that is based on electric octupole (E3) transition possesses a favorable combination of small systematic uncertainty and high sensitivity for such tests because of the strongly relativistic character of the excited ²F_7/2 state.  A comparison of two Yb⁺(E3) clocks with 4 x 10^-18 uncertainty provides stringent limits on Lorentz symmetry violation parameters for electrons [1]. Further improvements can be expected from experiments in which the Lorentz violating energy at the millihertz-level or below can be determined without a 642 THz clock frequency offset. Here, we concentrate on spectroscopy of the hyperfine structure of the ²F_7/2 state using microwave radiation enabling seconds long coherence times. These investigations make use of the extraordinary long lifetime of the ²F_7/2 level which we measure with a new method as 1.58(8) years [2].

Using two caesium fountain clocks, we determine the E3 transition frequency at 642 THz with 80 mHz uncertainty, the most accurate determination of an optical transition frequency to date. In addition to the E3 transition, the Yb⁺ ion also provides an electric quadrupole (E2) that can be employed in an optical clock. The ratio of the E3 and E2 transition frequencies has been determined in experiments with single laser-cooled trapped ions with a fractional uncertainty of 3×10^-17. Repeated measurements of both quantities over several years are analyzed for potential violations of local position invariance [3]. Including recent results, we improve by factors of more than 20 and 2 the limits on temporal variations of the fine structure constant α to below 10^-18/yr and of the proton-to-electron mass ratio to 4×10^-17/yr.

[1] C. Sanner, et al., Nature 567, 204 (2019).

[2] R. Lange, et al., PRL 127, 213001 (2021).

[3] R. Lange et al., PRL 126, 011102 (2021).