Scientists have developed a new spectroscopic interrogation protocol that significantly suppresses frequency shifts induced by residual ion motion in optical clocks. These errors, arising from second-order Doppler and quadratic Stark effects, are a major source of uncertainty in state-of-the-art atomic clocks. The new method achieves first-order auto-suppression of these frequency shifts, even with unknown and varying motional energy gain rates, removing the requirement for specific configurations that rely on cancelling opposite-signed effects.

The technique was experimentally demonstrated on a new ytterbium ion optical clock (Yb⁺), probing the 467 nm electric octupole (E3) transition. The combined fractional uncertainty contribution from motion-induced frequency shifts was reduced from 1.3×10⁻¹⁸ to 0.3×10⁻¹⁸. This advancement represents a substantial improvement in the precision of these devices, which are already the most accurate timekeepers known.

In addition to improved stability, the team performed an interleaved optical frequency ratio measurement between the E3 and ytterbium's electric quadrupole (E2) transition at 435 nm, yielding a value of 0.932 829 404 530 965 340 (39). Combining this data with previously published ratio data sets a new limit for a potential fractional temporal variation of the fine-structure constant of 2.4 (2.7) × 10⁻¹⁹/year, consistent with existing bounds and contributing to the search for new physics beyond the Standard Model.