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  • Open Access

Soft information decoding with superconducting qubits

Maurice D. Hanisch1,2, Bence Hetényi1,*, and James R. Wootton1,3,†

  • *Contact author: bence.hetenyi@ibm.com
  • †Contact author: james.wootton@unibas.ch
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APS Open Sci. 1, 000019 – Published 8 May, 2026

DOI: https://doi.org/10.1103/y9fh-4x6n

Abstract

Quantum error correction promises a viable path to fault-tolerant computations, enabling exponential error suppression when the device's error rates remain below the protocol's threshold. However, this threshold strongly depends on the classical method used to decode the measurements of the syndrome. These classical algorithms traditionally only interpret binary data, ignoring valuable information contained in complete analog measurement data. We present the first large-scale experimental verification of soft decoding by implementing repetition codes up to distance 51 on superconducting hardware. Unlike previous demonstrations limited to small code distances, this extensive range allows us to confirm a fundamental improvement in the exponential scaling of error suppression rather than a mere constant-factor reduction in error rates. We observe a 25% increase in the threshold, which yields logical error rates up to 30 times lower than standard hard decoding. Analyzing the trade-off between information volume and decoding performance, we show that a single byte of information per measurement suffices to reach optimal decoding. This underscores the effectiveness and practicality of soft decoding on hardware, including in time-sensitive contexts such as real-time decoding.

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Outline

Information

APS Open Sci. 1, 000019– Published 8 May, 2026
Vol. 1
  • Received 26 June 2025
  • Accepted 22 April 2026
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Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

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