In Ba⁺, the long lived 5D_5/2 metastable level, together with the 6S_1/2 ground level, allows for the implementation of multiple qubit types using only one ion species [1]. This enables advanced control techniques such as in-sequence cooling and mid-circuit measurements. These techniques are required for many error correction schemes. For qubits implemented in either level, a two-photon Raman process can be used to drive transitions with very low scattering error [2].

Here we present a laser system that uses only 532nm light to drive two-photon Raman transitions between hyperfine states for both the ground level and the metastable level qubits in ¹³⁷Ba⁺. We generate coherent beat-notes between the two Raman beams with frequency differences of ∼8GHz (ground level transition) and ∼70MHz (metastable level transition). To produce the 8GHz beat-note with sufficiently low phase noise for useful coherence times, we implement an active phase stabilisation system, leading to a significantly improved coherence time of the ground level qubit.

Finally, we present an all-fibre system coupled to a laser-written waveguide device that allows individual addressing of the Raman beams in long ion chains with very low cross-talk.

References

[1] D. Allcock et al. "omg blueprint for trapped ion quantum computing with metastable states", Applied Physics Letters, vol. 119, p. 214 002, 2021.

[2] I. Moore et al. “Photon scattering errors during stimulated raman transitions in trapped-ion qubits,” arXiv preprint arXiv:2211.00744, 2022.