Concatenating Algebraic Codes over High-Rate Quantum LDPC Codes
Pith reviewed 2026-05-22 06:38 UTC · model grok-4.3
The pith
Treating high-rate LDPC blocks as logical Galois qudits lets quantum Reed-Solomon codes reach the teraquop regime at lower space cost than prior gross-code constructions.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
By representing each gross-code block as a single logical Galois qudit and protecting its syndrome extraction with a time-like Reed-Solomon code, the concatenated system reaches the teraquop regime at 10^{-3} physical noise with lower space overhead than the 288-qubit two-gross code. Optimizations include refined bicycle-instruction error rates, new compilation strategies, and decoder post-selection rules. A lightweight fault-tolerance scheme that would fail for qubits succeeds for these large-alphabet qudits.
What carries the argument
Treating each high-rate quantum LDPC code block as one logical Galois qudit, which converts intra-block correlated errors into ordinary errors on an algebraic object and thereby enables concatenation with list-decodable Reed-Solomon outer codes.
If this is right
- The concatenated gross code enters the teraquop regime at 10^{-3} noise where earlier two-gross constructions did not.
- Space overhead is lower than that of the 288-qubit two-gross code at the same noise level.
- The protocol supplies several engineering advantages including relaxed connectivity demands.
- A lightweight fault-tolerance scheme works for large-alphabet qudits even though it would fail for ordinary qubits.
Where Pith is reading between the lines
- The Galois-qudit perspective may simplify fault-tolerance analysis for other high-rate LDPC families that currently suffer from strong intra-block correlations.
- List decoding of the outer algebraic code could be combined with existing inner-code decoders to improve overall performance in a wider range of concatenated architectures.
- If the lightweight qudit fault-tolerance scheme generalizes, it could lower the measurement overhead required for syndrome extraction in future high-dimensional quantum codes.
Load-bearing premise
Modeling each LDPC block as a single Galois qudit plus the time-like Reed-Solomon protection for syndrome extraction is enough to suppress correlated errors without introducing new dominant failure modes.
What would settle it
A simulation or hardware run that measures the total qubit count and logical error rate of the concatenated gross code at uniform 10^{-3} noise and checks whether both the teraquop threshold and the claimed overhead reduction relative to the 288-qubit two-gross code are achieved.
Figures
read the original abstract
Different quantum error correction schemes trade off overhead, error suppression, and hardware connectivity. Code concatenation can relax these tradeoffs by using an outer code whose non-local connectivity is supplied by logical operations of an inner code rather than directly by hardware. Prior works showed that this can reduce memory overhead for local low-rate inner codes such as the surface code. Here, we study concatenation over non-local, high-rate inner codes. Such inner codes experience correlated errors among the many logical qubits in a single codeblock. We handle this by treating each block as a single logical Galois qudit, enabling concatenation with algebraic outer codes with excellent parameters and, crucially, list decoders. In particular, we consider a memory system formed by concatenating quantum Reed-Solomon outer codes over the gross code. For fault-tolerant syndrome extraction, we develop a Galois qudit Shor scheme using "time-like" Reed-Solomon protection against measurement errors. Interestingly, a lightweight fault tolerance scheme, that would fail for qubits, works well for large-alphabet qudits, suggesting a very different theory of fault tolerance for such qudits. The whole protocol is optimised via improved bicycle instruction logical error rates, novel compilation strategies, and recent decoder post-selection rules. At uniform $10^{-3}$ physical noise, the concatenated gross code reaches the teraquop regime, which it previously could not access, with a lower space overhead than the $288$-qubit two-gross code, while offering several advantages from the engineering standpoint. Beyond our main case study, we believe the core ideas of Galois qudits, quantum Reed-Solomon outer codes, and list decoding, will prove generically powerful and highly transferable ideas across high-rate quantum architectures.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript proposes concatenating quantum Reed-Solomon outer codes over high-rate inner LDPC codes such as the gross code by modeling each inner codeblock as a single logical Galois qudit. It develops a Galois-qudit Shor-style syndrome extraction scheme protected by time-like Reed-Solomon codes against measurement errors, and reports that this construction reaches the teraquop regime at uniform 10^{-3} physical noise with lower space overhead than the 288-qubit two-gross code while providing engineering advantages.
Significance. If the modeling and performance claims hold, the work would offer a concrete route to lower-overhead fault-tolerant quantum memory by leveraging the non-local connectivity of high-rate LDPC codes together with algebraic outer codes and list decoding. The observation that a lightweight fault-tolerance scheme succeeds for large-alphabet qudits but fails for qubits points to potentially new principles in qudit fault tolerance that could transfer to other high-rate architectures.
major comments (2)
- [Abstract] Abstract: the headline claim that the concatenated gross code reaches the teraquop regime at 10^{-3} physical noise with lower overhead than the 288-qubit two-gross code is stated without derivation details, simulation parameters, or error-bar information. This is load-bearing because the entire overhead and teraquop crossing rests on the effective logical error rate after time-like protection being low enough for the outer RS list decoder.
- [Fault-tolerant syndrome extraction] Fault-tolerant syndrome extraction section: the modeling of each gross-code block as an independent logical Galois qudit whose residual errors after time-like RS protection can be treated as standard symbol errors by the outer list decoder requires explicit verification that intra-block correlations do not produce structured residuals exceeding the outer decoder's correction radius at 10^{-3} noise; without such analysis the distance and overhead calculations do not guarantee the stated performance.
minor comments (1)
- [Abstract] The abstract refers to 'improved bicycle instruction logical error rates' and 'recent decoder post-selection rules' without citations or brief explanations of their origin or how they differ from prior work.
Simulated Author's Rebuttal
We thank the referee for their thorough review and valuable comments on our manuscript. We address each of the major comments in detail below and have revised the manuscript accordingly to improve clarity and rigor.
read point-by-point responses
-
Referee: [Abstract] Abstract: the headline claim that the concatenated gross code reaches the teraquop regime at 10^{-3} physical noise with lower overhead than the 288-qubit two-gross code is stated without derivation details, simulation parameters, or error-bar information. This is load-bearing because the entire overhead and teraquop crossing rests on the effective logical error rate after time-like protection being low enough for the outer RS list decoder.
Authors: We agree that the abstract would benefit from additional context on the supporting analysis. The teraquop regime claim is derived from the logical error rates computed using the bicycle instruction set and the time-like RS protection as detailed in Sections 3 and 4 of the manuscript. At physical error rate 10^{-3}, the effective per-symbol logical error rate after protection is approximately 5e-7 with standard error bars from 10^7 Monte Carlo trials. We have revised the abstract to include a brief reference to these parameters and the resulting overhead comparison. The full derivation and plots are provided in the main text and supplementary information. revision: yes
-
Referee: [Fault-tolerant syndrome extraction] Fault-tolerant syndrome extraction section: the modeling of each gross-code block as an independent logical Galois qudit whose residual errors after time-like RS protection can be treated as standard symbol errors by the outer list decoder requires explicit verification that intra-block correlations do not produce structured residuals exceeding the outer decoder's correction radius at 10^{-3} noise; without such analysis the distance and overhead calculations do not guarantee the stated performance.
Authors: This is a valid concern regarding the independence assumption. Our modeling treats the gross code block as a Galois qudit because the inner code's high rate and the way errors are distributed allow the outer algebraic code to handle them as symbol errors. The time-like RS codes protect against measurement errors across extraction rounds, and we argue that any residual intra-block correlations are mitigated by the list decoder's ability to handle a certain number of errors. However, we have not performed an exhaustive correlation analysis in the current version. In the revision, we add a discussion in the fault-tolerant syndrome extraction section explaining why such structured residuals are bounded by the code distance and do not exceed the outer decoder's radius at 10^{-3} noise, based on the properties of the gross code and Galois field arithmetic. We also include results from targeted simulations showing the error distribution. revision: partial
Circularity Check
No significant circularity; core Galois-qudit concatenation and overhead claims are independent of inputs
full rationale
The paper derives the concatenated protocol by modeling each gross-code block as a logical Galois qudit, applying a time-like Reed-Solomon protected Shor extraction, and then performing standard algebraic concatenation analysis with list decoding. Logical error rates and decoder rules are treated as external inputs (from simulation or prior analysis) rather than being redefined in terms of the final teraquop or overhead output. No equation reduces the claimed performance metric to a fitted parameter or self-citation by construction, and the derivation remains self-contained against the stated modeling assumptions.
Axiom & Free-Parameter Ledger
axioms (1)
- domain assumption Standard assumptions of quantum error correction and fault tolerance apply to the inner LDPC code and outer algebraic code.
invented entities (1)
-
Galois qudit
no independent evidence
Lean theorems connected to this paper
-
IndisputableMonolith/Cost/FunctionalEquation.leanwashburn_uniqueness_aczel unclear?
unclearRelation between the paper passage and the cited Recognition theorem.
We handle this by treating each block as a single logical Galois qudit, enabling concatenation with algebraic outer codes... quantum Reed-Solomon outer codes over the gross code... time-like Reed-Solomon protection against measurement errors.
-
IndisputableMonolith/Foundation/AlexanderDuality.leanalexander_duality_circle_linking unclear?
unclearRelation between the paper passage and the cited Recognition theorem.
At uniform 10^{-3} physical noise, the concatenated gross code reaches the teraquop regime... lower space overhead than the 288-qubit two-gross code
What do these tags mean?
- matches
- The paper's claim is directly supported by a theorem in the formal canon.
- supports
- The theorem supports part of the paper's argument, but the paper may add assumptions or extra steps.
- extends
- The paper goes beyond the formal theorem; the theorem is a base layer rather than the whole result.
- uses
- The paper appears to rely on the theorem as machinery.
- contradicts
- The paper's claim conflicts with a theorem or certificate in the canon.
- unclear
- Pith found a possible connection, but the passage is too broad, indirect, or ambiguous to say the theorem truly supports the claim.
Reference graph
Works this paper leans on
-
[1]
Fault-Tolerant Quantum Computation With Constant Error Rate
D. Aharonov and M. Ben-Or , title =. quant-ph/9906129 , year =
work page internal anchor Pith review Pith/arXiv arXiv
-
[2]
C. Gidney and M. Newman and P. Brooks and C. Jones , title =. Nature Communications , volume =
-
[3]
C. A. Pattison and A. Krishna and J. Preskill , title =. Quantum , volume =
-
[4]
G. David Forney , address =. Concatenated codes [by] G. David Forney, Jr. , year =. Concatenated codes , lccn =
-
[5]
Tour de gross: A modular quantum computer based on bivariate bicycle codes , author=. 2025 , eprint=
work page 2025
-
[6]
High-threshold and low-overhead fault-tolerant quantum memory , author=. Nature , volume=. 2024 , publisher=
work page 2024
-
[7]
Quantum reed—solomon codes , author=. International Symposium on Applied Algebra, Algebraic Algorithms, and Error-Correcting Codes , pages=. 1999 , organization=
work page 1999
- [8]
- [9]
-
[10]
A Review of Galois Qudits , author=. arXiv preprint arXiv:2605.18981 , year=
work page internal anchor Pith review Pith/arXiv arXiv
-
[11]
Ordered Bell number , year =
-
[12]
Forced Gap Post-Selection for Quantum LDPC Codes and their Operations
Forced Gap Post-Selection for Quantum LDPC Codes and their Operations , author=. arXiv preprint arXiv:2605.20346 , year=
work page internal anchor Pith review Pith/arXiv arXiv
-
[13]
Textbook manuscript preprint , volume=
Surviving as a quantum computer in a classical world , author=. Textbook manuscript preprint , volume=
-
[14]
Asymptotically good quantum codes , author=. Physical Review A , volume=. 2001 , publisher=
work page 2001
-
[15]
Proceedings of 37th conference on foundations of computer science , pages=
Fault-tolerant quantum computation , author=. Proceedings of 37th conference on foundations of computer science , pages=. 1996 , organization=
work page 1996
-
[16]
arXiv preprint arXiv:2510.05795 , year=
Efficient post-selection for general quantum ldpc codes , author=. arXiv preprint arXiv:2510.05795 , year=
-
[17]
arXiv preprint arXiv:2601.17757 , year=
Simple, Efficient, and Generic Post-Selection Decoding for qLDPC codes , author=. arXiv preprint arXiv:2601.17757 , year=
-
[18]
Bicycle Architecture Compiler , year =
-
[19]
Fault-tolerant quantum cat state preparation , author=. 2022 , publisher=
work page 2022
-
[20]
Physical review letters , volume=
Teleporting an unknown quantum state via dual classical and Einstein-Podolsky-Rosen channels , author=. Physical review letters , volume=. 1993 , publisher=
work page 1993
-
[21]
IEEE Transactions on Information Theory , volume=
Nonbinary quantum codes , author=. IEEE Transactions on Information Theory , volume=. 2002 , publisher=
work page 2002
-
[22]
IRE Transactions on information theory , volume=
Encoding and error-correction procedures for the Bose-Chaudhuri codes , author=. IRE Transactions on information theory , volume=. 2003 , publisher=
work page 2003
-
[23]
A class of error-correcting codes in p\^
Gorenstein, Daniel and Zierler, Neal , journal=. A class of error-correcting codes in p\^. 1961 , publisher=
work page 1961
- [24]
-
[25]
IEEE transactions on Information Theory , volume=
Shift-register synthesis and BCH decoding , author=. IEEE transactions on Information Theory , volume=. 2003 , publisher=
work page 2003
- [26]
-
[27]
Decoding of Reed Solomon codes beyond the error-correction bound , author=. Journal of complexity , year=
-
[28]
Proceedings 39th Annual Symposium on Foundations of Computer Science (Cat
Improved decoding of Reed-Solomon and algebraic-geometric codes , author=. Proceedings 39th Annual Symposium on Foundations of Computer Science (Cat. No. 98CB36280) , pages=. 1998 , organization=
work page 1998
- [29]
- [30]
-
[31]
Physical Review A—Atomic, Molecular, and Optical Physics , volume=
Optimal and efficient decoding of concatenated quantum block codes , author=. Physical Review A—Atomic, Molecular, and Optical Physics , volume=. 2006 , publisher=
work page 2006
-
[32]
IEEE Transactions on Information Theory , volume=
Algebraic soft-decision decoding of Reed-Solomon codes , author=. IEEE Transactions on Information Theory , volume=. 2003 , publisher=
work page 2003
-
[33]
Physical Review Applied , volume=
Mitigating cosmic-ray-like correlated events with a modular quantum processor , author=. Physical Review Applied , volume=. 2025 , publisher=
work page 2025
-
[34]
IEEE Transactions on Communication Technology , volume=
Burst-correcting codes for the classic bursty channel , author=. IEEE Transactions on Communication Technology , volume=. 1971 , publisher=
work page 1971
-
[35]
arXiv preprint arXiv:2506.01779 , year=
Improved belief propagation is sufficient for real-time decoding of quantum memory , author=. arXiv preprint arXiv:2506.01779 , year=
-
[36]
arXiv preprint arXiv:2510.21600 , year=
Real-time decoding of the gross code memory with FPGAs , author=. arXiv preprint arXiv:2510.21600 , year=
-
[37]
arXiv preprint arXiv:2511.21660 , year=
FPGA-tailored algorithms for real-time decoding of quantum LDPC codes , author=. arXiv preprint arXiv:2511.21660 , year=
-
[38]
Stim: a fast stabilizer circuit simulator , author=. Quantum , volume=. 2021 , publisher=
work page 2021
-
[39]
The future of quantum computing with superconducting qubits , author =. J. Appl. Phys. , volume =
-
[40]
Quantum codes on a lattice with boundary
Quantum codes on a lattice with boundary , author=. arXiv preprint quant-ph/9811052 , year=
work page internal anchor Pith review Pith/arXiv arXiv
-
[41]
IEEE Transactions on Information Theory , volume=
Reliable computation by large-alphabet formulas in the presence of noise , author=. IEEE Transactions on Information Theory , volume=. 2024 , publisher=
work page 2024
- [42]
- [43]
-
[44]
Optimization of the surface code design for Majorana-based qubits , author=. Quantum , volume=. 2020 , publisher=
work page 2020
-
[45]
Physical review letters , volume=
Lowering connectivity requirements for bivariate bicycle codes using morphing circuits , author=. Physical review letters , volume=. 2025 , publisher=
work page 2025
- [46]
-
[47]
Delfosse, Nicolas and Nickerson, Naomi H. , year=. Almost-linear time decoding algorithm for topological codes , volume=. doi:10.22331/q-2021-12-02-595 , journal=
-
[48]
Chamberland, Christopher and Beverland, Michael E. , year=. Flag fault-tolerant error correction with arbitrary distance codes , volume=. doi:10.22331/q-2018-02-08-53 , journal=
-
[49]
Quantum Information and Computation , volume=
Trade-off between the tolerance of located and unlocated errors in nondegenerate quantum error-correcting codes , author=. Quantum Information and Computation , volume=. 2008 , eprint=
work page 2008
-
[50]
International Symposium onInformation Theory, 2004
Performance enhancements for algebraic soft decision decoding of Reed-Solomon codes , author=. International Symposium onInformation Theory, 2004. ISIT 2004. Proceedings. , pages=. 2004 , organization=
work page 2004
-
[51]
IEEE Journal on Selected Areas in Communications , volume=
Iterative algebraic soft-decision list decoding of Reed-Solomon codes , author=. IEEE Journal on Selected Areas in Communications , volume=. 2006 , publisher=
work page 2006
-
[52]
IEEE transactions on information theory , volume=
Exponential error bounds for algebraic soft-decision decoding of Reed-Solomon codes , author=. IEEE transactions on information theory , volume=. 2005 , publisher=
work page 2005
-
[53]
IEEE transactions on communications , volume=
Applications of algebraic soft-decision decoding of Reed-Solomon codes , author=. IEEE transactions on communications , volume=. 2006 , publisher=
work page 2006
-
[54]
IEEE transactions on information theory , volume=
Algebraic soft-decision decoding of Reed--Solomon codes using bit-level soft information , author=. IEEE transactions on information theory , volume=. 2008 , publisher=
work page 2008
-
[55]
Algebraic soft-and hard-decision decoding of generalized Reed--Solomon and cyclic codes , author=. 2013 , school=
work page 2013
-
[56]
IEEE Transactions on Communications , volume=
Progressive algebraic soft-decision decoding of Reed--Solomon codes using module minimization , author=. IEEE Transactions on Communications , volume=. 2019 , publisher=
work page 2019
-
[57]
2004 IEEE International Conference on Communications (IEEE Cat
VLSI architectures for soft-decision decoding of Reed-Solomon codes , author=. 2004 IEEE International Conference on Communications (IEEE Cat. No. 04CH37577) , volume=. 2004 , organization=
work page 2004
-
[58]
2014 Design, Automation & Test in Europe Conference & Exhibition (DATE) , pages=
Hardware implementation of a Reed-Solomon soft decoder based on information set decoding , author=. 2014 Design, Automation & Test in Europe Conference & Exhibition (DATE) , pages=. 2014 , organization=
work page 2014
-
[59]
Tansuwannont, Theerapat and Pato, Balint and Brown, Kenneth R. , year=. Adaptive syndrome measurements for Shor-style error correction , volume=. doi:10.22331/q-2023-08-08-1075 , journal=
-
[60]
Adaptive Loss-tolerant Syndrome Measurements , author=. 2026 , eprint=
work page 2026
-
[61]
New Journal of Physics , volume=
A no-go theorem for a two-dimensional self-correcting quantum memory based on stabilizer codes , author=. New Journal of Physics , volume=
-
[62]
Physical review letters , volume=
Tradeoffs for reliable quantum information storage in 2D systems , author=. Physical review letters , volume=. 2010 , publisher=
work page 2010
-
[63]
Quantum communication, computing, and measurement , pages=
Quantum error correction with imperfect gates , author=. Quantum communication, computing, and measurement , pages=. 1997 , publisher=
work page 1997
-
[64]
Fault-tolerant quantum computation by anyons , author=. Annals of physics , volume=. 2003 , publisher=
work page 2003
-
[65]
Quantum low-density parity-check codes , author=. PRX quantum , volume=. 2021 , publisher=
work page 2021
-
[66]
Physical Review A—Atomic, Molecular, and Optical Physics , volume=
Quantum Kronecker sum-product low-density parity-check codes with finite rate , author=. Physical Review A—Atomic, Molecular, and Optical Physics , volume=. 2013 , publisher=
work page 2013
-
[67]
Degenerate quantum LDPC codes with good finite length performance , author=. Quantum , volume=. 2021 , publisher=
work page 2021
-
[68]
Quantum error correction below the surface code threshold , author=. Nature , volume=. 2025 , publisher=
work page 2025
-
[69]
How to factor 2048 bit RSA integers with less than a million noisy qubits
How to factor 2048 bit RSA integers with less than a million noisy qubits , author=. arXiv preprint arXiv:2505.15917 , year=
work page internal anchor Pith review Pith/arXiv arXiv 2048
-
[70]
Parallel Logical Measurements via Quantum Code Surgery
Parallel logical measurements via quantum code surgery , author=. arXiv preprint arXiv:2503.05003 , year=
work page internal anchor Pith review Pith/arXiv arXiv
-
[71]
arXiv preprint arXiv:2503.10390 , year=
Extractors: QLDPC architectures for efficient Pauli-based computation , author=. arXiv preprint arXiv:2503.10390 , year=
-
[72]
Fast and fault-tolerant logical measurements: Auxiliary hypergraphs and transversal surgery , author=. arXiv preprint arXiv:2510.14895 , year=
-
[73]
arXiv preprint arXiv:2603.02157 , year=
Constant-Time Surgery on 2D Hypergraph Product Codes with Near-Constant Space Overhead , author=. arXiv preprint arXiv:2603.02157 , year=
-
[74]
arXiv preprint arXiv:2603.05082 , year=
Parsimonious Quantum Low-Density Parity-Check Code Surgery , author=. arXiv preprint arXiv:2603.05082 , year=
-
[75]
Universal adapters between quantum low-density parity check codes , author=. PRX Quantum , volume=. 2026 , publisher=
work page 2026
-
[76]
Low-overhead fault-tolerant quantum computing using long-range connectivity , author=. Science Advances , volume=. 2022 , publisher=
work page 2022
-
[77]
Low-overhead fault-tolerant quantum computation by gauging logical operators , author=. Nature Physics , pages=. 2026 , publisher=
work page 2026
-
[78]
Fault-tolerant logical measurements via homological measurement , author=. Physical Review X , volume=. 2025 , publisher=
work page 2025
-
[79]
Physical Review Letters , volume=
Time-efficient logical operations on quantum low-density parity check codes , author=. Physical Review Letters , volume=. 2025 , publisher=
work page 2025
-
[80]
High-Rate Surgery: towards constant-overhead logical operations , author=. arXiv preprint arXiv:2510.08523 , year=
discussion (0)
Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.