Gustavo Lima

Time-bin quantum cryptography systems have a fundamental post-selection loss at the detection stage, which increases with the dimension and that limits its application over long distances. Here, we are able to solve this long-standing hurdle by employing a few-mode fibre space-division multiplexing platform working with orbital angular momentum modes. In our scheme, we maintain the practicability provided by the time-bin scheme, while the quantum states are transmitted through a few-mode fibre in a configuration that does not introduce post-selection losses. We experimentally demonstrate our proposal by successfully transmitting phase-encoded single-photon states for quantum cryptography over 500 m of few-mode fibre, thus opening up new paths for quantum communication systems.

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.