Polarization-insensitive state preparation for trapped-ion hyperfine qubits

Quantum state preparation for trapped-ion qubits often relies on high-quality circularly polarized light, which may be difficult to achieve with chip-based integrated optics technology. We propose and implement a hybrid optical and microwave scheme for intermediate-field hyperfine qubits which instead relies on frequency selectivity. Experimentally, we achieve <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:mrow><a:mn>99.94</a:mn><a:mo>%</a:mo></a:mrow></a:math> fidelity for linearly polarized <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:mrow><b:mo>(</b:mo><b:msup><b:mi>σ</b:mi><b:mo>+</b:mo></b:msup><b:mo>/</b:mo><b:msup><b:mi>σ</b:mi><b:mo>−</b:mo></b:msup><b:mo>)</b:mo></b:mrow></b:math> light, using <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:mmultiscripts><c:mi>Ca</c:mi><c:none/><c:mo>+</c:mo><c:mprescripts/><c:none/><c:mn>43</c:mn></c:mmultiscripts></c:math> at 28.8 mT. We find that the fidelity remains above <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:mrow><d:mn>99.8</d:mn><d:mo>%</d:mo></d:mrow></d:math> for a mixture of all polarizations <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:mrow><e:mo>(</e:mo><e:msup><e:mi>σ</e:mi><e:mo>+</e:mo></e:msup><e:mo>/</e:mo><e:msup><e:mi>σ</e:mi><e:mo>−</e:mo></e:msup><e:mo>/</e:mo><e:mi>π</e:mi><e:mo>)</e:mo></e:mrow></e:math>. We calculate that the method is capable of <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:mrow><f:mn>99.99</f:mn><f:mo>%</f:mo></f:mrow></f:math> fidelity in <g:math xmlns:g="http://www.w3.org/1998/Math/MathML"><g:mmultiscripts><g:mi>Ca</g:mi><g:none/><g:mo>+</g:mo><g:mprescripts/><g:none/><g:mn>43</g:mn></g:mmultiscripts></g:math>, and even higher fidelities in heavier ions such as <h:math xmlns:h="http://www.w3.org/1998/Math/MathML"><h:mmultiscripts><h:mi>Ba</h:mi><h:none/><h:mo>+</h:mo><h:mprescripts/><h:none/><h:mtext>137</h:mtext></h:mmultiscripts></h:math>. Published by the American Physical Society 2024