Cited by 3,325Quantum Computing Algorithms and ArchitectureQuantum Information and CryptographyQuantum many-body systems

Quantum Algorithm for Linear Systems of Equations

Open access
Harrow, Aram W., Hassidim, Avinatan, Lloyd, Seth·2009·Physical Review Letters

Abstract

Solving linear systems of equations is a common problem that arises both on its own and as a subroutine in more complex problems: given a matrix $A$ and a vector $\stackrel{\ensuremath{\rightarrow}}{b}$, find a vector $\stackrel{\ensuremath{\rightarrow}}{x}$ such that $A\stackrel{\ensuremath{\rightarrow}}{x}=\stackrel{\ensuremath{\rightarrow}}{b}$. We consider the case where one does not need to know the solution $\stackrel{\ensuremath{\rightarrow}}{x}$ itself, but rather an approximation of the expectation value of some operator associated with $\stackrel{\ensuremath{\rightarrow}}{x}$, e.g., ${\stackrel{\ensuremath{\rightarrow}}{x}}^{\ifmmode\dagger\else\textdagger\fi{}}M\stackrel{\ensuremath{\rightarrow}}{x}$ for some matrix $M$. In this case, when $A$ is sparse, $N\ifmmode\times\else\texttimes\fi{}N$ and has condition number $\ensuremath{\kappa}$, the fastest known classical algorithms can find $\stackrel{\ensuremath{\rightarrow}}{x}$ and estimate ${\stackrel{\ensuremath{\rightarrow}}{x}}^{\ifmmode\dagger\else\textdagger\fi{}}M\stackrel{\ensuremath{\rightarrow}}{x}$ in time scaling roughly as $N\sqrt{\ensuremath{\kappa}}$. Here, we exhibit a quantum algorithm for estimating ${\stackrel{\ensuremath{\rightarrow}}{x}}^{\ifmmode\dagger\else\textdagger\fi{}}M\stackrel{\ensuremath{\rightarrow}}{x}$ whose runtime is a polynomial of $\mathrm{log}(N)$ and $\ensuremath{\kappa}$. Indeed, for small values of $\ensuremath{\kappa}$ [i.e., $\mathrm{poly}\mathrm{log}(N)$], we prove (using some common complexity-theoretic assumptions) that any classical algorithm for this problem generically requires exponentially more time than our quantum algorithm.

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Crossref · 10.1103/physrevlett.103.150502

Cites

35 of 35 references verified0 citation errors · 0 not checkable

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  • On specific applications of Harrow et al.'s algorithm, its use is possible in computational biology, where numeric models can be solved, and also in the domain of machine learning techniques.

    “On the specific applications of this algorithm [75], its use is possible in computational biology, where numeric models can be solved, and also in the domain of machine learning techniques.”

    From Quantum Computing in the Next-Generation Computational Biology Landscape: From Protein Folding to Molecular Dynamics (2024) · cites this paper as [75]

    The Introduction and Discussion (page not specified in our copy) describe using the algorithm to estimate features of solutions to linear equations, including equations arising from numerical models. They do not identify computational biology or machine-learning techniques as applications, so the claimed domains are not supported by this paper's text. (LLM, conf=0.83)

    Experimental — accuracy not yet measured
  • As proposed by Harrow et al., the quantum algorithm is capable enough to give solutions to definite linear systems at exponentially rapid speed compared to any existing algorithm.

    “As proposed by Harrow etal. [75], the quantum algorithm is capable enough to give solutions to definite linear systems at exponentially rapid speed compared to any existing algorithm.”

    From Quantum Computing in the Next-Generation Computational Biology Landscape: From Protein Folding to Molecular Dynamics (2024) · cites this paper as [75]

    The Abstract describes a quantum algorithm that can be exponentially faster than classical algorithms for certain linear-system tasks. But it estimates a feature of the solution rather than producing the full solution, and the speedup depends on conditions including sparsity and a sufficiently small condition number; the claim omits those limits. (LLM, conf=0.89)

    Experimental — accuracy not yet measured
  • For an N x N linear system, classical algorithmic complexity scales polynomially in N, whereas the HHL quantum algorithm has complexity scaling logarithmically in N, with some scaling improvements proposed.

    “The classical algorithmic complexity for this task scales polynomially in the dimension N , whereas the now-famous Harrow-Hassidim-Lloyd (HHL) quantum algorithm [3] has a complexity that scales logarithmically in N , with some scaling improvements having been proposed [134-137].”

    From Variational quantum algorithms (2021) · cites this paper as [3]

    The Abstract says classical time scales polynomially with N and that the HHL algorithm can scale logarithmically with N. But it limits that comparison to cases where the goal is to estimate a feature of the solution rather than output the full vector, and its runtime also depends on sparsity, condition number, and precision. The later scaling improvements are cited to other references in the citing sentence, not to HHL. (LLM, conf=0.86)

    Experimental — accuracy not yet measured
  • The task of matrix inversion admits an exponential quantum speedup.

    “In particular, the task of matrix inversion admits an exponential quantum speedup, which could have many applications.”

    From Quantum Computing in the NISQ era and beyond (2018) · cites this paper as [40]

  • The HHL algorithm runs in time O(log N), an exponential speedup relative to classical matrix inversion, for fixed error and for fixed sparsity and condition number of A.

    “This quantum algorithm runs in time O (log N ) , an exponential speedup relative to classical matrix inversion (for fixed error and for fixed sparsity and condition number of A ).”

    From Quantum Computing in the NISQ era and beyond (2018) · cites this paper as [40]

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Cites

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Research works printed but not linked: 14

  1. [1]1. P. W. Shor, Proc. of the 35th FOCS (IEEE, New York, 1994), pp. 124-134.
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  9. [12]12. See EPAPS Document No. E-PRLTAO-103-055942 for detailed proofs of the claims in our manuscript. For more information on EPAPS, see http://www.aip.org/pubservs/ epaps.html.
  10. [16]16. D. R. Simon, SIAM J. Comput. 26, 1474 (1997).
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Authors
Harrow, A. W., Hassidim, A., & Lloyd, S.
Affiliations
  • University of Bristol (GB)
  • Massachusetts Institute of Technology (US)
Venue
Physical Review Letters
Published
2009-10-07
Volume
103
Issue
15
Pages
150502-150502
ISSN
  • 0031-9007 (ISSN-L)
  • 1079-7114
OpenAlex
W1492999010
Cited by
3,325
References
34
Citations by year
2026: 355 · 2025: 553 · 2024: 478 · 2023: 429 · 2022: 332 · 2021: 337 · 2020: 231 · 2019: 181 · 2018: 98 · 2017: 102 · 2016: 53 · 2015: 35 · 2014: 39 · 2013: 33 · 2012: 40
FWCI
23.75
Type
journal article
Language
en
Open access
Locations
  • Physical Review Letters (publishedVersion)
  • arXiv (Cornell University) (publishedVersion)
  • PubMed (publishedVersion)
  • DSpace@MIT (Massachusetts Institute of Technology) (submittedVersion, cc-by-nc)
  • arXiv (Cornell University)
  • PIRSA
  • Bulletin of the American Physical Society
Funders
  • W. M. Keck Foundation
  • Engineering and Physical Sciences Research Council (GR/S82169/01)
Topics
  • Physical Sciences › Computer Science › Artificial Intelligence › Quantum Computing Algorithms and Architecture
  • Physical Sciences › Computer Science › Artificial Intelligence › Quantum Information and Cryptography
  • Physical Sciences › Physics and Astronomy › Atomic and Molecular Physics, and Optics › Quantum many-body systems
Keywords
quantum algorithms, systems of linear equations, sparse matrices, condition number, expectation value estimation, quantum advantage

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