Measurement of Scattering Errors During Far Detuned Stimulated Raman Gates

Stimulated Raman transitions are widely used in trapped atom and ion systems for applications such as interferometry, spectroscopy, and as quantum logic gates. These transitions allow for laser addressability of otherwise forbidden or energy inaccessible (rf and microwave) transitions. Current implementation of quantum logic gates by stimulated Raman transitions are limited by errors caused by spontaneous Raman scattering (SRS). Understanding the mechanism of SRS is crucial in order to characterize these gate errors. It was previously believed that SRS placed a fundamental lower bound on the fidelity achievable with Raman gates, but recently published theoretical models [1] suggest that gates implemented with far-detuned lasers are not limited by scattering. We present measurement of SRS rates in metastable, hyperfine ion qubits encoded in ¹³⁷Ba⁺ due to red-, blue-, and near-detuned lasers (-43, 162, -2.5 THz, respectively). We show a scattering error rate of 6×10^-6 per single-qubit, σ_x gate for red-detuned Raman beams, in good agreement with theory. We extend these results to predict error rates for two qubit, entangling gates implemented with Raman lasers in ¹³⁷Ba⁺, a promising candidate for high-fidelity quantum information processing.

[1] I. D. Moore, et al., Phys. Rev. A 107, 032413 (2023).