Towards Scalable Quaternary Message-Passing Decoding for Quantum Error Correction
Pith reviewed 2026-06-30 15:53 UTC · model grok-4.3
The pith
A dilution method lets quaternary Min-Sum decoders reach apparent 16% depolarizing thresholds up to distance 20.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
We introduce a dilution method, which allows a quaternary Min-Sum (MS) decoder to exhibit an apparent depolarizing threshold of 16% up to distance 20, outperforming Minimum-Weight Perfect Matching in finite-length regimes. Notably, for X-noise, the standard MS decoder under dilution has worst-case complexity O(N log² d) and outperforms BP-OSD at d=65. The observed ∼9% threshold may correspond to a true asymptotic threshold. Finally, we give a graph-dilution argument that interprets the success of the dilution method and offers insight into when MP algorithms can genuinely scale.
What carries the argument
The dilution method applied to the decoding graph for quaternary Min-Sum decoding, together with the accompanying graph-dilution argument.
Load-bearing premise
The dilution procedure and graph-dilution argument remain effective at distances much larger than 65 and the reported thresholds are not artifacts of the specific simulation parameters or code instances used.
What would settle it
Running the diluted Min-Sum decoder on surface codes of distance 100 or higher under depolarizing noise to determine if the threshold stays near 16% or falls.
Figures
read the original abstract
The scalability and interpretability of message-passing (MP) decoding, such as (quaternary) Belief Propagation, remain open challenges in quantum error correction. Even for surface codes, arguably the first testbed for decoding methods, studies of improved MP decoders have mostly been restricted to small distances ($d \lesssim 19$). Moreover, the mismatch with established message-passing theory limits the decoder's interpretability, making it unclear whether MP decoding can sustain its effectiveness at large system sizes. This work takes a step toward a more principled and interpretable MP decoding framework, with the goal of making MP-based decoding more reliable and bridging theory and practice. We introduce a dilution method, which allows a quaternary Min-Sum (MS) decoder to exhibit an apparent depolarizing threshold of $16\%$ up to distance $20$, outperforming Minimum-Weight Perfect Matching in finite-length regimes. Notably, for $X$-noise, the standard MS decoder under dilution has worst-case complexity $O(N \log^2 d)$ and outperforms BP-OSD at $d=65$. The observed $\sim 9\%$ threshold may correspond to a true asymptotic threshold. Finally, we give a graph-dilution argument that interprets the success of the dilution method and offers insight into when MP algorithms can genuinely scale. Taken together, these results provide encouraging progress toward scalable and interpretable MP decoding in quantum error correction.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript proposes a dilution method for quaternary Min-Sum (MS) message-passing decoding of quantum surface codes. It reports an apparent depolarizing threshold of 16% up to distance 20 (outperforming MWPM in finite-length regimes), an O(N log² d) complexity for X-noise that outperforms BP-OSD at d=65, and a possible ~9% asymptotic threshold, interpreted via a graph-dilution argument that aims to explain when MP decoders can scale.
Significance. If the reported thresholds and complexity advantages hold asymptotically and generalize beyond the tested distances and code instances, the work would provide a concrete step toward scalable, interpretable MP decoding in QEC, addressing the current restriction of such studies to small distances and offering a principled way to suppress trapping sets via dilution.
major comments (3)
- [Simulation results] Results on depolarizing noise (up to d=20): the central 16% threshold claim and its comparison to MWPM rest on Monte-Carlo data whose statistical reliability cannot be assessed without reported trial counts, error bars, or the precise dilution implementation (e.g., how edges or messages are diluted).
- [X-noise performance] X-noise results at d=65: the O(N log² d) MS decoder outperforming BP-OSD is load-bearing for the scalability claim, yet the manuscript provides no details on the surface-code instances, cycle structure after dilution, or whether the observed advantage persists when the underlying Tanner graph contains longer cycles.
- [Graph-dilution argument] Graph-dilution argument: presented as interpretive rather than a closed-form bound, it does not yet demonstrate that short-cycle suppression remains effective at distances ≫65; without such analysis the extrapolation to a possible 9% asymptotic threshold remains an open risk for the central scaling claim.
minor comments (2)
- [Preliminaries] Notation for the quaternary alphabet and message alphabet should be defined once at first use and used consistently.
- [Figures] Figure captions for threshold plots should explicitly state the number of logical errors observed per data point.
Simulated Author's Rebuttal
We thank the referee for the careful reading and constructive comments. We address each major comment below.
read point-by-point responses
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Referee: [Simulation results] Results on depolarizing noise (up to d=20): the central 16% threshold claim and its comparison to MWPM rest on Monte-Carlo data whose statistical reliability cannot be assessed without reported trial counts, error bars, or the precise dilution implementation (e.g., how edges or messages are diluted).
Authors: We agree these statistical and implementation details are necessary to assess reliability. In the revised manuscript we will report the number of Monte-Carlo trials per data point, include error bars on all logical-error-rate plots, and give a precise description of the dilution procedure including the selection of edges and messages. revision: yes
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Referee: [X-noise performance] X-noise results at d=65: the O(N log² d) MS decoder outperforming BP-OSD is load-bearing for the scalability claim, yet the manuscript provides no details on the surface-code instances, cycle structure after dilution, or whether the observed advantage persists when the underlying Tanner graph contains longer cycles.
Authors: We will add the requested details: the specific surface-code instances used at d=65, the cycle structure of the diluted Tanner graph, and a discussion of decoder performance when longer cycles are present. These additions will be placed in the relevant results section. revision: yes
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Referee: [Graph-dilution argument] Graph-dilution argument: presented as interpretive rather than a closed-form bound, it does not yet demonstrate that short-cycle suppression remains effective at distances ≫65; without such analysis the extrapolation to a possible 9% asymptotic threshold remains an open risk for the central scaling claim.
Authors: The graph-dilution argument is offered as an interpretive lens rather than a closed-form proof. We will revise the text to state its limitations explicitly, clarify that it does not constitute a demonstration for distances ≫65, and note that the ~9% figure remains a conjecture pending further numerical or analytic work at larger distances. revision: partial
Circularity Check
No circularity; thresholds from finite-distance Monte Carlo simulations and graph-dilution argument offered as interpretation only.
full rationale
The paper's central results consist of empirical performance curves obtained via Monte Carlo decoding simulations on surface codes up to d=65 (X-noise) and d=20 (depolarizing), together with an interpretive graph-dilution argument that explains observed behavior without claiming a closed-form derivation or asymptotic proof. No equations reduce any reported threshold to a fitted parameter by construction, no self-citation chain bears the load of the main claim, and the dilution method is presented as an algorithmic modification whose effectiveness is measured directly rather than assumed. The derivation chain is therefore self-contained against external benchmarks.
Axiom & Free-Parameter Ledger
axioms (1)
- domain assumption Surface codes under depolarizing or X noise behave according to established quantum error correction models.
invented entities (1)
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Dilution method
no independent evidence
Reference graph
Works this paper leans on
-
[1]
On the iterative decoding of sparse quantum codes
D. Poulin and Y. Chung, “On the iterative decoding of sparse quantum codes,”arXiv preprint arXiv:0801.1241, 2008
work page internal anchor Pith review Pith/arXiv arXiv 2008
-
[2]
T. Müller, T. Alexander, M. E. Beverland, M. Bühler, B. R. Johnson, T. Maurer, and D. Vandeth, “Improved belief propagation is sufficient for real-time decoding of quantum memory,”arXiv preprint arXiv:2506.01779, 2025
-
[3]
Fifteen years of quantum ldpc coding and improved decoding strategies,
Z. Babar, P. Botsinis, D. Alanis, S. X. Ng, and L. Hanzo, “Fifteen years of quantum ldpc coding and improved decoding strategies,”iEEE Access, vol. 3, pp. 2492–2519, 2015
2015
-
[4]
Trapping sets of quantum ldpc codes,
N. Raveendran and B. Vasić, “Trapping sets of quantum ldpc codes,”Quantum, vol. 5, p. 562, 2021
2021
-
[5]
Degeneracy and its impact on the decoding of sparse quantum codes,
P. Fuentes, J. E. Martinez, P. M. Crespo, and J. Garcia-Frías, “Degeneracy and its impact on the decoding of sparse quantum codes,”IEEE Access, vol. 9, pp. 89093–89119, 2021
2021
-
[6]
Richardson and R
T. Richardson and R. Urbanke,Modern coding theory. Cambridge university press, 2008
2008
-
[7]
Mezard and A
M. Mezard and A. Montanari,Information, physics, and computation. Oxford University Press, 2009
2009
-
[8]
Quantum ldpc codes with positive rate and minimum distance proportional to the square root of the blocklength,
J.-P. Tillich and G. Zémor, “Quantum ldpc codes with positive rate and minimum distance proportional to the square root of the blocklength,”IEEE Transactions on Information Theory, vol. 60, no. 2, pp. 1193–1202, 2013
2013
-
[9]
Quantum ldpc codes with almost linear minimum distance,
P. Panteleev and G. Kalachev, “Quantum ldpc codes with almost linear minimum distance,”IEEE Transactions on Information Theory, vol. 68, no. 1, pp. 213–229, 2021
2021
-
[10]
Balanced product quantum codes,
N. P. Breuckmann and J. N. Eberhardt, “Balanced product quantum codes,”IEEE Transactions on Information Theory, vol. 67, no. 10, pp. 6653–6674, 2021
2021
-
[11]
Belief propagation decoding of quantum ldpc codes with guided decimation,
H. Yao, W. A. Laban, C. Häger, A. G. i Amat, and H. D. Pfister, “Belief propagation decoding of quantum ldpc codes with guided decimation,” in2024 IEEE International Symposium on Information Theory (ISIT). IEEE, 2024, pp. 2478–2483
2024
-
[12]
Exploiting degeneracy in belief propagation decoding of quantum codes,
K.-Y. Kuo and C.-Y. Lai, “Exploiting degeneracy in belief propagation decoding of quantum codes,” npj Quantum Information, vol. 8, no. 1, p. 111, 2022
2022
-
[13]
Generalized belief propagation algorithms for decoding of surface codes,
J. Old and M. Rispler, “Generalized belief propagation algorithms for decoding of surface codes,”Quan- tum, vol. 7, p. 1037, 2023
2023
-
[14]
Enhanced feedback iterative decoding of sparse quantum codes,
Y.-J. Wang, B. C. Sanders, B.-M. Bai, and X.-M. Wang, “Enhanced feedback iterative decoding of sparse quantum codes,”IEEE transactions on information theory, vol. 58, no. 2, pp. 1231–1241, 2012
2012
-
[15]
Stabilizer inactivation for message-passing decoding of quantum ldpc codes,
J. Du Crest, M. Mhalla, and V. Savin, “Stabilizer inactivation for message-passing decoding of quantum ldpc codes,” in2022 IEEE Information Theory Workshop (ITW). IEEE, 2022, pp. 488–493
2022
-
[16]
Topological quantum memory,
E. Dennis, A. Kitaev, A. Landahl, and J. Preskill, “Topological quantum memory,”Journal of Mathe- matical Physics, vol. 43, no. 9, pp. 4452–4505, 2002
2002
-
[17]
Almost-linear time decoding algorithm for topological codes,
N. Delfosse and N. H. Nickerson, “Almost-linear time decoding algorithm for topological codes,”Quan- tum, vol. 5, p. 595, 2021
2021
-
[18]
Proof of finite surface code threshold for matching,
A. G. Fowler, “Proof of finite surface code threshold for matching,”Physical review letters, vol. 109, no. 18, p. 180502, 2012
2012
-
[19]
Proof of a finite threshold for the union-find decoder
S. Yoshida, E. Lake, and H. Yamasaki, “Proof of a finite threshold for the union-find decoder,”arXiv preprint arXiv:2602.20238, 2026
work page internal anchor Pith review Pith/arXiv arXiv 2026
-
[20]
Degenerate quantum ldpc codes with good finite length performance,
P. Panteleev and G. Kalachev, “Degenerate quantum ldpc codes with good finite length performance,” Quantum, vol. 5, p. 585, 2021. 17
2021
-
[21]
Localized statistics decoding 13 for quantum low-density parity-check codes
T. Hillmann, L. Berent, A. O. Quintavalle, J. Eisert, R. Wille, and J. Roffe, “Localized statistics decoding: A parallel decoding algorithm for quantum low-density parity-check codes,”arXiv preprint arXiv:2406.18655, 2024
-
[22]
Improved single-shot decoding of higher-dimensional hypergraph- product codes,
O. Higgott and N. P. Breuckmann, “Improved single-shot decoding of higher-dimensional hypergraph- product codes,”PRX Quantum, vol. 4, no. 2, p. 020332, 2023
2023
-
[23]
Statistical physics of inference: Thresholds and algorithms,
L. Zdeborová and F. Krzakala, “Statistical physics of inference: Thresholds and algorithms,”Advances in Physics, vol. 65, no. 5, pp. 453–552, 2016
2016
-
[24]
Good quantum error-correcting codes exist,
A. R. Calderbank and P. W. Shor, “Good quantum error-correcting codes exist,”Phys. Rev. A, vol. 54, pp. 1098–1105, Aug 1996. [Online]. Available: https://link.aps.org/doi/10.1103/PhysRevA.54.1098
-
[25]
Multiple-particleinterferenceandquantumerrorcorrection,
A.Steane, “Multiple-particleinterferenceandquantumerrorcorrection,”ProceedingsoftheRoyalSociety ofLondon.SeriesA:Mathematical, PhysicalandEngineeringSciences, vol.452, no.1954, pp.2551–2577,
1954
-
[26]
Available: https://royalsocietypublishing.org/doi/abs/10.1098/rspa.1996.0136
[Online]. Available: https://royalsocietypublishing.org/doi/abs/10.1098/rspa.1996.0136
-
[27]
Refined belief propagation decoding of sparse-graph quantum codes,
K.-Y. Kuo and C.-Y. Lai, “Refined belief propagation decoding of sparse-graph quantum codes,”IEEE Journal on Selected Areas in Information Theory, vol. 1, no. 2, pp. 487–498, 2020
2020
-
[28]
Log-domain decoding of quantum ldpc codes over binary finite fields,
C.-Y. Lai and K.-Y. Kuo, “Log-domain decoding of quantum ldpc codes over binary finite fields,”IEEE Transactions on Quantum Engineering, vol. 2, pp. 1–15, 2021
2021
-
[29]
Quaternary neural belief propagation decoding of quantum ldpc codes with overcomplete check matrices,
S. Miao, A. Schnerring, H. Li, and L. Schmalen, “Quaternary neural belief propagation decoding of quantum ldpc codes with overcomplete check matrices,”IEEE Access, 2025
2025
-
[30]
Stabilizer Codes and Quantum Error Correction
D.Gottesman, “Stabilizercodesandquantumerrorcorrection.caltechph.d,” Ph.D.dissertation, Thesis, eprint: quant-ph/9705052, 1997
work page internal anchor Pith review Pith/arXiv arXiv 1997
-
[31]
Fault-tolerant quantum computation by anyons,
A. Y. Kitaev, “Fault-tolerant quantum computation by anyons,”Annals of physics, vol. 303, no. 1, pp. 2–30, 2003
2003
-
[32]
A message-passing algorithm with damping,
M. Pretti, “A message-passing algorithm with damping,”Journal of Statistical Mechanics: Theory and Experiment, vol. 2005, no. 11, pp. P11008–P11008, 2005
2005
-
[33]
Sufficient conditions for convergence of Loopy Belief Propagation
J. Mooij and H. Kappen, “Sufficient conditions for convergence of loopy belief propagation,”arXiv preprint arXiv:1207.1405, 2012
work page internal anchor Pith review Pith/arXiv arXiv 2012
-
[34]
Beyond trees: Analysis and convergence of belief propagation in graphs with multiple cycles,
R. Zivan, O. Lev, and R. Galiki, “Beyond trees: Analysis and convergence of belief propagation in graphs with multiple cycles,” inProceedings of the AAAI Conference on Artificial Intelligence, vol. 34, no. 05, 2020, pp. 7333–7340
2020
-
[35]
Belief propagation decoding on a sparsified graph ensemble of the surface code,
B. Zhang, H. Yao, and H. D. Pfister, “Belief propagation decoding on a sparsified graph ensemble of the surface code,” in2025 IEEE International Symposium on Information Theory (ISIT). IEEE, 2025, pp. 1–6. 18 A Proof of Theorem 1 We use two different local decompositions for the two sparsification patterns, as shown in the Fig. 15. For the diagonal patter...
2025
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