Benchmarking a High-Fidelity Mixed-Species Entangling Gate

We implement a two-qubit logic gate between a ⁴³Ca⁺ hyperfine qubit and a ⁸⁸Sr⁺ Zeeman qubit. For this pair of ion species, the S-P optical transitions are close enough that a single laser of wavelength 402 nm can be used to drive the gate but sufficiently well separated to give good spectral isolation and low photon scattering errors. We characterize the gate by full randomized benchmarking, gate set tomography, and Bell state analysis. The latter method gives a fidelity of 99.8(1)%, comparable to that of the best same-species gates and consistent with known sources of error.