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High-dimensional spatial entanglement distribution using custom-made optical fibres

Fernandez Cano, Javier

Published Sep 5, 2024• 0 citations
OpenAlex

Abstract

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.

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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.
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 entangled photons pairs are interesting for quantum information and cryptography: Compared to the well-known 2D polarization case, the stronger non-local quantum correlations could improve noise resistance or security, and the larger amount of information per photon increases the available bandwidth. One implementation is to use entanglement in the spatial degree of freedom of twin photons created by spontaneous parametric down-conversion, which is equivalent to orbital angular momentum entanglement, this has been proven to be an excellent model system. The use of optical fiber technology for distribution of such photons has only very recently been practically demonstrated and is of fundamental and applied interest. It poses a big challenge compared to the established time and frequency domain methods: For spatially entangled photons, fiber transport requires the use of multimode fibers, and mode coupling and intermodal dispersion therein must be minimized not to destroy the spatial quantum correlations. We demonstrate that these shortcomings of conventional multimode fibers can be overcome by using a hollow-core photonic crystal fiber, which follows the paradigm to mimic free-space transport as good as possible, and are able to confirm entanglement of the fiber-transported photons. Fiber transport of spatially entangled photons is largely unexplored yet, therefore we discuss the main complications, the interplay of intermodal dispersion and mode mixing, the influence of external stress and core deformations, and consider the pros and cons of various fiber types.
The global quantum network requires the distribution of entangled states over long distances, with significant advances already demonstrated using polarization, reaching approximately 1200 km in free space and 100 km in optical fiber. 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 fiber and is limited by decoherence induced mode coupling. Here we circumvent this by transporting multi-dimensional spatial entangled states down conventional single-mode fiber (SMF). We achieve this by entangling the spin-orbit degrees of freedom of a bi-photon pair, passing the polarization (spin) photon down the SMF while accessing multiple orbital angular momentum (orbital) sub-spaces with the other, thereby realizing multi-dimensional spatial entanglement transport. We show high fidelity hybrid entanglement preservation down 250 m of SMF across multiple 2 x 2 dimensions, which we confirm by quantum state tomography and Bell violation measures. This work offers an alternative approach to spatial mode entanglement transport that facilitates deployment in legacy networks across conventional fiber optic links.
The distribution of entanglement in quantum networks will enable the next generation of technologies in quantum-secured communications, distributed quantum computing and sensing. Future quantum networks will require dense connectivity, allowing multiple users to share entanglement in a reconfigurable and multiplexed manner, while long-distance connections are established through the teleportation of entanglement, or entanglement swapping. Although several recent works have demonstrated fully connected, local multi-user networks based on multiplexing, extending such networks to a global network architecture of interconnected local networks remains an outstanding challenge. Here we demonstrate the next step in the evolution of multiplexed quantum networks-a prototype global reconfigurable network in which entanglement is routed and teleported in a flexible and multiplexed manner between two local four-user networks. At the heart of our network is a programmable 8 × 8-dimensional multi-port circuit that harnesses the natural mode-mixing process inside of a multi-mode fibre to implement on-demand high-dimensional operations on two independent photons carrying eight transverse-spatial modes. Our circuit design allows us to break away from the limited planar geometry and bypass the control and fabrication challenges of conventional integrated photonic platforms. Our demonstration highlights the potential of this architecture for enabling large-scale, global quantum networks that offer versatile connectivity while being fully compatible with an existing communications infrastructure.