Quantum low-density parity-check (LDPC) codes are stabilizer codes in which each stabilizer generator acts on O(1) qubits and each qubit participates in O(1) generators. Some famous classes of codes such as the toric code are quantum LDPC codes. There are several reasons that explain why this subclass of stabilizer codes is particularly interesting. For instance, last year it was shown by Gottesman that such codes would in principle allow fault-tolerant quantum computing with constant overhead.However, for this application we need quantum LDPC codes that are able to encode a linear fraction of qubitswith good minimum distance properties and that are equipped with an efficient decoding algorithm.The fact that their classical counterpart enjoys all these nice properties could lead to think that it will be easyto obtain the same result in the quantum setting. However it has turned out that decoding quantum LDPC codes is a much more involved issue than decoding classical LDPC codes.The latter class of codes is decoded successfully by low complexity iterative decoding algorithms.In principle, such algorithms could also be used to decode quantum LDPC codes.However, this approach fails for quantum LDPC codes due to the fact that suchcodes are inherently highly degenerate. I will survey and discuss in this talk alternative approaches that have been successful for some subclasses of quantum LDPC codes.
Quantum low-density parity-check (LDPC) codes are stabilizer codes in which each stabilizer generator acts on O(1) qubits and each qubit participates in O(1) generators. Some famous classes of codes such as the toric code are quantum LDPC codes. There are several reasons that explain why this subclass of stabilizer codes is particularly interesting. For instance, last year it was shown by Gottesman that such codes would in principle allow fault-tolerant quantum computing with constant overhead.However, for this application we need quantum LDPC codes that are able to encode a linear fraction of qubitswith good minimum distance properties and that are equipped with an efficient decoding algorithm.The fact that their classical counterpart enjoys all these nice properties could lead to think that it will be easyto obtain the same result in the quantum setting. However it has turned out that decoding quantum LDPC codes is a much more involved issue than decoding classical LDPC codes.The latter class of codes is decoded successfully by low complexity iterative decoding algorithms.In principle, such algorithms could also be used to decode quantum LDPC codes.However, this approach fails for quantum LDPC codes due to the fact that suchcodes are inherently highly degenerate. I will survey and discuss in this talk alternative approaches that have been successful for some subclasses of quantum LDPC codes.
Quantum error correction (QEC) is a cornerstone of quantum computing, enabling reliable information processing in the presence of noise. Sparse stabilizer codes -- referred to generally as quantum low-density parity-check (QLDPC) codes -- have risen to the forefront of QEC research in recent years. This can be attributed to several key factors. First, classical LDPC codes admit low-complexity belief propagation iterative decoding and near-capacity performance, which contributed to the early interest in QLDPC codes. Then, the result promising constant overhead fault tolerance using QLDPC codes led to the search for code families that go beyond the long-holding n scaling barrier of minimum distance for codelength n. This resulted in recent breakthroughs in the construction of QLDPC codes, which, combined with efficient decoding algorithms and the development of fault-tolerant protocols operating on QLDPC-encoded quantum information, provide a promising pathway to low-overhead, fault-tolerant quantum computation. However, despite their potential, challenges remain, particularly in constructing and decoding finite-length codes that account for, or efficiently leverage, specific characteristics of quantum hardware, such as connectivity, topology, native gate sets, and noise models. This article provides an in-depth examination of QLDPC codes and their iterative decoders, catering to an information theory audience with no or limited background in quantum mechanics. We discuss the theoretical underpinnings, explore unique characteristics of quantum channels, and delineate key code constructions and decoding algorithms, ultimately highlighting the impact and future prospects of QLDPC codes in quantum information science.
Quantum Low-Density Parity-Check (LDPC) codes provide a promising solution to protect quantum information from errors and are therefore considered an important step in the realisation of a fullscale fault-tolerant quantum computer. In this thesis, we study quantum LDPC codes under three different approaches, providing insights into renormalisation decoding for Kitaev’s toric code, establishing bounds on the minimum distance of geometrically-local codes, and constructing small instances of quantum Tanner codes. Kitaev’s toric code is a prominent quantum LDPC code that is currently the most widely pursued for experimental implementation. We focus on the probabilistic renormalisation decoders introduced by Duclos- Cianci and Poulin, a family of decoders that exhibit one of the best trade-offs between accuracy and efficiency. We study how they handle adversarial errors by introducing a deterministic renormalisation decoder that does not use a priori probabilities of the error model. We find that this renormalisation decoder allows for fractal-like uncorrectable error patterns, and we obtain a lower bound on the weight of such errors. Due to physical constraints, a lot of attention has gone into quantum LDPC codes whose stabilisers act only on a limited number of nearby qubits. We extend the well-known Bravyi-Terhal bound to quantum codes defined by local constraints on a lattice quotient, providing a crucial insight into the limitations of geometrically-local stabiliser codes. As an application, we provide upper bounds on the minimum distance of Abelian Two-Block Group Algebra codes, a family of quantum LDPC codes for which the minimum distance is currently unknown. In recent years, asymptotically good quantum LDPC codes were constructed, but provable results on their minimum distances only apply to impractically long codes. Following the effort in the design of near-term implementable codes, we construct short instances of quantum Tanner codes using square complexes obtained from lifted products of regular graphs. Several explicit instances of these codes exhibit minimum distances surpassing the square root of the code length.
Quantum low-density parity-check (qLDPC) codes are an important component in the quest for quantum fault tolerance. Dramatic recent progress on qLDPC codes has led to constructions which are asymptotically good, and which admit linear-time decoders to correct errors affecting a constant fraction of codeword qubits. These constructions, while theoretically explicit, rely on inner codes with strong properties only shown to exist by probabilistic arguments, resulting in lengths that are too large to be practically relevant. In practice, the surface/toric codes, which are the product of two repetition codes, are still often the qLDPC codes of choice. A previous construction of qLDPC codes based on the lifted product of an expander-based classical LDPC code with a repetition code (Panteleev and Kalachev, 2020) achieved a near-linear distance, and avoids the need for such intractable inner codes. Our main result is an efficient decoding algorithm for these codes that corrects a near-linear number of adversarial errors. En route, we give a similar algorithm for the hypergraph product version these codes, which are simpler but have distance growing only as the square root of the block length. Our decoding algorithms leverage the fact that the codes we consider are quasi-cyclic, meaning that they respect a cyclic group symmetry. Since the repetition code is not based on expanders, previous approaches to decoding expander-based qLDPC codes, which typically worked by greedily flipping code bits to reduce some potential function, do not apply in our setting. Instead, we reduce our decoding problem (in a black-box manner) to that of decoding classical expander-based LDPC codes under noisy parity-check syndromes. For completeness, we also include a treatment of such classical noisy-syndrome decoding that is sufficient for our application to the quantum setting.
Due to their fast decoding algorithms, quantum generalizations of low-density parity check, or LDPC, codes have been invesitgated as a solution to the problem of decoherence in fragile quantum states [1, 2]. However, the additional twisted inner product requirements of quantum stabilizer codes force four-cycles and eliminate the possibility of randomly generated quantum LDPC codes. Moreover, the classes of quantum LDPC codes discovered thus far generally have unknown or small minimum distance, or a fixed rate (see [3, 4] and references therin). This paper presents several new classes of quantum LDPC codes constructed from finite projective planes. These codes have rates that increase with the block length n and minimum weights proportional to n1=2. For the sake of completeness, we include an introduction to classical error correction and LDPC codes, and provide a review of quantum communication, quantum stabilizer codes, and finite projective geometry.
Quantum error correction codes are needed to protect quantum systems from interference. An important family of quantum error correction codes are stabilizer codes. Many stabilizer codes in the literature belong to a subclass of stabilizer codes, referred to as CSS codes since they are based on the so-called CSS formalism. CSS codes have received significant attention because the dual-containing condition for the CSS formalism is relatively straightforward to satisfy and also because it is easy to produce CSS codes from classical error correction codes. However, some stabilizer codes in the literature show superior error correction performance than existing CSS codes, and are based on binary low-density parity-check (LDPC) codes. Since it has been shown that CSS codes based on nonbinary LDPC codes have good performance over quantum qubit channels, in this paper we investigate stabilizer codes based on nonbinary LDPC codes. In particular, we propose constructions of stabilizer codes based on nonbinary quasi-cyclic low-density parity-check (QC-LDPC) codes for qubit channels. The simulation results show that our QC-LDPC stabilizer codes decoded by a nonbinary sum-product algorithm have better performance than their binary counterparts.