Paolo Mataloni

Practical and secure quantum communications must not depend on any physical assumptions on the employed devices and be able to cover long distances, preferably without a line-of-sight requirement. The violation of a Bell’s inequality offers a method to certify the security of the link. Energy-time entanglement-based quantum communications can fulfill all the practical requirements. Unfortunately the main configuration employed in many demonstrations suffers from a post-selection loophole, which can be used to break the security of the communication. Here we experimentally demonstrate for the first time a Bell violation with energy-time entangled photon pairs distributed over 1 km optical fibers, in a new “hug” interferometric configuration, which is free of the post-selection loophole. For the two-photon interference curves we obtain average raw and net visibilities of 84.4 and 95.1% respectively, with a corresponding violation of the Bell CHSH inequality of S = 2.39 for the raw results, surpassing the classical limit by 3.25 standard deviations. Our results indicate that quantum key distribution using energy-time entangled pairs free of the post-selection loophole is feasible over long-distances.