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Energy and time entangled photons at a wavelength of 1310 nm are produced by parametric down-conversion in a 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.
Tittel et al. (Fri,) studied this question.