Polarisation-insensitive state preparation for trapped-ion hyperfine qubits

TL;DR

This paper introduces a hybrid optical/microwave method for preparing trapped-ion hyperfine qubit states that is insensitive to polarization, achieving high fidelity with simpler optical requirements.

Contribution

The authors demonstrate a polarization-insensitive state preparation technique using frequency selectivity, achieving over 99.8% fidelity with linearly polarized light in trapped calcium ions.

Findings

Achieved 99.94% fidelity with linearly polarized light.

Fidelity remains above 99.8% for mixed polarisations.

Method theoretically capable of 99.99% fidelity in calcium ions.

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$^\text{+}$.