Quantum information processing in mixed qubit type registers of individually addressed ¹³⁷Ba⁺ions

We present a quantum computing system that allows for storage and individual addressing of long ¹³⁷Ba⁺ chains. The long (~30 s) lifetime of the metastable D_5/2 level in barium makes it well-suited for schemes utilising both ground and metastable level qubits[1]. We drive both qubit types with a two-photon Raman process using 532 nm light. We achieve individual addressing with very low cross-talk of our 532 nm beams via a laser-written waveguide device. We also exploit the Stark shifts induced by the individual beams to execute fundamental operations, including mapping between the ground and metastable levels, state preparation, readout, and cooling, on a targeted subset of qubits.

Employing mixed qubits types within the same register, combined with our individual addressing capabilities, enables techniques like mid-circuit measurements and in-sequence cooling. These techniques are essential for most error correction and error mitigation schemes, as well as dynamic quantum circuit designs. We demonstrate post-selection schemes that enhance the fidelity of state preparation and effectively identify and eliminate errors due to population leakage. We further present preliminary results on a direct entangling gate between a ground level and a metastable level qubit. Such gates make certain quantum algorithms more efficient by minimising the number of necessary operations. Lastly, we discuss how utilising qudit[2] or polyqubit[3] encodings has the potential to simplify some quantum information protocols by reducing the number of ions required.

[1] Allcock, D.T.C., Campbell, W.C., Chiaverini, J., Chuang, I.L., Hudson, E.R., Moore, I.D., Ransford, A., Roman, C., Sage, J.M. and Wineland, D.J., 2021. Omg blueprint for trapped ion quantum computing with metastable states. Applied Physics Letters, 119(21), p.214002.

[2] Low, P.J., White, B.M., Cox, A.A., Day, M.L. and Senko, C., 2020. Practical trapped-ion protocols for universal qudit-based quantum computing. Physical Review Research, 2(3), p.033128.

[3] Campbell, W.C. and Hudson, E.R., 2022. Polyqubit quantum processing. arXiv preprint arXiv:2210.15484.