Researchers demonstrated that multiple quantum patterns of twisted light can be transmitted across a conventional fibre link that, paradoxically, supports only one pattern. The implication is a new approach to realizing a future quantum network, harnessing multiple dimensions of entangled quantum light.
Above – Two photons are entangled, one in polarization and the other in orbital angular momentum — twisted light. By passing the polarization photon through the fiber and keeping the twisted light in air, multi-dimensional entanglement transport is possible even over single mode fiber. CREDIT Wits University
Multi-dimensional twisted patterns can carry orbital angular momentum (OAM) which could be used for very secure encrypted messages.
This work demonstrated multi-dimensional entanglement transport in conventional single-mode fibre. The light is twisted in two degrees of freedom: the polarisation is twisted to form spirally light, and so is the pattern. This is referred to as spin-orbit coupling, here exploited for quantum communication. Each transmission is still only a qubit (2D) but there are an infinite number of them because of the infinite number of twisted patterns we could entangle in the other photon.
The team demonstrated transfer of multi-dimensional entanglement states over 250 m of single-mode fibre, showing that an infinite number of two-dimensional subspaces could be realized. Each subspace could be used for sending information, or multiplexing information to multiple receivers.
The global quantum network requires the distribution of entangled states over long distances, with significant advances already demonstrated using polarisation, reaching approximately 1200 km in free space and 100 km in optical fibre. While Hilbert spaces with higher dimensionality, e.g., spatial modes of light, allows higher information capacity per photon, such spatial mode entanglement transport requires custom multimode fibre and is limited by decoherence induced mode coupling. Here we circumvent this by transporting 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 multiple orbital angular momentum (orbital) subspaces with the other, thereby realising multi-dimensional entanglement transport. We show high fidelity hybrid entanglement preservation down 250 m of SMF across multiple 2 × 2 dimensions, which we confirm by quantum state tomography, Bell violation measures and a novel quantum eraser scheme. This work offers an alternative approach to spatial mode entanglement transport that facilitates deployment in legacy networks across conventional fibre.