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Physical Review Letters

Violation of Bell's Inequality over 4 km of Optical Fiber

P. R. Tapster, John G. Rarity, P. C. M. Owens

Published Oct 3, 1994• 237 citations
Publisher

Abstract

Using energy correlated photon pairs created in parametric down-conversion we demonstrate two-photon interference effects in fiber interferometers separated by 4.3 km of optical fiber. The measured 86% interference visibilities confirm the purely quantum mechanical origin of these correlations.
DOI
10.1103/physrevlett.73.1923

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An experimental demonstration of quantum-correlations is presented. Energy and time entangled photons at wavelengths of 704 and 1310 nm are produced by parametric downconversion in KNbO 3 and are sent through optical fibers into a bulk-optical (704 nm) and an all-fiber Michelson-interferometer (1310 nm), respectively. The two interferometers are located 35 meters aside from one another. Using Faraday mirrors in the fiber interferometer, all birefringence effects in the fibers are automatically compensated. We obtained two-photon fringe visibilities of up to 95% from which one can project a violation of Bell's inequality by 8 standard deviations. The good performance and the auto-aligning feature of Faraday-mirror interferometers show their potential for a future test of Bell's inequalities in order to examine quantum correlations over long distances.
Energy and time entangled photons at a wavelength of 1310 nm are produced by parametric down-conversion in a KNbO3{\mathrm{KNbO}}_{3}KNbO3​ crystal and are sent into all-fiber interferometers using a telecommunications fiber network. The two interferometers of this Franson-type test of the Bell inequality are located 10.9 km apart from one another. Two-photon fringe visibilities of up to 81.6% are obtained. These strong nonlocal correlations support the nonlocal predictions of quantum mechanics and provide evidence that entanglement between photons can be maintained over long distances.
Summary form only given.In this presentation experimental results of a two-photon interference experiment will be presented and discussed. The two photons, produced by parametric downconversion in a KNbO/sub 3/ crystal, were analyzed with two interferometers physically separated by more than 10 km with the source roughly at the center. The results clearly indicate that the famous spooky action at a distance extends over very large distances. Indeed, from the measured two-photon fringe visibility of 81.6% we infer a violation of the Bell inequality by 10 standard deviations.
We report the measurement of nonclassical photon statistics at the outputs of 2\ifmmode\times\else\texttimes\fi{}2 and 3\ifmmode\times\else\texttimes\fi{}3 fiber couplers resulting from quantum-mechanical second-order interference of entangled photon pairs from a parametric down-conversion source. This is a demonstration of the bunching of bosons at the outputs of fiber multiports. Single-mode fibers provide precise wavelength selection from the down-conversion source spectrum, thus yielding very long coherence times and good collection efficiency without the need of interference filters. Two-photon interference visibilities close to the theoretical maximum were achieved in the experiments. \textcopyright{} 1996 The American Physical Society.