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#REDIRECT[[Continuous-variable quantum information]]
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'''Continuous-variable quantum computation''', also called '''continuous quantum computation''', is the area of [[quantum computation]] that makes use of [[Observable|physical observables]], like the strength of an [[electromagnetic field]], whose numerical values belong to [[List of continuity-related mathematical topics|continuous]] [[Interval (mathematics)|intervals]].<ref>{{Cite journal|last=Lloyd|first=Seth|last2=Braunstein|first2=Samuel L.|date=1999-01-01|title=Quantum Computation over Continuous Variables|url=https://link.aps.org/doi/10.1103/PhysRevLett.82.1784|journal=Physical Review Letters|volume=82|issue=8|pages=1784–1787|arxiv=quant-ph/9810082|doi=10.1103/PhysRevLett.82.1784}}</ref><ref>{{Cite journal|last=Bartlett|first=Stephen D.|last2=Sanders|first2=Barry C.|date=2002-01-01|title=Universal continuous-variable quantum computation: Requirement of optical nonlinearity for photon counting|url=https://link.aps.org/doi/10.1103/PhysRevA.65.042304|journal=Physical Review A|volume=65|issue=4|pages=|arxiv=quant-ph/0110039|doi=10.1103/PhysRevA.65.042304}}</ref><ref>{{Cite journal|last=Menicucci|first=Nicolas C.|last2=van Loock|first2=Peter|last3=Gu|first3=Mile|last4=Weedbrook|first4=Christian|last5=Ralph|first5=Timothy C.|last6=Nielsen|first6=Michael A.|date=2006-09-13|title=Universal Quantum Computation with Continuous-Variable Cluster States|url=https://link.aps.org/doi/10.1103/PhysRevLett.97.110501|journal=Physical Review Letters|volume=97|issue=11|pages=110501|arxiv=quant-ph/0605198|doi=10.1103/PhysRevLett.97.110501}}</ref><ref>{{Cite journal|last=Tasca|first=D. S.|last2=Gomes|first2=R. M.|last3=Toscano|first3=F.|last4=Souto Ribeiro|first4=P. H.|last5=Walborn|first5=S. P.|date=2011-01-01|title=Continuous-variable quantum computation with spatial degrees of freedom of photons|url=https://link.aps.org/doi/10.1103/PhysRevA.83.052325|journal=Physical Review A|volume=83|issue=5|pages=|arxiv=1106.3049|doi=10.1103/PhysRevA.83.052325}}</ref> In a sense, continuous-variable quantum computation is "analogue," while quantum computation using [[Qubit|qubits]] is "digital." In more technical terms, the former makes use of [[Hilbert space|Hilbert spaces]] that are [[Dimension|infinite-dimensional]], while the Hilbert spaces for systems comprising collections of qubits are finite-dimensional.<ref>{{Cite book|url=|title=Quantum Information with Continuous Variables|last=Braunstein|first=S. L.|last2=Pati|first2=A. K.|date=2012-12-06|publisher=Springer Science & Business Media|year=|isbn=9789401512589|___location=|pages=|language=en|doi=10.1007/978-94-015-1258-9}}</ref> One major motivation for studying continuous-variable quantum computation is that many scientific problems have mathematical formulations that are naturally continuous in character. Another motivation is to understand in what ways quantum computers are more capable or more powerful than classical computers.<ref>{{Cite journal|last=Adesso|first=Gerardo|last2=Ragy|first2=Sammy|last3=Lee|first3=Antony R.|date=2014-03-12|title=Continuous Variable Quantum Information: Gaussian States and Beyond|url=http://www.worldscientific.com/doi/abs/10.1142/S1230161214400010|journal=Open Systems & Information Dynamics|volume=21|issue=01n02|pages=1440001|arxiv=1401.4679|doi=10.1142/S1230161214400010|issn=1230-1612}}</ref>
== Applications ==
One example of a scientific problem that is naturally expressed in continuous terms is [[path integration]]. The general technique of path integration has numerous applications including [[quantum mechanics]], [[quantum chemistry]], [[statistical mechanics]], and [[computational finance]]. Because randomness is present throughout quantum theory, one typically requires that a quantum computational procedure yield the correct answer, not with certainty, but with high probability. For example, one might aim for a procedure that computes the correct answer with probability at least 3/4. In the case of continuous-variable quantum computation, one also specifies a degree of uncertainty, typically by setting the maximum acceptable error. Thus, the goal of a continuous-variable quantum computation could be to compute the numerical result of a path-integration problem to within an error of at most ε with probability 3/4 or more. In this context, it is known<ref>{{Cite journal|last=Traub|first=J. F.|last2=Woźniakowski|first2=H.|date=2002-10-01|title=Path Integration on a Quantum Computer|url=https://link.springer.com/article/10.1023/A:1023417813916|journal=Quantum Information Processing|language=en|volume=1|issue=5|pages=365–388|arxiv=quant-ph/0109113|doi=10.1023/A:1023417813916|issn=1570-0755}}</ref> that quantum algorithms can outperform their classical counterparts, and the computational complexity of the problem, as measured by the number of times one would expect to have to query a quantum computer to get a good answer, grows as the inverse of ε.
In the standard model of quantum computation the probabilistic nature of quantum computation enters only through measurement; the queries are deterministic. In analogy with classical Monte Carlo, Woźniakowski introduced the idea of a quantum setting with [http://arXiv.org/quant-ph/0601196 randomized queries]. He showed that in this setting the qubit complexity is of order <math>\scriptstyle \log\varepsilon^{-1}</math>, thus achieving an exponential improvement over the qubit complexity in the standard quantum computing setting.
Besides path integration there have been numerous recent papers studying algorithms and quantum speedups for continuous problems. These include finding matrix [[Eigenvalues and eigenvectors|eigenvalues]], phase estimation, the Sturm–Liouville eigenvalue problem, solving [[Differential equation|differential equations]] with the [[Feynman–Kac formula]], initial value problems, function approximation and high-dimensional integration.
==External links==
*http://quantum.cs.columbia.edu – Continuous quantum computing web page at [[Columbia University]]
==References==
*Bessen, A. J. (2005), A lower bound for phase estimation, Physical Review A, 71(4), 042313. Also [https://arXiv.org/abs/quant-ph/0412008 arXiv:quant-ph/0412008].
*Heinrich, S. (2002), Quantum Summation with an Application to Integration, J. Complexity, 18(1), 1–50. Also [https://arXiv.org/abs/quant-ph/0105116 arXiv:quant-ph/0105116].
*Heinrich, S. (2003), Quantum integration in Sobolev spaces, J. Complexity, 19, 19–42.
*Heinrich, S. (2004), Quantum Approximation I. Embeddings of Finite Dimensional <math>L_p</math> Spaces, J. Complexity, 20, 5–26. Also [https://arXiv.org/abs/quant-ph/0305030 arXiv:quant-ph/0305030].
*Heinrich, S. (2004), Quantum Approximation II. Sobolev Embeddings, J. Complexity, 20, 27–45. Also [https://arXiv.org/abs/quant-ph/0305031 arXiv:quant-ph/0305031].
*Jaksch, P. and Papageorgiou, A. (2003), Eigenvector approximation leading to exponential speedup of quantum eigenvalue calculation, Phys. Rev. Lett., 91, 257902. Also [https://arXiv.org/abs/quant-ph/0308016 arXiv:quant-ph/0308016].
*Kacewicz, B. Z. (2005), Randomized and quantum solution of initial value problems, J. Complexity, 21, 740–756.
*Kwas, M., Complexity of multivariate Feynman–Kac Path Integration in Randomized and Quantum settings, 2004. Also [https://arXiv.org/abs/quant-ph/0410134 arXiv:quant-ph/0410134].
*Novak, E. (2001), Quantum complexity of integration, J. Complexity, 17, 2–16. Also [https://arXiv.org/abs/quant-ph/0008124 arXiv:quant-ph/0008124].
*Novak, E., Sloan, I. H., and Wozniakowski, H., Tractability of Approximation for Weighted Korobov Spaces on Classical and Quantum Computers, J. Foundations of Computational Mathematics, 4, 121-156, 2004. Also [https://arXiv.org/abs/quant-ph/0206023 arXiv:quant-ph/0206023]
*Papageorgiou, A. and Wo´zniakowski, H. (2005), Classical and Quantum Complexity of the Sturm–Liouville Eigenvalue Problem, Quantum Information Processing, 4(2), 87–127. Also [https://arXiv.org/abs/quant-ph/0502054 arXiv:quant-ph/0502054].
*Papageorgiou, A. and Wo´zniakowski, H. (2007), The Sturm–Liouville Eigenvalue Problem and NP-Complete Problems in the Quantum Setting with Queries, Quantum Information Processing, 6(2), 101–120. Also [https://arXiv.org/abs/quant-ph/0504194 arXiv:quant-ph/0504194].
*Woźniakowski, H. (2006), The Quantum Setting with Randomized Queries for Continuous Problems, Quantum Information Processing, 5(2), 83–130. Also [https://arXiv.org/abs/quant-ph/0601196 arXiv:quant-ph/0601196].
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[[Category:Quantum information science]]
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