Few-mode-fiber technology fine-tunes losses of quantum communication systems

A natural choice for quantum communication is to use the relative phase between two paths of a single-photon for information encoding. This method was nevertheless quickly identified as impractical over long distances and thus a modification based on single-photon time-bins has then become widely adopted. It however, introduces a fundamental loss, 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 fiber 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 fiber 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 fiber, showing the feasibility of our scheme.