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Conference on Lasers and Electro-Optics

Quantum correlation between photons separated by more than 10 km

Nicolas Gisin, Wolfgang Tittel, Jurgen Brendel, Hugo Zbinden

Published Nov 27, 2002• 0 citations
Publisher

Abstract

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.

DOI
10.1109/iqec.1998.680184

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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.
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.
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.
A Franson-type test of Bell inequalities by photons 10.9 km apart is presented. Energy-time entangled photon pairs are measured using two-channel analyzers, leading to a violation of the inequalities by 16 standard deviations without subtracting accidental coincidences. Subtracting them, a two-photon interference visibility of 95.595.5%95.5 is observed, demonstrating that distances up to 10 km have no significant effect on entanglement. This sets quantum cryptography with photon pairs as a practical competitor to the schemes based on weak pulses.