# Quantum low-density parity-check (QLDPC) code[1]

## Description

Also called a sparse quantum code. 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 as \(n\to\infty\). 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

## Rate

Asymptotic scaling of \(k\) and \(d\) with \(n\) depends heavily on the code construction.

Geometrically local qubit codes are limited by the Bravyi-Poulin-Terhal (BPT) bound [2] (see also [3–5]), which states that \(d \leq O(n^{1-1/D})\) and \(k d^{2/D-1} = O(n)\) for \(D\)-dimensional lattice geometries. Codes on a \(D\)-dimensional Riemannian manifold with diameter \(L\) satisfy \(k = O(L^{D-2})\) [6].

For general graphs, distance is limited by graphs' connectivity, and a constant relative minimum distance can be achieved only for graphs that contain expanders [7]. Conversely, a code with parameters \(k\) and \(d\) requires a graph with \(\Omega(d)\) edges of length \(\Omega(d/n^{1/D})\) [8].

## Gates

## Decoding

## Fault Tolerance

## Code Capacity Threshold

## Threshold

## Notes

## Parents

## Children

- Fracton code — Fracton codes admit geometrically local stabilizer generators on a cubic lattice.
- 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
- Translationally invariant stabilizer code — Translationally-invariant stabilizer codes are geometrically local.
- Classical-product code
- Hierarchical 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. On the other hand, chain complexes describing some QLDPC codes can be 'lifted' into higher-dimensional manifolds admitting some notion of geometric locality [28]. This opens up the possibility that some QLDPC codes, despite not being geometrically local, can in fact be associated with a geometrically local theory described by a category.
- Dynamically-generated QECC — QLDPC codes can arise from a dynamical process [29].
- 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.
- Honeycomb Floquet code — The Floquet check operators are weight-two, and each qubit participates in one check each round.
- Generalized bicycle (GB) code — A code GB\((a,b)\) is given by the sum of weights of polynomials \(a(x)\) and \(b(x)\). The GB code ansatz is convenient for designing quantum LDPC codes.

## References

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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 low-density parity-check (QLDPC) code”, The Error Correction Zoo (V. V. Albert & P. Faist, eds.), 2022. https://errorcorrectionzoo.org/c/qldpc