Multi-dimensional entanglement transport through single-mode fibre

TL;DR

This paper demonstrates the transport of multi-dimensional entangled states through standard single-mode fibre by entangling spin and orbital angular momentum degrees of freedom, enabling high-dimensional quantum communication over existing fibre infrastructure.

Contribution

It introduces a method to transmit multi-dimensional entanglement through conventional single-mode fibre by hybrid encoding of spin and orbital degrees of freedom, avoiding the need for custom multimode fibre.

Findings

High fidelity entanglement preserved over 250 m of SMF

Successful implementation of quantum key distribution protocols

Quantum state tomography and eraser experiments confirm entanglement integrity

Abstract

The global quantum network requires the distribution of entangled states over long distances, with significant advances already demonstrated using entangled polarisation states, reaching approximately 1200 km in free space and 100 km in optical fibre. Packing more information into each photon requires Hilbert spaces with higher dimensionality, for example, that of spatial modes of light. However spatial mode entanglement transport requires custom multimode fibre and is limited by decoherence induced mode coupling. Here we transport multi-dimensional entangled states down conventional single-mode fibre (SMF). We achieve this by entangling the spin-orbit degrees of freedom of a bi-photon pair, passing the polarisation (spin) photon down the SMF while accessing multi-dimensional orbital angular momentum (orbital) subspaces with the other. We show high fidelity hybrid entanglement preservation down 250 m of SMF across multiple 2x2 dimensions, demonstrating quantum key distribution protocols, quantum state tomographies and quantum erasers. This work offers an alternative approach to spatial mode entanglement transport that facilitates deployment in legacy networks across conventional fibre.