Direct comparison of multi-ion optical clocks based on ${}^{40}$Ca${}^{+}$ and ${}^{88}$Sr${}^{+}$

Yosef Sokolik, Roee Ozeri, Nitzan Akerman

We report the first direct frequency comparison between two multi-ion optical clocks based on the S${}_{1/2}$ to D${}_{5/2}$ transition in \Ca and \Sr ions. Using linear chains of up t…