G. Carvacho, A. Cuevas, G. Saavedra, Jaime Cariñe, W.A.T. Nogueira , M. Figueroa, A. Cabello, P. Matalloni, G. Lima and G. B. Xavier
NATURE COMMUNICATIONS 4 (2013)
Any practical realization of entanglement-based quantum communication must be
intrinsically secure and able to span long distances avoiding the need of a straight line between the communicating parties. The violation of Bell’s inequality offers a method for the certification of quantum links without knowing the inner workings of the devices. Energy-time entanglement quantum communication satisfies all these requirements. However, currently there is a fundamental obstacle with the standard configuration adopted: an intrinsic geometrical loophole that can be exploited to break the security of the communication, in addition to other loopholes. Here we show the first experimental Bell violation with energytime entanglement distributed over 1 km of optical fibres that is free of this geometrical loophole. This is achieved by adopting a new experimental design, and by using an actively stabilized fibre-based long interferometer. Our results represent an important step towards long-distance secure quantum communication in optical fibres.