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Scientific Reports

Experimental Demonstration of Four-Dimensional Photonic Spatial Entanglement between Multi-core Optical Fibres

Hee Jung Lee, Sang-Kyung Choi, Hee Su Park

Published Jun 21, 2017• 31 citations
Publisher

Abstract

Fibre transport of multi-dimensional photonic quantum states promises high information capacity per photon without space restriction. This work experimentally demonstrates transmission of spatial ququarts through multi-core optical fibres and measurement of the entanglement between two fibres with quantum state analyzers, each composed of a spatial light modulator and a single-mode fibre. Quantum state tomography reconstructs the four-dimension entangled state that verifies the nonlocality through concurrences in two-dimensional subspaces, a lower bound of four-dimensional concurrence and a Bell-type CGLMP inequality.

DOI
10.1038/s41598-017-04444-8

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Fibre transport of multi-dimensional photonic quantum states promises high information capacity per photon without space restriction. This work experimentally demonstrates transmission of spatial ququarts through multi-core optical fibres and measurement of the entanglement between two fibres with quantum state analyzers, each composed of a spatial light modulator and a single-mode fibre. Quantum state tomography reconstructs the four-dimension entangled state that verifies the nonlocality through concurrences in two-dimensional subspaces, a lower bound of four-dimensional concurrence and a Bell-type CGLMP inequality.
Fiber transport of multi-dimensional photonic qudits promises high information capacity per photon without space restriction. This work experimentally demonstrates transmission of spatial qudits through multi-core optical fibers and measurement of the entanglement between two fibers with quantum state analyzers, each composed of a spatial light modulator and a single-mode fiber. Quantum state tomography reconstructs the four-dimension entangled state that verifies the non-locality through concurrences in two-dimensional subspaces and a high-dimensional Bell-type CGLMP inequality.
High-dimensional entanglement is a valuable resource for secure and efficient quantum information processing. A major challenge for practical use of multidimensional quantum systems is the establishment of controls over arbitrary superposition states in realistic conditions. This work demonstrates spatially entangled photon pairs propagating through two separate four-core optical fibers with the amplitudes and phases of the superposition being independently controllable. Using quantum state analyzers that can detect arbitrary multicore superposition states, Bell-type CGLMP inequalities in two, three, and four dimensions are directly tested. Enhanced violation of the inequality by slight nonmaximality of entanglement is also demonstrated.
As quantum computers emerge and the demand for higher data transmission rates grows, spatial multiplexing of high-dimensional quantum states is essential for the development of quantum networks and communication technologies in our globalized society. This study presents advancements in the quantum transfer of high-dimensional spatial photon states and encompasses five months of research focused on constructing, from the ground up, an experimental setup for testing custom-made fibres of interest for potential use in high-dimensional long-distance entanglement distribution. Three setups were built in total, with improvements made to different components before assembling them into an entangled photon pair source. The experimental results demonstrate successful high-dimensional quantum transport in free-space and successful classical transport through a custom-made tapered fibre. Simulations in Python and COMSOL for structured modes, their superpositions, and for finding the eigenmodes of fibres with different core radii and cladding indices provided critical insights into the necessary physical corrections required in the experimental setup. Additionally, an interface was developed for manipulating the spatial light modulator and power readings, with built-in processes of interest. Future work will explore different fibre core sizes, indices, and topologies to achieve quantum state transport, with upcoming experiments comparing fibre and free-space spatial entanglement distributions.
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.
Spatial division multiplexing is a key technology for increasing the capacity of the classical optical fiber networks. It also provides additional degrees of freedom for photon entanglement and transmission in quantum communication networks. This paper describes steps toward generating a four-dimensional, spatially-entangled state and its distribution over multi-km-long four-core fibers. We generate four mutually coherent beams using a spatial light modulator and investigate their simultaneous coupling into a four-core fiber. We also verify the stability of both the relative phase and polarization between the cores over a 10 km of a four-core fiber.