Polarisation-insensitive state preparation for trapped-ion hyperfine qubits

Abstract

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 (${\sigma }^{+}$/${\sigma }^{-}$) light, using ${}^{43}$Ca${}^{+}$ at 28.8 mT. We find that the fidelity remains above $99.8\%$ for a mixture of all polarisations (${\sigma }^{+}$/${\sigma }^{-}$/$\pi$). We calculate that the method is capable of $99.99\%$ fidelity in ${}^{43}$Ca${}^{+}$, and even higher fidelities in heavier ions such as ${}^{\text{137}}$Ba${}^{\text{+}}$.