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arXiv:2609.39121

Bivariate Bicycle Codes and Metachecks: Syndrome Repair, Measurement-Fault Ambiguity, and Logical Obstructions

Mohammad Rowshan

quant-phcs.ITSubmitted Sep 30, 2026• Announced Oct 1, 2026• 0 citations
v1 · Sep 30, 2026v2 · Oct 1, 2026
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Abstract

Faulty syndrome measurements can corrupt an otherwise correct quantum-error correction step. Bivariate bicycle (BB) codes contain dependent stabilizer checks, so their measured syndromes obey parity constraints that can be used as metachecks. We study when this built-in redundancy actually supports syndrome repair and how it interacts with the logical structure of the code. Annihilator quotients identify single-block logical classes and show when a mixed-block search is unavoidable. For syndrome repair, the metasyndrome is equivalent to reduction modulo the valid-syndrome ideal. The translation group therefore maps into the unit group of a quotient algebra of dimension $k/2$, with kernel $K_M$. This gives a distance-two characterization, a unit-group bound on distinguishable single faults, and a family-level obstruction: for bounded-$k$ BB families with growing block length, metasyndrome-only exact repair fails with probability tending to one at any fixed measurement-error rate. With no intervening data fault, the same orbit count also gives the minimum partial second measurement needed to remove every single-fault ambiguity. Exact calculations separate the standard examples. The $[[72,12,6]]$ code has syndrome distance three and minimum-weight repair corrects every single measurement fault, whereas Gross $[[144,12,12]]$ has 36 indistinguishable single-fault pairs. Sustained phenomenological experiments show the same qualitative contrast and favor joint data--measurement decoding over a separated repair stage on the codes with stronger ambiguity. The analysis extends our earlier coprime-period treatment to general two-block BB codes and separates measurement ambiguity from the data error it can induce.

Comments
14 pages, 5 figures, 9 tables

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Faulty syndrome measurements can corrupt an otherwise correct quantum-error correction step. Bivariate bicycle (BB) codes contain dependent stabilizer checks, so their measured syndromes obey parity constraints that can be used as metachecks. We study when this built-in redundancy actually supports syndrome repair and how it interacts with the logical structure of the code. Annihilator quotients identify single-block logical classes and show when a mixed-block search is unavoidable. For syndrome repair, the metasyndrome is equivalent to reduction modulo the valid-syndrome ideal. The translation group therefore maps into the unit group of a quotient algebra of dimension k/2k/2k/2, with kernel KMK_MKM​. This gives a distance-two characterization, a unit-group bound on distinguishable single faults, and a family-level obstruction: for bounded-kkk BB families with growing block length, metasyndrome-only exact repair fails with probability tending to one at any fixed measurement-error rate. With no intervening data fault, the same orbit count also gives the minimum partial second measurement needed to remove every single-fault ambiguity. Exact calculations separate the standard examples. The [[72,12,6]][[72,12,6]][[72,12,6]] code has syndrome distance three and minimum-weight repair corrects every single measurement fault, whereas Gross [[144,12,12]][[144,12,12]][[144,12,12]] has 36 indistinguishable single-fault pairs. Sustained phenomenological experiments show the same qualitative contrast and favor joint data--measurement decoding over a separated repair stage on the codes with stronger ambiguity. The analysis extends our earlier coprime-period treatment to general two-block BB codes and separates measurement ambiguity from the data error it can induce.
Bivariate bicycle (BB) codes are a prominent class of quantum LDPC codes constructed from group algebras. While the logical dimension and quantum distance of \emph{coprime} BB codes are known to be determined by a greatest common divisor polynomial g(z)g(z)g(z), the properties governing their fault tolerance under noisy measurement have remained implicit. In this work, we prove that this same polynomial g(z)g(z)g(z) dictates the code's stabilizer redundancy and the structure of the classical \emph{syndrome codes} required for single-shot decoding. We derive a strict equality between the quantum rate and the stabilizer redundancy density, and we provide BCH-like bounds on the achievable single-shot measurement error tolerance. Guided by this framework, we construct small coprime BB codes with significantly improved syndrome distance (dSd_SdS​) and evaluate them using BP+OSD. Our analysis reveals a structural bottleneck: within the coprime BB ansatz, high quantum rate imposes an upper bound on syndrome distance, limiting single-shot performance. These results provide concrete algebraic design rules for next-generation 2BGA codes in measurement-limited architectures.
Bivariate bicycle (BB) quantum codes are a prominent finite-length family of quantum low-density parity-check codes, but their minimum distance is usually established numerically rather than read from the defining polynomials. We study the ZZZ-logical quotient K/SK/SK/S over F2[x,y]/(xℓ−1,ym−1)\mathbb F_2[x,y]/(x^\ell-1,y^m-1)F2​[x,y]/(xℓ−1,ym−1) and show that it fits into a short exact sequence with an annihilator quotient as kernel and a colon quotient as cokernel. The sequence gives an explicit logical basis, a dimension formula, and a componentwise distance identity dZ=min⁡(dA,dC)d_Z=\min(d_{\mathcal A},d_{\mathcal C})dZ​=min(dA​,dC​). Using the Frobenius structure of the finite group algebra, we prove rA=rC=k/2r_{\mathcal A}=r_{\mathcal C}=k/2rA​=rC​=k/2 for every BB code, including repeated-root cases. The algebraic component of a logical class is distinct from the support shape of its lightest representatives: an annihilator class can have a lighter two-block representative, and a colon class can have a one-sided minimum. For lower bounds we show that every proper subset of a minimum-weight logical operator has nonzero syndrome, and that this property persists inside the colon component but not inside the annihilator component. A translation-anchored cluster search built on it proves the distances 4,6,10,10,12,184,6,10,10,12,184,6,10,10,12,18 of the six standard BB codes of lengths 181818 to 288288288 and enumerates every minimum-weight logical operator. The resulting census shows that [[108,8,10]][[108,8,10]][[108,8,10]] is the only one of the six whose distance is attained in a single component, with dC=10d_{\mathcal C}=10dC​=10 and dA=12d_{\mathcal A}=12dA​=12.
High-rate bivariate bicycle (BB) codes are promising low-overhead quantum memories, but their stabilizer checks typically have weight 666 or higher, making syndrome extraction challenging. We introduce subsystem bivariate bicycle (SBB) codes, a translation-invariant CSS subsystem construction that realizes BB-code logical structure using local weight-444 gauge measurements. Their stabilizer syndromes are inferred by multiplying the corresponding gauge outcomes. We further show that nonlocal stabilizers in translation-invariant CSS subsystem codes can be detected using a determinantal-ideal criterion based on the gauge-operator commutation matrix. When this criterion excludes nonlocal stabilizers, a finite-depth Clifford circuit decouples gauge qubits and identifies the protected subsystem with a corresponding BB stabilizer code. An SBB code is topological, meaning that it has no nontrivial local logical operators, if and only if the corresponding BB code is topological. A finite search yields low-overhead examples including [[27,6,3]][[27,6,3]][[27,6,3]], [[75,10,5]][[75,10,5]][[75,10,5]], and [[108,12,6]][[108,12,6]][[108,12,6]]; the latter encodes six times more logical qubits than a subsystem surface code at the same block length and distance. These results show how gauge degrees of freedom can make high-rate BB logical structure compatible with local weight-444 syndrome extraction.
Bivariate bicycle (BB) codes of Bravyi \emph{et al.}~\cite{Bravyi2024} are quantum low-density parity-check codes with weight-666 checks, exemplified by [[144,12,12]][[144,12,12]][[144,12,12]] with kd2/n=12kd^2/n=12kd2/n=12. We develop the algebraic structure theory of BB-type codes with weight-888 checks (weight-444 generator polynomials) and use it, together with an exactly validated search pipeline, to construct and certify new codes. We prove an exact dimension formula k=2dim⁡R/(A,B)k=2\dim R/(A,B)k=2dimR/(A,B) (forcing even kkk), a 4ℓm4\ell m4ℓm-element symmetry group on generator pairs, an X/ZX/ZX/Z distance equality dX=dZd_X=d_ZdX​=dZ​, and a family of subgroup-coset kernel vectors giving rigorous distance upper bounds and a design rule for high-distance constructions; all distances are computed exhaustively by a cross-validated bit-mask verifier. At n=144n=144n=144 the pipeline returns a census of 535353 codes whose strongest members surpass the BB benchmark: [[144,6,d≥15]][[144,6,d\ge 15]][[144,6,d≥15]] exceeds the benchmark distance 121212 (certified d≥15d\ge 15d≥15), [[144,10,12]][[144,10,12]][[144,10,12]] reaches it with weight-888 checks, and [[144,16,10]][[144,16,10]][[144,16,10]] encodes a third more logical qubits at kd2/n=11.11kd^2/n=11.11kd2/n=11.11 (7.4%7.4\%7.4% below benchmark) while decoding no worse. At n=72n=72n=72, [[72,14,8]][[72,14,8]][[72,14,8]] attains kd2/n=12.44kd^2/n=12.44kd2/n=12.44---more than twice the same-length BB code---and decodes better; a circuit-level memory experiment places our weight-888 codes at ≈0.1%\approx 0.1\%≈0.1% pseudo-threshold versus ≈0.4%\approx 0.4\%≈0.4% for the BB reference under an identical model, quantifying the threshold cost of the heavier checks. All structural statements are verified numerically on the whole census.
Decoding Bivariate Bicycle (BB) quantum error correction codes typically requires Belief Propagation (BP) followed by Ordered Statistics Decoding (OSD) post-processing when BP fails to converge. Whether BP will converge on a given syndrome is currently determined only after running BP to completion. We show that convergence can be predicted in advance by a single modulo operation: if the syndrome defect count is divisible by the code's column weight w, BP converges with high probability (100% at p <= 0.001, degrading to 87% at p = 0.01); otherwise, BP fails with probability >= 90%. The mechanism is structural: each physical data error activates exactly w stabilizers, so a defect count not divisible by w implies the presence of measurement errors outside BP's model space. Validated on five BB codes with column weights w = 2, 3, and 4, mod-w achieves AUC = 0.995 as a convergence classifier at p = 0.001 under phenomenological noise, dominating all other syndrome features (next best: AUC = 0.52). The false positive rate scales empirically as O(p^2.05) (R^2 = 0.98), confirming the analytical bound from Proposition 2. Among BP failures on mod-w = 0 syndromes, 82% contain weight-2 data error clusters, directly confirming the dominant failure mechanism. The prediction is invariant under BP scheduling strategy and decoder variant, including Relay-BP - the strongest known BP enhancement for quantum LDPC codes. These results apply directly to IBM's Gross code [[144, 12, 12]] and Two-Gross code [[288, 12, 18]], targeted for deployment in 2026-2028.
Syndrome histories are normally used to detect and correct errors in fault-tolerant quantum computation, but they can also reveal information about the logical operations executed during computation. We study logical-operation identification using syndrome histories alone or together with other classical measurement outcomes. We develop an exact inference method and a scalable approximation that account for unknown circuit faults and ideal measurement randomness. We evaluate automorphisms, logical Pauli measurements, T injections, and logical circuits combining these instructions on [[18,4,2]][[18,4,2]][[18,4,2]] and [[72,12,6]][[72,12,6]][[72,12,6]] bivariate bicycle (BB) codes under multiple noise models and error rates. A single logical instruction can be identified with over 94\% accuracy under circuit-level noise with error rates between 10−510^ {-5} 10−5 and 10−310^{-3}10−3. We also achieve over 99\% accuracy for automorphism-based logical circuits and 78.32\% for logical circuits combining all three instructions. We further analyze identification ambiguities arising from insufficient or indistinguishable measurement information, fault timing, and approximation inference. Together, these results extend the role of syndromes beyond error decoding to identifying executed logical operations and circuits during fault-tolerant quantum computation. They also suggest that access to syndrome histories and related classical measurement outcomes may unintentionally reveal information about the executed computation.