Experimental demonstration of quantum correlations over more than 10 km

Energy and time entangled photons at a wavelength of 1310 nm are produced by parametric down-conversion in a ${\mathrm{KNbO}}_3$ 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.