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Violation of Bell inequalities by photons more than 10 km apart

W. Tittel·J. Brendel·H. Zbinden·N. Gisin
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Abstract

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 2-photon interference visibility of 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.

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W. Tittel, J. Brendel, H. Zbinden, N. Gisin. 1998-06-12. Violation of Bell inequalities by photons more than 10 km apart. https://doi.org/10.1103/physrevlett.81.3563

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