Towards microwave-driven trapped ions with superconducting circuits

Hyperfine qubits realized in trapped ions allow for high-fidelity single- and two-qubit gate operations. Universal control of those qubits has traditionally been achieved through the utilization of optical Raman transitions with carefully controlled multiple laser beams. Recently, microwave-based entangling methods without lasers have been demonstrated using on-chip circuitry with surface-trapped ions. The direct access to the hyperfine transitions enabled by microwave fields provides high-fidelity operations that are not limited by spontaneous optical Raman emission. Moreover, utilizing mature microwave off-the-shelf components enhances scalability due to their compact sizes and superior controllability of frequencies, phases, and amplitudes. A challenge encountered in microwave-based entangling operations is the generation of joule heat resulting from sub-ampere AC current flowing through relatively narrow microwave waveguides. This substantial amount of joule heat from normal conductors would impose limitations on entanglement fidelity and scalability. To address this issue, we demonstrate an all-superconducting surface trap chip incorporating high-Q superconducting microwave resonators. A sub-ampere current flowing through narrow superconducting electrodes generates a high magnetic field gradient at the ion position, potentially enabling fast microwave Mølmer–Sørensen gates with low joule heat generation and reduced input microwave power.