## Description

Member of a family of \([[n,k,d]]\) modular-qudit or Galois-qudit stabilizer codes for which the number of sites participating in each stabilizer generator and the number of stabilizer generators that each site participates in are both bounded by a constant \(w\) as \(n\to\infty\); can be denoted by \([[n,k,d,w]]\). Sometimes, the two parameters are explicitly stated: each site of an an \((l,w)\)-regular QLDPC code is acted on by \(\leq l\) generators of weight \(\leq w\). QLDPC codes can correct many stochastic errors far beyond the distance, which may not scale as favorably. Together with more accurate, faster, and easier-to-parallelize measurements than those of general stabilizer codes, this property makes QLDPC codes interesting in practice.

A geometrically local stabilizer code is a QLDPC code where the sites involved in any syndrome bit are contained in a fixed volume that does not scale with \(n\). As opposed to general stabilizer codes, syndrome extraction of the constant-weight check operators of a QLDPC codes can be done using a constant-depth circuit.

Notable \([[n,k,d]]\) QLDPC codes are summarized in Table I, demonstrating the steady improvement in code parameters that culminated in the first asymptotically good QLDPC codes.

\(k\) | \(d\) | Code |
---|---|---|

\(2\) | \(\sqrt{n}\) | |

\(2\) | \(\sqrt{n\sqrt{\log n}}\) | |

\(\Theta(n)\) | \(\sqrt{n}\) | |

\(\sqrt{n}/\log n\) | \(\sqrt{n} \log n\) | |

\(\sqrt{n}\) | \(\sqrt{n} \log^c n\) | |

\(n^{3/5}/\text{polylog}(n)\) | \(n^{3/5}/\text{polylog}(n)\) | |

\(\log n\) | \(n/\log n\) | |

\(\Theta(n)\) | \(\Theta(n)\) | |

\(\Theta(n)\) | \(\Theta(n)\) | |

\(\Theta(n)\) | \(\Theta(n)\) |

Strictly speaking, the term parity check describes only bitwise qubit error syndromes. Nevertheless, qudit stabilizer codes satisfying the above criteria are also called QLDPC codes.

## Protection

Detects errors on \(d-1\) sites, corrects errors on \(\left\lfloor (d-1)/2 \right\rfloor\) sites. Code distance may not be a reliable marker of code performance.

Since QLDPC codes are stabilizer QLRCs whose locality \(r = w\), their relative distance is bounded by [1; Thm. 4] \begin{align} \delta = \frac{d}{n} \leq \frac{1}{2} - \Omega\left(\frac{1}{r}\right)~. \tag*{(1)}\end{align}

## Rate

Asymptotic scaling of \(k\) and \(d\) with \(n\) depends heavily on the code construction. Bounds generalizing the BPT bound to QLDPC codes depend on the separation profile of the code's underlying connectivity graph [2,3]. A constant relative minimum distance can be achieved only for graphs that contain expanders [2]. Conversely, a code with parameters \(k\) and \(d\) requires a graph with order \(\Omega(d)\) edges of length of order \(\Omega(d/n^{1/D})\) [4]. Random QLDPC codes found by solving certain constraint satisfaction problems (CSPs) practically achieve the capacity of the erasure channel [5].

Qubit QLDPC codes cannot attain the capacity of the erasure channel [6], but this capacity can be attained by code families with weight \(w = O(\text{polylog}n)\) [7].

## Encoding

## Gates

## Decoding

## Fault Tolerance

## Code Capacity Threshold

## Threshold

## Notes

## Parents

- Quantum low-weight check (QLWC) code — QLDPC codes are QLWC codes for which the number of stabilizer generators that each site participates in is bounded by a constant as \(n\to\infty\).
- Quantum locally recoverable code (QLRC) — QLDPC codes are stabilizer QLRCs whose locality \(r \leq w\), the maximum number of subsystems that a stabilizer generator participates in [1].

## Children

- Generalized homological-product code — Homological products are a primary tool for generating QLDPC codes with favorable parameters. Typically, whenever the input codes are LDPC or QLDPC, the resulting code will be QLDPC with non geometrically local stabilizer generators.
- Good QLDPC code
- Classical-product code
- Hierarchical code
- Yoked surface code
- Bivariate bicycle (BB) code
- Finite-geometry (FG) QLDPC code
- Quasi-cyclic QLDPC code

## Cousins

- Low-density parity-check (LDPC) code
- Topological code — Topological codes are not generally defined using Pauli strings. However, for appropriate tesselations, the codespace is the ground-state subspace of a geometrically local Hamiltonian. In this sense, topological codes are QLDPC codes.
- Dynamically-generated QECC — QLDPC codes can arise from a dynamical process [53].
- Hamiltonian-based code — QLDPC code Hamiltonians can be simulated, with the help of perturbation theory, by two-dimensional Hamiltonians with non-commuting terms whose interactions scale with \(n\) [54].
- Single-shot code — Qubit QLDPC codes satisfying linear confinement are single shot [55]. Any code that admits a local greedy decoder also satisfies linear confinement, and so is single shot [43].
- Low-density generator-matrix (LDGM) code — LDGM codes can yield CSS [56–59] and non-CSS [60,61] QLDPC codes. Some of the LDGM-based CSS codes have \(n\)-independent minimum distance and no code capacity threshold [62; Sec. 4.2].
- Random stabilizer code — Random QLDPC codes found by solving certain constraint satisfaction problems (CSPs) practically achieve the capacity of the erasure channel [5].
- Quasi-cyclic LDPC (QC-LDPC) code — QC-LDPC codes can be used to make QLDPC codes using various non-CSS constructions [63].
- Quantum locally testable code (QLTC) — Stabilizer LTCs are QLDPC. More general QLTCs are not defined using Pauli strings, but the codespace is the ground-state subspace of a local Hamiltonian. In this sense, QLTCs are QLDPC codes.
- 2D lattice stabilizer code — Chain complexes describing QLDPC codes can be converted to 2D lattice stabilizer codes [64].
- Lattice stabilizer code — Chain complexes describing some QLDPC codes can be 'lifted' into higher-dimensional manifolds admitting some notion of geometric locality [65]. In addition, chain complexes describing QLDPC codes can be converted to 2D lattice stabilizer codes [64].
- Sparse subsystem code — Sparse subsystem codes reduce to QLDPC codes when there are no gauge qudits.
- Honeycomb Floquet code — The Floquet check operators are weight-two, and each qubit participates in one check each round.
- Spacetime circuit code — General spacetime circuit codes can be sparsified to yield QLDPC spacetime circuit codes [66].
- EA QLDPC code
- Bicycle code — Bicycle codes are the first QLDPC codes [67].
- \(D\)-dimensional twisted toric code — It is conjectured that appropriate twisted boundary conditions yield multi-dimensional toric code families with linear distance and logarithmic-weight stabilizer generators [68].
- Galois-qudit BCH code — Some Galois-qudit BCH codes can be constructed as QLDPC codes [69].
- Two-block CSS code — When matrices \(A\) and \(B\) have row and column weights bounded by \(W\), a two-block CSS code is a quantum LDPC code with stabilizer generators bounded by \(2W\).
- Distance-balanced code — Lattice surgery techniques for QLDPC codes [10,43] utilize weight reduction. Single-ancilla syndrome extraction circuits that, for the most part, preserve the effective distance of weight-reduced qLDPC codes [70].
- Two-block group-algebra (2BGA) codes — Given group algebra elements \(a,b\in \mathbb{F}_q[G]\) with weights \(W_a\) and \(W_b\) (i.e., number of non-zero terms in the expansion), the 2BGA code LP\((a,b)\) has stabilizer generators of uniform weight \(W_a+W_b\).
- Generalized bicycle (GB) code — Stabilizer generators of the code GB\((a,b)\) have weights given by the sum of weights of polynomials \(a(x)\) and \(b(x)\). The GB code ansatz is convenient for designing QLDPC codes and several extensions exist [71].

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## Page edit log

- Victor V. Albert (2022-10-02) — most recent
- Eugene Tang (2022-10-02)
- Victor V. Albert (2022-09-16)
- Victor V. Albert (2022-05-13)
- Victor V. Albert (2022-01-24)
- Xiaozhen Fu (2021-12-12)

## Cite as:

“Quantum LDPC (QLDPC) code”, The Error Correction Zoo (V. V. Albert & P. Faist, eds.), 2022. https://errorcorrectionzoo.org/c/qldpc