High-fidelity mixed-species entangling gates for ion-trap quantum computing

Working with mixed-species ion crystals is advantageous for scaling up trapped ion systems for quantum computing and networking. A high-fidelity entangling gate between ions of different species gives the freedom to select ions with preferable attributes for different tasks, and to coherently map the information from one to the other depending on the required task.

We present a Mølmer-Sørensen gate entangling a ⁴³Ca⁺ and an ⁸⁸Sr⁺ ion using bichromatic Raman laser beams which are near-resonant with qubit transitions in the hyperfine manifold and in the Zeeman-split levels respectively. We measure a Bell-state  fidelity of 99.6(2)%, which is close to the fidelity  previously obtained using a mixed-species σ_z⊗σ_z light-shift gate in the same experimental setup (99.8(2)% [1]).

The comparison between the two gates allows selection of the more robust mechanism for use in a future networking experiment. The advantage of the Mølmer-Sørensen mechanism is that it can be used on first-order field-insensitive ‘clock’ qubit transitions, which appear in ⁴³Ca⁺. However, we find that the mixed-species Mølmer-Sørensen gate is far more sensitive to slow drifts in the magnetic field than the light-shift gate.

[1] Hughes, A. C., et al. "Benchmarking of a high-fidelity mixed-species entangling gate." (2020)