Isotope Shift Measurements in Ca$^+$

We report progress towards a precise measurement of the isotope shifts in the 4$^2$S$_{1/2}\rightarrow$ 3$^2$D$_{3/2}$ 732 nm electric quadrupole transition 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 or better; when combined into a King Plot, these two measurements can enable exacting tests of King's linearity, offering a path toward probing new physics beyond the Standard Model and also providing benchmarks for ever-improving theory of atomic and nuclear structure.