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Science 4 May 2007:
Vol. 316. no. 5825, pp. 726 - 729
DOI: 10.1126/science.1138007
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Reports

Beating the Standard Quantum Limit with Four-Entangled Photons

Tomohisa Nagata,1 Ryo Okamoto,1,2 Jeremy L. O'Brien,3,4 Keiji Sasaki,1 Shigeki Takeuchi1,2*

Precision measurements are important across all fields of science. In particular, optical phase measurements can be used to measure distance, position, displacement, acceleration, and optical path length. Quantum entanglement enables higher precision than would otherwise be possible. We demonstrated an optical phase measurement with an entangled four-photon interference visibility greater than the threshold to beat the standard quantum limit—the limit attainable without entanglement. These results open the way for new high-precision measurement applications.

1 Research Institute for Electronic Science, Hokkaido University, Sapporo 060-0812, Japan.
2 Japan Science and Technology Agency, Honcho, Kawaguchi-shi, Saitama, 332-0012, Japan.
3 Department of Electrical and Electronic Engineering, University of Bristol, Merchant Venturers Building, Woodland Road, Bristol BS8 1UB, UK.
4 H. H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol BS8 1TL, UK.

* To whom correspondence should be addressed. E-mail: takeuchi{at}es.hokudai.ac.jp

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THIS ARTICLE HAS BEEN CITED BY OTHER ARTICLES:
Magnetic Field Sensing Beyond the Standard Quantum Limit Using 10-Spin NOON States.
J. A. Jones, S. D. Karlen, J. Fitzsimons, A. Ardavan, S. C. Benjamin, G. A. D. Briggs, and J. J. L. Morton (2009)
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An Entanglement Filter.
R. Okamoto, J. L. O'Brien, H. F. Hofmann, T. Nagata, K. Sasaki, and S. Takeuchi (2009)
Science 323, 483-485
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Silica-on-Silicon Waveguide Quantum Circuits.
A. Politi, M. J. Cryan, J. G. Rarity, S. Yu, and J. L. O'Brien (2008)
Science 320, 646-649
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