Towards Ca⁺-assisted manipulation of Mg⁺ ions in a surface-electrode Paul trap

We discuss progress towards implementing mixed-species operation of Ca⁺-Mg⁺ ion crystals in a surface-electrode radio-frequency trap. The trap has been used to demonstrate universal laser-free control of single and multiple Mg⁺ ions using trap-integrated current-carrying electrodes [1]. Doppler cooling for Mg⁺ is performed with 280 nm ultraviolet light which can cause significant trap charging and thus rapid drifts in the ions' motional frequencies. By introducing a `helper' Ca<sup>+</sup> ion, quantum logic techniques based on rf and microwave magnetic field gradients can be used for sympathetic cooling, as well as state preparation and readout, of a co-trapped `data' Mg⁺ ion using the ions' shared motional modes [2], without requiring 280 nm light. By removing the need for lasers addressing the Mg⁺ ion (apart from a photoionization beam), trap charging effects can be significantly reduced and decoherence of the `data' ion from light that is used to manipulate the Ca⁺ ion will be minimal. We have been adding features to our Ca⁺ ion toolkit at our operating magnetic field of 212.8 G while working towards multi-ion crystal cooling and internal state manipulation.

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

[2] 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).