Low-excitation transport and separation of high-mass-ratio mixed-species ion chains

We demonstrate low-excitation transport and separation of two-ion crystals consisting of one ${}^9\mathrm{Be}^{+}$ and one ${}^{40}\mathrm{Ca}^{+}$ ion, with a high mass ratio of 4.4. The full separation involves transport of the mixed-species chain, splitting each ion into separate potential wells, and then transport of each ion prior to detection. We find the high mass ratio makes the protocol sensitive to mode crossings between axial and radial modes, as well as to uncontrolled radial electric fields that induce mass-dependent twists of the ion chain, which initially gave excitations $\overline{n}\gg 10$. By controlling these stages, we achieve excitation as low as $\overline{n}=1.40\pm 0.08\mathrm{phonons}$ for the calcium ion and $\overline{n}=1.44\pm 0.09\mathrm{phonons}$ for the beryllium ion. Separation and transport of mixed-species chains are key elements of the quantum charge-coupled device architecture and may also be applicable to quantum-logic-based spectroscopy of exotic species.