Towards tests of King's Linearity in Ca⁺

We report progress towards a test of King's Linearity in the calcium system via precise measurement of isotope shifts in the 4²S_1/2→ 3²D_3/2 (732 nm) and 4²S_1/2→ 3²D_5/2 (729 nm) electric quadrupole transitions in Ca+. We co-trap two isotopes and simultaneously excite both ions using frequency sidebands on a single laser, dramatically reducing systematic uncertainties from many sources such as laser frequency drift and magnetic field instabilities. Such measurements have the potential to reach Hz-level precision (part-per-billion) or better.  A King Plot made at this level of precision could offer unprecedented sensitivity to probe for the existence of light bosonic Dark Matter and other new physics beyond the Standard Model while also providing benchmarks for ever-improving theory of atomic and nuclear structure.