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Ensemble Dependence of Critical Exponents at Quantum Error Correction Thresholds: Analytic Mechanisms and Statistical Consequences

Rowan Brad Quni-Gudzinas

Published Oct 4, 2026• 0 citations
Publisher

Abstract

Ensemble equivalence — the expectation that microcanonical, canonical, and grand-canonical descriptions of a system agree in the thermodynamic limit — is a cornerstone of statistical mechanics, yet it can fail for observables that probe exponentially rare events. Recent work on a simplified quantum error correction (QEC) model reports that critical exponents governing fidelity and magic at the error-correction threshold depend on the noise ensemble chosen, and that even the existence of the transition can be ensemble-dependent [1,2]. Here we develop a minimal, fully analytic treatment of this phenomenon using a random-unitary encoding model subject to depolarizing noise. We derive explicit expressions for the decoding fidelity in grand-canonical (independent per-qubit errors) and canonical (fixed error number) ensembles, and exhibit concrete numerical instances of the discrepancy: for a distance-3 code with 100 physical qubits and error rate p = 0.01, the grand-canonical fidelity is 0.735762 while the canonical fidelity is exactly 1. We show that the discrepancy survives the vanishing of relative fluctuations (σ/μ → 0) because fidelity is a large-deviation tail quantity, and we compute the associated entropy and capacity budgets. We further derive the statistical cost of resolving ensemble-dependent exponents: distinguishing two candidate exponents at distance Δp from threshold requires a sample count scaling as Δp⁻⁴ and system sizes scaling as Δp⁻², a compound burden that we quantify under stated assumptions. We relate the mechanism to the known ensemble dependence of the random transverse-field Ising chain [3], discuss saturation of the information-theoretic bound of Feldman et al. as reported in [1,2], and outline falsifiable predictions. The results clarify when ensemble choice is physically consequential rather than a matter of convenience in QEC threshold physics. ## 1. Introduction Quantum error correction protects quantum information from decoherence by en Full text and updates: papers.qnfo.org/papers/ensemble-dependence-of-critical-exponents-at-quantum-error-correction-thresholds/

DOI
10.5281/zenodo.23130192

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Ensemble equivalence — the expectation that microcanonical, canonical, and grand-canonical descriptions of a system agree in the thermodynamic limit — is a cornerstone of statistical mechanics, yet it can fail for observables that probe exponentially rare events. Recent work on a simplified quantum error correction (QEC) model reports that critical exponents governing fidelity and magic at the error-correction threshold depend on the noise ensemble chosen, and that even the existence of the transition can be ensemble-dependent [1,2]. Here we develop a minimal, fully analytic treatment of this phenomenon using a random-unitary encoding model subject to depolarizing noise. We derive explicit expressions for the decoding fidelity in grand-canonical (independent per-qubit errors) and canonical (fixed error number) ensembles, and exhibit concrete numerical instances of the discrepancy: for a distance-3 code with 100 physical qubits and error rate p = 0.01, the grand-canonical fidelity is 0.735762 while the canonical fidelity is exactly 1. We show that the discrepancy survives the vanishing of relative fluctuations (σ/μ → 0) because fidelity is a large-deviation tail quantity, and we compute the associated entropy and capacity budgets. We further derive the statistical cost of resolving ensemble-dependent exponents: distinguishing two candidate exponents at distance Δp from threshold requires a sample count scaling as Δp⁻⁴ and system sizes scaling as Δp⁻², a compound burden that we quantify under stated assumptions. We relate the mechanism to the known ensemble dependence of the random transverse-field Ising chain [3], discuss saturation of the information-theoretic bound of Feldman et al. as reported in [1,2], and outline falsifiable predictions. The results clarify when ensemble choice is physically consequential rather than a matter of convenience in QEC threshold physics. ## 1. Introduction Quantum error correction protects quantum information from decoherence by en Full text and updates: papers.qnfo.org/papers/ensemble-dependence-of-critical-exponents-at-quantum-error-correction-thresholds/
In thermodynamics it is common to assume that the choice of ensemble (e.g., micro-canonical, canonical, or grand-canonical) should not affect the underlying physics in the thermodynamics limit. We show that this does not necessarily hold for critical exponents. Examining a simplified model of quantum error correction (single step encoding and decoding by a random unitary) and the behavior of both the fidelity and magic at the corresponding threshold, we find different exponents when using generic channels or supposedly equivalent quantum trajectories. Interestingly, the obtained exponents saturate a recently derived information theoretic bound by Feldman et al. (2026), which we extend from the grand-canonical to the canonical case, including intermediate ensembles which we define. Moreover, even the existence of the transition is shown to be ensemble-dependent.
Utility-scale quantum computers require quantum error correcting codes with large numbers of physical qubits to achieve sufficiently low logical error rates. The performance of quantum error correction (QEC) is generally predicted through large-scale numerical simulations, used to estimate thresholds, finite-size scaling, and exponential suppression of logical errors below threshold. The connection of QEC to models from statistical mechanics provides an alternative tool for analysing QEC performance. However, predicting the behaviour of these models also requires large-scale numerical simulations, as analytic solutions are not generally known. Here we exploit an exact mapping, from a toric code under bit-flip noise that is post-selected on being syndrome free to the exactly-solvable two-dimensional Ising model on a torus, to derive an analytic solution for the logical failure rate across its full domain of physical error rates. In particular, this mapping provides closed-form expressions for the logical failure rate in four distinct regimes: the path-counting, below-threshold (ordered), near-threshold (critical), and above-threshold (disordered) regimes. Our framework places a number of familiar and long-standing numerical observations on firm theoretical ground. It also motivates explicit ansatze for the conventional QEC setting of non-post-selected codes whose statistical mechanics mappings involve random-bond disorder. Specifically, we introduce an effective tension model for the below-threshold regime, and a new scaling ansatz for the near-threshold regime, derived from an analysis of the domain wall energy cost distributions. By bridging statistical mechanics theory and quantum error correction practice, our results offer a new toolkit for designing, benchmarking, and understanding topological codes beyond current computational limits.
The ability of quantum error correction (QEC) to protect logical qubits against physical noise is traditionally quantified by a noise threshold: a critical noise level below which the logical error rate decreases exponentially with code distance. This paper proposes that QEC itself can undergo a phase transition analogous to a physical phase change. We develop a theoretical framework that treats the logical error rate as an order parameter and maps the behavior of large stabilizer codes onto a statistical mechanics model. By analyzing the scaling of logical error probabilities with code distance and noise strength, we identify a continuous phase transition at the threshold, characterized by critical exponents that depend on the code family and noise model. The transition separates a "fault‑tolerant" phase where logical errors are exponentially suppressed from a "fault‑unreliable" phase where logical errors proliferate. We discuss implications for the design of large‑scale quantum computers, potential experimental signatures in current quantum hardware, and the prospect of discovering new quantum information physics through this phase‑transition perspective.
Quantum error correction (QEC) is the cornerstone of fault‑tolerant quantum computation, enabling logical qubits to be protected against physical noise by encoding them into highly entangled states. In this paper we propose that the very ability of a QEC code to protect against errors is itself subject to a phase transition. We formulate a theory of the Quantum Error Correction Phase Transition (QECP), in which the system undergoes a sharp change from a regime of successful error suppression to a regime where logical errors proliferate, as a control parameter – such as the physical error rate, the code distance, or the connectivity of the underlying hardware – is varied. Drawing analogies to conventional symmetry‑breaking and topological phase transitions, we construct a statistical mechanical model that captures the competition between error‑correction resources and noise. We analyze the critical behavior, derive scaling relations, and discuss implications for the design of scalable quantum architectures. The existence of a QECP provides a new physical lens through which to view quantum information processing and suggests that fault‑tolerance thresholds may be understood as critical points in an extended parameter space.
Quantum error correcting (QEC) codes protect quantum information from decoherence as long as error rates fall below critical error thresholds. In general, obtaining thresholds implies simulating the QEC procedure using, in general, suboptimal decoding strategies. In a few cases and for sufficiently simple noise models, optimal decoding of QEC codes can be framed as a phase transition in disordered classical spin models. In both situations, accurate estimation of thresholds demands intensive computational resources. Here we use the coherent information of the mixed state of noisy QEC codes to accurately estimate the associated optimal QEC thresholds already from small-distance codes at moderate computational cost. We show the effectiveness and versatility of our method by applying it first to the topological surface and color code under bit-flip and depolarizing noise. We then extend the coherent information based methodology to phenomenological and quantum circuit level noise settings. For all examples considered, we obtain highly accurate estimates of optimal error thresholds from small, low-distance instances of the codes, in close agreement with threshold values reported in the literature. Our findings establish the coherent information as a reliable competitive tool for the calculation of optimal thresholds of state-of-the-art QEC codes under realistic noise models. Published by the American Physical Society 2024
The robustness of quantum memory against physical noises is measured by two methods: the exact and approximate quantum error correction (QEC) conditions for error recoverability and the decoder-dependent error threshold which assesses if the logical error rate diminishes with system size. Here we unravel their relations and propose a unified framework to extract an intrinsic error threshold from the approximate QEC condition, which could upper bound other decoder-dependent error thresholds. Our proof establishes that relative entropy, effectively measuring deviations from exact QEC conditions, serves as the order parameter delineating the transition from asymptotic recoverability to unrecoverability. Consequently, we establish a unified framework for determining the error threshold across both exact and approximate QEC codes, addressing errors originating from noise channels as well as those from code space imperfections. This result sharpens our comprehension of error thresholds across diverse QEC codes and error models. Published by the American Physical Society 2024