Quantum key establishment via a multimode fiber

TL;DR

This paper introduces a novel quantum key distribution method using multimode fibers that encodes high-dimensional information through spatial modes, enabling secure, long-distance optical communication without the need for basis switching.

Contribution

The authors propose and experimentally demonstrate a new quantum key establishment technique leveraging multimode fibers and spatial mode encoding, enhancing security and distance capabilities.

Findings

Successfully demonstrated secure key exchange over multimode fiber.

Detected eavesdropper attacks without basis switching.

Achieved high-dimensional encoding with few-photon pulses.

Abstract

Quantum communication aims to provide absolutely secure transmission of secret information. State-of-the-art methods encode symbols into single photons or coherent light with much less than one photon on average. For long distance communication, typically a single-mode fiber is used and significant effort has been devoted already to increase the data carrying capacity of a single optical line. Here we propose and demonstrate a fundamentally new concept for remote key establishment. Our method allows high-dimensional alphabets using spatial degrees of freedom by transmitting information through a light-scrambling multimode fiber and exploiting the no-cloning theorem. Eavesdropper attacks can be detected without using randomly switched mutually unbiased bases. We prove the security against a common class of intercept-resend and beam-splitting attacks with single-photon Fock states and with weak coherent light. Since it is optical fiber based, our method allows to naturally extend secure communication to larger distances. We experimentally demonstrate this new type of key exchange method by encoding information into a few-photon light pulse decomposed over guided modes of an easily available multimode fiber.