Quantum information protocols with metastable trapped-ion barium qubits

We can harness the rich internal structure of ions to enhance quantum information processing in trapped-ion devices by using unorthodox qubit states [1]. Barium is particularly well-suited for such schemes due to its visible-light optical transitions and long-lived metastable states [2]. Furthermore, 532 nm light can be used to drive both the ground S_1/2 state and metastable D_5/2 state qubit transitions via a 2-photon Raman process with very low scattering error.

We present a new method to further mitigate these Raman scattering errors that often limit the fidelity of laser-driven gates. We discuss schemes utilising registers of mixed qubit types to implement partial projective measurements, mid-circuit measurements, and in-sequence cooling of the ions. Finally, we show progress towards implementing an entangling gate between a ground-state and a metastable state qubit, and individual optical addressing using a novel laser-written waveguide system.

[1] D. T. C. Allcock, et al. Applied Physics Letters, 119(21):214002, 2021.

[2] J. E. Christensen, et al. npj Quantum Information, 6(1):35, 2020.