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arXiv:2507.10395

Fault-Tolerant Quantum Error Correction for Constant-Excitation Stabilizer Codes under Coherent Noise

Ching–Yi Lai, Pei-Hao Liou, Yingkai Ouyang

Published Jul 14, 2025• 0 citations
arXivHTMLPDF

Abstract

Collective coherent (CC) noise poses challenges for fault-tolerant error correction (FTEC), as it is not captured by conventional stochastic noise models. Constant-excitation (CE) codes are inherently immune to CC errors, but a fault-tolerant framework for operating these codes under circuit-level noise has not yet been established. Here, we develop an FTEC framework for CE CSS codes based on dual-rail concatenation. We show that conventional transversal CNOT gates violate the CE constraint and develop CE-preserving logical CNOT gates together with modified Shor- and Steane-type syndrome extraction schemes using zero-controlled NOT gates and CE-compatible ancilla states. We further develop an extended stabilizer simulation algorithm that tracks both stochastic and CC noise. Using this framework, we identify small distance-3 CE CSS codes demonstrate that the $[[14,1,3]]$ code maintains robust performance under coherent noise. Our results establish a fault-tolerant framework for CE codes under circuit-level noise and demonstrate their potential for quantum processors affected by CC noise.

DOI
10.48550/arxiv.2507.10395

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Collective coherent (CC) noise poses challenges for fault-tolerant error correction (FTEC), as it is not captured by conventional stochastic noise models. Constant-excitation (CE) codes are inherently immune to CC errors, but a fault-tolerant framework for operating these codes under circuit-level noise has not yet been established. Here, we develop an FTEC framework for CE CSS codes based on dual-rail concatenation. We show that conventional transversal CNOT gates violate the CE constraint and develop CE-preserving logical CNOT gates together with modified Shor- and Steane-type syndrome extraction schemes using zero-controlled NOT gates and CE-compatible ancilla states. We further develop an extended stabilizer simulation algorithm that tracks both stochastic and CC noise. Using this framework, we identify small distance-3 CE CSS codes demonstrate that the [[14,1,3]][[14,1,3]][[14,1,3]] code maintains robust performance under coherent noise. Our results establish a fault-tolerant framework for CE codes under circuit-level noise and demonstrate their potential for quantum processors affected by CC noise.
Fault-tolerant quantum error correction (FTEC) protocol is one of the most important components for a fault-tolerant simulation of a quantum circuit. It helps prevent error accumulation during the process which may cause the computation to fail. In a physical implementation of fault-tolerant circuits, the number of required ancilla qubits for the FTEC protocol can be a major concern because it grows exponentially as the level of concatenation increases whereas the resources are limited. Recently, the idea of using flag qubits to detect high-weight errors caused by fewer faults in syndrome extraction circuits has been proposed. The use of flag qubits allows the construction of fault-tolerant protocols with the fewest number of ancillas known to-date. In this work, some critical properties of CSS codes constructed from classical cyclic codes that enable the construction of a flag fault-tolerant error correction scheme are proved, and a fault-tolerant protocol along with a family of circuits for flag fault-tolerant error correction are given. The flag-FTEC protocol proposed in this work requires only four ancilla qubits and applicable to CSS codes of distance 3 constructed from classical cyclic codes.
Fault-tolerant error-correction (FTEC) circuit is the foundation for achieving reliable quantum computation and remote communication. However, designing a fault-tolerant error correction scheme with a solid error-correction ability and low overhead remains a significant challenge. In this paper, a low-overhead fault-tolerant error correction scheme is proposed for quantum communication systems. Firstly, syndrome ancillas are prepared into Bell states to detect errors caused by channel noise. We propose a detection approach that reduces the propagation path of quantum gate fault and reduces the circuit depth by splitting the stabilizer generator into X-type and Z-type. Additionally, a syndrome extraction circuit is equipped with two flag qubits to detect quantum gate faults, which may also introduce errors into the code block during the error detection process. Finally, analytical results are provided to demonstrate the fault-tolerant performance of the proposed FTEC scheme with the lower overhead of the ancillary qubits and circuit depth.
Small high-rate non-CSS stabilizer codes provide compact platforms for encoded quantum computation, but mixed-Pauli checks and limited native transversal logical gates complicate fault-tolerant dynamics. Block-level constructions offer an alternative by mapping an entire logical block to a physical circuit rather than compiling separately protected logical gates. We investigate this approach using the high-rate [[8,3,3]] non-CSS code and logical Trotter circuits as a testbed. We construct flagged syndrome-extraction circuits and establish a circuit-level memory pseudo-threshold near 1.5×10−31.5\times10^{-3}1.5×10−3. We then apply our symplectic-transvection construction, which maps a logical Trotter circuit to a physical circuit with the same block pattern for any stabilizer code. Although this mapping preserves the intended unitary algebraically, encoded Trotter circuits exhibit asymmetry between logical-XXX and logical-ZZZ failure channels. Single-fault analysis identifies the mechanism: a fault on the shared parity ancilla can propagate through the uncomputation network into an undetectable logical operator, reducing the effective circuit distance in the affected sector. We evaluate flag-conditioned recovery, biased-noise decoding, CliNR resource verification, flag postselection, and asymmetric gate-noise models. These methods suppress propagated faults but do not simultaneously suppress both logical sectors in the realistic configurations studied. A diagnostic protected limit removing the identified malignant first-order locations restores pseudo-threshold behavior in both sectors, approaching memory performance. These results demonstrate the potential of block-level logical constructions for non-CSS codes without rich native transversal gate sets and the joint protection of the parity network, analog rotation, and recovery required to preserve fault-tolerant distance.
Stabilizer states are extensively studied in quantum information theory for their structures based on the Pauli group. Calderbank-Shor-Steane (CSS) stabilizer states are of particular importance in their application to fault-tolerant quantum computation (FTQC). However, how to fault-tolerantly prepare arbitrary CSS stabilizer states for general CSS stabilizer codes is still unknown, and their preparation can be highly costly in computational resources. In this paper, we show how to prepare a large class of CSS stabilizer states useful for FTQC. We propose distillation protocols using syndrome encoding by classical codes or quantum CSS codes. Along the same lines, we show that classical coding techniques can reduce the ancilla consumption in Steane syndrome extraction by using additional transversal controlled-not gates and classical computing power. In the scenario of a fixed ancilla consumption rate, we can increase the frequency of quantum error correction and effectively lower the error rate.
Abstract Coherent errors, which arise from collective couplings, are a dominant form of noise in many realistic quantum systems, and are more damaging than oft considered stochastic errors. Here, we propose integrating stabilizer codes with constant-excitation codes by code concatenation. Namely, by concatenating an [[n, k, d]] stabilizer outer code with dual-rail inner codes, we obtain a [[2n, k, d]] constant-excitation code immune from coherent phase errors and also equivalent to a Pauli-rotated stabilizer code. When the stabilizer outer code is fault-tolerant, the constant-excitation code has a positive fault-tolerant threshold against stochastic errors. Setting the outer code as a four-qubit amplitude damping code yields an eight-qubit constant-excitation code that corrects a single amplitude damping error, and we analyze this code’s potential as a quantum memory.
Fault-tolerant quantum computation traditionally incurs substantial resource overhead, with both qubit and time overheads scaling polylogarithmically with the size of the computation. While prior work by Gottesman showed that constant qubit overhead is achievable under stochastic noise using quantum low-density parity-check (QLDPC) codes, it has remained an open question whether similar guarantees hold under more general, non-stochastic noise models. In this work, we address this question by considering a general circuit-level noise model defined via the diamond norm, which captures both stochastic and non-stochastic noise, including coherent and amplitude damping noise. We prove that constant qubit overhead fault-tolerant quantum computation is achievable in this general setting, using QLDPC codes with constant rate and linear minimum distance. To establish our result, we develop a fault-tolerant error correction scheme and a method for implementing logic gates under general circuit noise. These results extend the theoretical foundations of fault-tolerant quantum computation and offer new directions for fault-tolerant architectures under realistic noise models.