Markus Grassl, Peter W. Shor, Bei Zeng
We show how good quantum error-correcting codes can be constructed using generalized concatenation. The inner codes are quantum codes, the outer codes can be linear or nonlinear classical codes. Many new good codes are found, including both stabilizer codes as well as so-called nonadditive codes.
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As there is no quantum error correction code with universal set of transversal gates, several approaches have been proposed which, in combination of transversal gates, make universal fault-tolerant quantum computation possible. Magic state distillation, code switching, code concatenation and pieceable fault-tolerance are well-known examples of such approaches. However, the overhead of these approaches is one of the main bottlenecks for large-scale quantum computation. In this paper, two approaches for universal fault-tolerant quantum computation, mainly based on code concatenation, are propose…
Using transversal gates is a straightforward and efficient technique for fault-tolerant quantum computing. Since transversal gates alone cannot be computationally universal, they must be combined with other approaches such as magic state distillation, code switching or code concatenation in order to achieve universality. In this paper we propose an alternative approach for universal fault-tolerant quantum computing mainly based on the code concatenation approach proposed in [PRL 112, 010505 (2014)] but in a non-uniform fashion. The proposed approach is described based on non-uniform concatenat…
We examine the transformation of noise under a quantum error correcting code (QECC) concatenated repeatedly with itself, by analyzing the effects of a quantum channel after each level of concatenation using recovery operators that are optimally adapted to use error syndrome information from the previous levels of the code. We use the Shannon entropy of these channels to estimate the thresholds of correctable noise for QECCs and find considerable improvements under this adaptive concatenation. Similar methods could be used to increase quantum fault tolerant thresholds.
In this work, we introduce a new concatenation scheme which aims at protecting information against the occurrence of both computational errors and quantum erasures. According to our scheme, the internal code must be a quantum loss-correcting code that does not perform measurements, while the external code must be a quantum error-correcting code. We illustrate the concatenation proposed with an example in which one qubit of information is protected against the occurrence of two erasures and one computational error.
Graphs are closely related to quantum error-correcting codes: every stabilizer code is locally equivalent to a graph code, and every codeword stabilized code can be described by a graph and a classical code. For the construction of good quantum codes of relatively large block length, concatenated quantum codes and their generalizations play an important role. We develop a systematic method for constructing concatenated quantum codes based on "graph concatenation", where graphs representing the inner and outer codes are concatenated via a simple graph operation called "generalized local complem…
Gottesman, Kitaev and Preskill have proposed a scheme to encode a qubit in a harmonic oscillator, which is called the GKP code. It is designed to be resistant to small shift errors contained in momentum and position quadratures. Thus there's some intrinsic fault tolerance of the GKP code. In this thesis,we propose a method to utilize all the information contained in the continuous shifts, not just simply map a GKP-encoded qubit to a normal qubit. This method enables us to do maximum-likelihood decisions and thus increase fault-tolerance of the GKP code. This thesis shows that the continuous na…
We study the performance of simple error correcting and error avoiding quantum codes together with their concatenation for correlated noise models. Specifically, we consider two error models: i) a bit-flip (phase-flip) noisy Markovian memory channel (model I); ii) a memory channel defined as a memory degree dependent linear combination of memoryless channels with Kraus decompositions expressed solely in terms of tensor products of X-Pauli (Z-Pauli) operators (model II). The performance of both the three-qubit bit flip (phase flip) and the error avoiding codes suitable for the considered error …
A novel scheme is presented for fault-tolerant quantum computation based on the cluster model. Some relevant logical cluster states are constructed in concatenation by post-selection through verification, without necessity of recovery operation, where a suitable code such as the Steane's 7-qubit code is adopted for transversal operations. This simple concatenated construction of verified cluster states achieves a high noise threshold ~1%, and restrains the divergence of resources.
Concatenation Schemes for Topological Fault-tolerant Quantum Error Correction
本研究提出基于小型纠错/检错码与三维cluster态级联的拓扑容错量子纠错方案，通过容错态制备与解码将电路级错误转为擦除错误，最优经典码级联方案显著提升阈值、降低时空开销。
Concatenation of two quantum error correcting codes with complementary sets of transversal gates can provide a means towards universal fault-tolerant computation. We first show that it is generally preferable to choose the inner code with the higher pseudo-threshold in order to achieve lower logical failure rates. We then explore the threshold properties of a wide range of concatenation schemes. Notably, we demonstrate that the concatenation of complementary sets of Reed-Muller codes can increase the code capacity threshold under depolarizing noise when compared to extensions of previously pro…
Concatenating quantum error-correcting codes with decoherence-free subspaces and vice versa
本文研究量子纠错码与无退相干子空间码的两种级联顺序，证明强关联错误下DFS作内码更优、独立错误下QECC作内码更优，并通过具体码例验证了容错性能。
Exploring Asymmetric QEC Code Concatenation
本文形式化级联量子码的Hadamard变形空间，提出六种策略，其中一种在码容量噪声下平衡X/Z逻辑错误率并降低总错误率。
It has been known that quantum error correction via concatenated codes can be done with exponentially small failure rate if the error rate for physical qubits is below a certain accuracy threshold. Other, unconcatenated codes with their own attractive features-improved accuracy threshold, local operations-have also been studied. By iteratively distilling a certain two-qubit entangled state it is shown how to perform an encoded Toffoli gate, important for universal computation, on CSS codes that are either unconcatenated or, for a range of very large block sizes, singly concatenated.
Resource-efficient fault-tolerant one-way quantum repeater with code concatenation
提出一种基于码级联的单向量子中继器，结合树簇码与5量子比特码，以最小资源开销实现长达10,000公里的容错量子通信。
Qubit-oscillator concatenated codes: decoding formalism & code comparison
该研究针对玻色码与量子比特码的最优级联问题，为四类码的三种级联方案开发了高效最大似然译码器，经加性高斯白噪声测试验证，部分含GKP稳定子码的级联方案性能更优，还提出了无额外压缩的新型GKP稳定子码及其多能级版本。
We explicitly construct an infinite family of asymptotically good concatenated quantum stabilizer codes where the outer code uses CSS-type quantum Reed-Solomon code and the inner code uses a set of special quantum codes. In the field of quantum error-correcting codes, this is the first time that a family of asymptotically good quantum codes is derived from bad codes. Its occurrence supplies a gap in quantum coding theory.
Fault-tolerant protocols through spacetime concatenation
本文提出时空拼接框架，用于高效编译稳定码的 syndrome 提取电路，通过结构化布局和编码矩阵实现动态码的容错编译，并展示其在硬件约束下的适应性。
As there is no quantum error correction code with universal set of transversal gates, several approaches have been proposed which, in combination of transversal gates, make universal fault-tolerant quantum computation possible. Magic state distillation, code switching, code concatenation and pieceable fault-tolerance are well-known examples of such approaches. However, the overhead of these approaches is one of the main bottlenecks for large-scale quantum computation. In this paper, a hybrid approach is proposed which combines the code concatenation technique with the other mentioned approache…
Elevator Codes: Concatenation for resource-efficient quantum memory under biased noise
本研究提出一种二维局部码构造，通过拼接两个经典码，在偏置噪声下显著降低量子比特开销，实现更低的逻辑错误率。
When a logical qubit is protected using a quantum error-correcting code, the net effect of coding, decoherence (a physical channel acting on qubits in the codeword) and recovery can be represented exactly by an effective channel acting directly on the logical qubit. In this paper we describe a procedure for deriving the map between physical and effective channels that results from a given coding and recovery procedure. We show that the map for a concatenation of codes is given by the composition of the maps for the constituent codes. This perspective leads to an efficient means for calculating…
We derive the effective channel for a logical qubit protected by an arbitrary quantum error-correcting code, and derive the map between channels induced by concatenation. For certain codes in the presence of single-bit Pauli errors, we calculate the exact threshold error probability for perfect fidelity in the infinite concatenation limit. We then use the control theory technique of balanced truncation to find low-order non-asymptotic approximations for the effective channel dynamics.
Blocklet concatenation: Low-overhead fault-tolerant protocols for fusion-based quantum computation
本文提出Blocklet级联容错协议，利用码级联与横向门构造恒定尺寸资源态的FBQC融合网络，实现较高擦除阈值并给出逻辑操作、解码与光子硬件实现技术。
Concatenation of the Gottesman-Kitaev-Preskill code with the XZZX surface code
本文研究GKP码与XZZX表面码的级联，在两种噪声模型下分析其性能，引入不对称矩形GKP码，阈值达0.67，优于标准表面码，并降低逻辑错误率，减少开销。
Stitched surface codes: planar concatenation through GHZ corridors
该研究提出一种通过GHZ走廊实现的容错量子存储拼接表面码架构，实验在Willow超导处理器上验证，增大码距可降低逻辑错误率，为超越标准表面码的级联架构提供近期测试平台。
We propose two schemes to obtain Gottesman-Kitaev-Preskill (GKP) error syndromes by means of linear optical operations, homodyne measurements and GKP ancillae. This includes showing that for a concatenation of GKP codes with a $[n,k,d]$ stabilizer code only $2n$ measurements are needed in order to obtain the complete syndrome information, significantly reducing the number of measurements in comparison to the canonical concatenated measurement scheme and at the same time generalizing linear-optics-based syndrome detections to higher GKP codes. Furthermore, we analyze the possibility of building…
Analysis of quantum error correcting codes is typically done using a stochastic, Pauli channel error model for describing the noise on physical qubits. However, it was recently found that coherent errors (systematic rotations) on physical data qubits result in both physical and logical error rates that differ significantly from those predicted by a Pauli model. Here we examine the accuracy of the Pauli approximation for coherent errors on data qubits under the repetition code. We analytically evaluate the logical error as a function of concatenation level and code distance. We find that cohere…
We apply a dynamical systems approach to concatenation of quantum error correcting codes, extending and generalizing the results of Rahn et al. [1] to both diagonal and nondiagonal channels. Our point of view is global: instead of focusing on particular types of noise channels, we study the geometry of the coding map as a discrete-time dynamical system on the entire space of noise channels. In the case of diagonal channels, we show that any code with distance at least three corrects (in the infinite concatenation limit) an open set of errors. For Calderbank-Shor-Steane (CSS) codes, we give a m…
Deterministic generation of concatenated graph codes from quantum emitters
针对光子量子技术中光子损耗的核心挑战，提出利用双量子发射器或单带存储量子发射器的多光子发射生成级联图码的方案，可在高损耗、标准融合门且无辅助光子的场景下实现容错融合基量子计算。
We estimate the resource requirements for the quantum simulation of the ground state energy of the one dimensional quantum transverse Ising model (TIM), based on the surface code implementation of a fault tolerant quantum computer. The surface code approach has one of the highest known tolerable error rates (1%) which makes it currently one of the most practical quantum computing schemes. Compared to results of the same model using the concatenated Steane code, the current results indicate that the simulation time is comparable but the number of physical qubits for the surface code is 2-3 orde…
Letting the tiger out of its cage: bosonic coding without concatenation
本文提出一种无需级联的玻色子编码通用框架，利用满足类CSS同调条件的整数矩阵构造多模编码，统一了猫态、配对猫态、双轨等多种编码并产生新的类表面码。
Entanglement has shown promise in enhancing information processing tasks in a sensor network, via distributed quantum sensing protocols. As noise is ubiquitous in sensor networks, error correction schemes based on Gottesman, Kitaev and Preskill (GKP) states are required to enhance the performance, as shown in [New J. Phys. 22, 022001 (2020)] assuming homogeneous noise among sensors and perfect GKP states. Here, we extend the analyses of performance enhancement to finite squeezed GKP states in a heterogeneous noise model. To begin with, we study different concatenation schemes of GKP-two-mode-s…
We introduce the concept of generalized concatenated quantum codes. This generalized concatenation method provides a systematical way for constructing good quantum codes, both stabilizer codes and nonadditive codes. Using this method, we construct families of new single-error-correcting nonadditive quantum codes, in both binary and nonbinary cases, which not only outperform any stabilizer codes for finite block length, but also asymptotically achieve the quantum Hamming bound for large block length.
We discuss a method to construct quantum codes correcting amplitude damping errors via code concatenation. The inner codes are chosen as asymmetric Calderbank-Shor-Steane (CSS) codes. By concatenating with outer codes correcting symmetric errors, many new codes with good parameters are found, which are better than the amplitude damping codes obtained by any previously known construction.
We present a family of additive quantum error-correcting codes whose capacities exceeds that of quantum random coding (hashing) for very noisy channels. These codes provide non-zero capacity in a depolarizing channel for fidelity parameters $f$ when $f> .80944$. Random coding has non-zero capacity only for $f>.81071$; by analogy to the classical Shannon coding limit, this value had previously been conjectured to be a lower bound. We use the method introduced by Shor and Smolin of concatenating a non-random (cat) code within a random code to obtain good codes. The cat code with block size five …
Subsystem many-hypercube codes: High-rate concatenated codes with low-weight syndrome measurements
本文提出基于多超立方体码的子系统量子码，通过保持高编码率的同时实现恒定权重的综合征测量，提升解码性能。
We prove a new version of the quantum threshold theorem that applies to concatenation of a quantum code that corrects only one error, and we use this theorem to derive a rigorous lower bound on the quantum accuracy threshold epsilon_0. Our proof also applies to concatenation of higher-distance codes, and to noise models that allow faults to be correlated in space and in time. The proof uses new criteria for assessing the accuracy of fault-tolerant circuits, which are particularly conducive to the inductive analysis of recursive simulations. Our lower bound on the threshold, epsilon_0 > 2.73 \t…
Concatenating Algebraic Codes over High-Rate Quantum LDPC Codes
本文研究了在非局部高率内码上进行码拼接，通过将每个块视为单个逻辑Galois qudit，从而与具有优异参数和列表解码能力的代数外码进行拼接，实现了更低的空间开销和更优的工程优势。
We study the properties of error correcting codes for noise models in the presence of asymmetries and/or correlations by means of the entanglement fidelity and the code entropy. First, we consider a dephasing Markovian memory channel and characterize the performance of both a repetition code and an error avoiding code in terms of the entanglement fidelity. We also consider the concatenation of such codes and show that it is especially advantageous in the regime of partial correlations. Finally, we characterize the effectiveness of the codes and their concatenation by means of the code entropy …
Long-distance quantum communication using concatenated ring graph codes
针对长距离量子通信需处理传输损耗和操作错误但现有级联码硬件需求高的问题，提出基于级联环图码和线性光学贝尔态测量的单向量子中继器架构，物质量子比特数随级联深度线性扩展，单量子比特错误率ε>10⁻³时仍能以千赫兹速率实现>10⁴千米通信。
Fault Tolerant Quantum Error Mitigation
本研究提出容错量子错误缓解（FTQEM），通过容错操作和代码串联指数级抑制错误，提供解析阈值，并验证其优于传统方法。
The Gottesman-Kitaev-Preskill (GKP) code is an important type of bosonic quantum error-correcting code. Since the GKP code only protects against small shift errors in $\hat{p}$ and $\hat{q}$ quadratures, it is necessary to concatenate the GKP code with a stabilizer code for the larger error correction. In this paper, we consider the concatenation of the single-mode GKP code with the two-dimension (2D) color code (color-GKP code) on the square-octagon lattice. We use the Steane type scheme with a maximum-likelihood estimation (ME-Steane scheme) for GKP error correction and show its advantage fo…
Bosonic Pauli+: Efficient Simulation of Concatenated Gottesman-Kitaev-Preskill Codes
针对级联Gottesman-Kitaev-Preskill（GKP）码缺乏高效大规模仿真工具的问题，提出可高效仿真多模式且捕捉玻色子多模式动力学的玻色子Pauli+模型（BP+），验证了其准确性并展示其通用性。
Concatenated coding provides a general strategy to achieve the desired level of noise protection in quantum information storage and transmission. We report the implementation of a concatenated quantum error-correcting code able to correct against phase errors with a strong correlated component. The experiment was performed using liquid-state nuclear magnetic resonance techniques on a four spin subsystem of labeled crotonic acid. Our results show that concatenation between active and passive quantum error-correcting codes offers a practical tool to handle realistic noise contributed by both ind…
Biased-Noise Quantum Reed-Solomon Codes and a Tornado Concatenation for Cat Qubits
研究针对有偏噪声猫量子比特的纠错码，利用噪声偏差构建仅 Clifford 量子 Reed-Solomon 码，引入 Tornado 级联架构，经模拟其逻辑错误率低且缩放特性优，给出构造、模型分析及相关说明。
Time-Efficient Constant-Space-Overhead Fault-Tolerant Quantum Computation
针对现有容错量子计算常数空间开销协议处理时间呈多项式增长的问题，提出通过级联多个小规模量子汉明码的新方法，同时实现常数空间开销与准多项式对数时间开销，且在解码器非常数运行时间下仍保持容错。
We examine the performance of the single-mode GKP code and its concatenation with the toric code for a noise model of Gaussian shifts, or displacement errors. We show how one can optimize the tracking of errors in repeated noisy error correction for the GKP code. We do this by examining the maximum-likelihood problem for this setting and its mapping onto a 1D Euclidean path-integral modeling a particle in a random cosine potential. We demonstrate the efficiency of a minimum-energy decoding strategy as a proxy for the path integral evaluation. In the second part of this paper, we analyze and nu…
The advantage of the concatenated three-qubit codes
本文提出基于三量子比特码的级联量子纠错协议，通过总结级联规则构建适用于任意独立噪声的纠错方案，在非理想门操作下以更少物理资源实现更高有效信道保真度与容错阈值。
Universal quantum computation via scalable measurement-free error correction
本文提出Bacon-Shor码无测量形变协议与一次性Toffoli小装置，实现无中间测量、无前馈的容错通用量子计算，测得约0.12%的电路级退极化噪声容错阈值，适配中性原子平台。
We propose a method for universal fault-tolerant quantum computation using concatenated quantum error correcting codes. Namely, other than computational basis state preparation as required by the DiVincenzo criteria [1], our scheme requires no special ancillary state preparation to achieve universality, as opposed to schemes such as magic state distillation. The concatenation scheme exploits the transversal properties of two different codes, combining them to provide a means to protect against low-weight arbitrary errors. We give the required properties of the error correcting codes to ensure …
Blind Quantum Computation (BQC) is a delegation computing protocol that allows a client to utilize a remote quantum server to implement desired quantum computations while keeping her inputs, outputs, and algorithms private. However, qubit errors during the quantum computation are important issues that need to be addressed. In this paper, we propose a fault-tolerant blind quantum computation protocol with quantum error-correcting codes to avoid the accumulation and propagation of qubit errors during the computational process. We further propose to apply the concatenated codes in the protocol to…
We study the performance of Bacon-Shor codes, quantum subsystem codes which are well suited for applications to fault-tolerant quantum memory because the error syndrome can be extracted by performing two-qubit measurements. Assuming independent noise, we find the optimal block size in terms of the bit-flip error probability p_X and the phase error probability p_Z, and determine how the probability of a logical error depends on p_X and p_Z. We show that a single Bacon-Shor code block, used by itself without concatenation, can provide very effective protection against logical errors if the noise…
For every stabiliser $N$-qudit absolutely maximally entangled state, we present a method for determining the stabiliser generators and logical operators of a corresponding quantum error correction code. These codes encode $k$ qudits into $N-k$ qudits, with $k\leq \left \lfloor{N/2} \right \rfloor$, where the local dimension $d$ is prime. We use these methods to analyse the concatenation of such quantum codes and link this procedure to entanglement swapping. Using our techniques, we investigate the spread of quantum information on a tensor network code formerly used as a toy model for the AdS/C…
Bosonic rotation codes, introduced here, are a broad class of bosonic error-correcting codes based on phase-space rotation symmetry. We present a universal quantum computing scheme applicable to a subset of this class--number-phase codes--which includes the well-known cat and binomial codes, among many others. The entangling gate in our scheme is code-agnostic and can be used to interface different rotation-symmetric encodings. In addition to a universal set of operations, we propose a teleportation-based error correction scheme that allows recoveries to be tracked entirely in software. Focusi…
Distributed Quantum Error Correction with Permutation-Invariant Approximate Codes
针对模块化量子计算中噪声处理器间操作的纠错挑战，提出基于近似码的分布式量子纠错框架，允许跨模块级联局部码并使用处理器局部门，推导下界并模拟证明其在非均匀错误环境中优于现有方法，给出了置换不变W态码的编解码电路及制备方法。
We construct concatenated capacity-achieving quantum codes for noisy optical quantum channels. We demonstrate that the error-probability of capacity-achieving quantum polar encoding can be reduced by the proposed low-complexity concatenation scheme.
Fault-tolerant quantum computing with the parity code and noise-biased qubits
基于偏置噪声量子比特与奇偶架构的编码拼接，实现低物理量子比特开销的容错量子计算
We discuss long code problems in the Bennett-Brassard 1984 (BB84) quantum key distribution protocol and describe how they can be overcome by concatenation of the protocol. Observing that concatenated modified Lo-Chau protocol finally reduces to the concatenated BB84 protocol, we give the unconditional security of the concatenated BB84 protocol.
On Error Thresholds for Pauli Channels: Some answers with many more questions
本文研究Pauli通道的误差阈值，通过分析余集重量生成函数框架，探讨了稳定化码的非加性特性及拼接码的性能优化。
Descriptions of quantum algorithms, communication etc. protocols assume the existence of closed quantum system. However, real life quantum systems are open and are highly sensitive to errors. Hence error correction is of utmost importance if quantum computation is to be carried out in reality. Ideally, an error correction block should be placed after every gate operation in a quantum circuit. This increases the overhead and reduced the speedup of the quantum circuit. Moreover, the error correction blocks themselves may induce errors as the gates used for error correction may be noisy. In this …
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 posi…
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