Precision Measurement of Isotope Shifts in the $^2S_{1/2} \, \rightarrow \, ^2D_{5/2}$ 729 nm E2 transition in Ca$^+$

We report progress towards a precise measurement of the isotope shifts in the $4^2S_{1/2} \rightarrow 3^2D_{5/2}$ 729 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 or magnetic field instabilities. Such measurements hold the potential to reach sub-Hz precision, offering a path towards probing New Physics at long and intermediate length scales while also providing benchmarks for ever-improving atomic and nuclear theory.