Direct optical frequency comb spectroscopy and optical clock operation with trapped ⁴⁰Ca⁺ ions

We demonstrate direct optical frequency comb spectroscopy of the |4s ²S_1/2⟩ → |3d ²D_5/2⟩ clock transition in ⁴⁰Ca⁺ using a single ion confined in a radio frequency (rf) Paul trap. Here, we present spectra with Hz level linewidths using both Rabi and Ramsey spectroscopy protocols. Using a light shift cancellation method, we calibrate the AC Stark shift and electric quadrupole shift of this transition by performing a frequency ratio measurement against a local ¹⁷¹Yb⁺ optical clock. We demonstrate clock operation with Ca⁺ using direct optical frequency comb interrogation and achieve a statistical uncertainty below 10⁻¹⁶. We present an update on the status of optical clock comparisons between Ca⁺ and Yb⁺ at Colorado State University, including an absolute frequency determination of the ⁴⁰Ca⁺ clock transition using this technique.