Towards a scalable mixed-species ion trap quantum computer with one laser-free species

We discuss progress towards implementing mixed species operation of Ca⁺-Mg⁺ ion crystals in a surface-electrode radio-frequency trap. By utilizing quantum logic spectroscopy techniques, the ‘helper’ Ca⁺ ion can be used to sympathetically cool, prepare a hyperfine ground state of, and non-destructively read out the ‘data’ Mg⁺ ion [1]. Universal laser-free control of single and multiple Mg⁺ ions [2], as well as laser-free Ca⁺-Mg⁺ entangling operations, are performed using trap-integrated microwave electrodes. Laser-free addressing of individual ions in a trap zone, or controllable sets of ions in multiple zones, can be used to perform simultaneous operations in parallel on many qubits. By removing the need for lasers addressing the Mg⁺ ion, trap charging due to ultraviolet light and damage to the ‘data’ ion from resonant light crosstalk can both be eliminated.

[1] P. O. Schmidt, T. Rosenband, C. Langer, W. M. Itano, J. C. Bergquist, and D. J. Wineland. Spectroscopy using quantum logic. Science 309, 749 (2005).

[2] R. Srinivas et al. High-fidelity laser-free universal control of trapped ion qubits. Nature 597, 209–213 (2021)