Polarisation-insensitive state preparation for trapped-ion hyperfine qubits

Quantum state preparation for trapped-ion qubits often relies on high-quality circularly-polarised light, which may be difficult to achieve with chip-based integrated optics technology. We propose and implement a hybrid optical/microwave scheme for intermediate-field hyperfine qubits which instead relies on frequency selectivity. Experimentally, we achieve $99.94%$ fidelity for linearly-polarised (σ⁺/σ⁻) light, using ⁴³Ca⁺ at 28.8 mT. We find that the fidelity remains above $99.8%$ for a mixture of all polarisations (σ⁺/σ⁻/π). We calculate that the method is capable of $99.99%$ fidelity in ⁴³Ca⁺, and even higher fidelities in heavier ions such as ¹37Ba⁺.