Reducing the error rate of a superconducting logical qubit using analog readout information

Quantum error correction enables the preservation of logical qubits with a lower logical error rate than the physical error rate, with performance depending on the decoding method. Traditional decoding approaches rely on the binarization (“hardening”) of readout data, thereby ignoring valuable information embedded in the analog (“soft”) readout signal. We present experimental results showcasing the advantages of incorporating soft information into the decoding process of a distance-3 (<a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><a:mi>d</a:mi><a:mo>=</a:mo><a:mn>3</a:mn></a:math>) bit-flip surface code with flux-tunable transmons. We encode each of the 16 computational states that make up the logical state <d:math xmlns:d="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><d:mrow><d:mrow><d:mo stretchy="false">|</d:mo></d:mrow><d:mrow><d:msub><d:mn>0</d:mn><d:mi>L</d:mi></d:msub></d:mrow><d:mo fence="false" stretchy="false">⟩</d:mo></d:mrow></d:math>, and protect them against bit-flip errors by performing repeated <j:math xmlns:j="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><j:mi>Z</j:mi></j:math>-basis stabilizer measurements. To infer the logical fidelity for the <m:math xmlns:m="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"><m:mrow><m:mrow><m:mo stretchy="false">|</m:mo></m:mrow><m:mrow><m:msub><m:mn>0</m:mn><m:mi>L</m:mi></m:msub></m:mrow><m:mo fence="false" stretchy="false">⟩</m:mo></m:mrow></m:math> state, we average across the 16 computational states and employ two decoding strategies: minimum-weight perfect matching and a recurrent neural network. Our results show a reduction of up to 6.8% in the extracted logical error rate with the use of soft information. Decoding with soft information is widely applicable, independent of the physical qubit platform, and could allow for shorter readout durations, further minimizing logical error rates. Published by the American Physical Society 2024