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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

Stabilization of Phase and Polarization in Multiplexed Entangled States Through Multi-Channel Optical Fibre Arrays

Adrián S. Blanca, Giacomo Paganini, Evelyn A. Ortega, Robin Camphausen, Álvaro Cuevas, Valerio Pruneri

Published Jun 23, 2025• 0 citations
Publisher

Abstract

The second quantum revolution has driven rapid advancements in quantum technologies, particularly in quantum communication and computing, where the manipulation and distribution of quantum entanglement are crucial. Prior studies have explored entanglement distribution via multi-core fibres (MCF), which allow space-division multiplexing (SDM) and spatial information encoding. Particularly, in [1], polarization-entangled photon pairs from a spontaneous parametric down-conversion (SPDC)-based entangled photon source (EPS) were transmitted through opposite cores of a MCF. However, the ring-shaped intensity of the source output led to non-optimal core overlap, achieving coupling efficiencies below 1%. In this work, we focus on enhancing the coupling efficiency of 810 nm polarization-entangled photons into the six internal cores of a 19-core MCF designed for this wavelength. We achieve this by shaping the SPDC ring into Gaussian modes with a phase-only liquid crystal on Silicon (LCoS) spatial light modulator (SLM). The phase mask in Fig. 1(a) is used in the configuration shown in Fig. 1(b).

DOI
10.1109/cleo/europe-eqec65582.2025.11110705

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The second quantum revolution has driven rapid advancements in quantum technologies, particularly in quantum communication and computing, where the manipulation and distribution of quantum entanglement are crucial. Prior studies have explored entanglement distribution via multi-core fibres (MCF), which allow space-division multiplexing (SDM) and spatial information encoding. Particularly, in [1], polarization-entangled photon pairs from a spontaneous parametric down-conversion (SPDC)-based entangled photon source (EPS) were transmitted through opposite cores of a MCF. However, the ring-shaped intensity of the source output led to non-optimal core overlap, achieving coupling efficiencies below 1%. In this work, we focus on enhancing the coupling efficiency of 810 nm polarization-entangled photons into the six internal cores of a 19-core MCF designed for this wavelength. We achieve this by shaping the SPDC ring into Gaussian modes with a phase-only liquid crystal on Silicon (LCoS) spatial light modulator (SLM). The phase mask in Fig. 1(a) is used in the configuration shown in Fig. 1(b).
Rapid recent advances in quantum technology are giving rise to potential new applications in a wide range of fields. In particular, efficient generation and distribution of quantum entanglement is crucial for many use cases in quantum communication. In this field, multi-core optical fibers (MCF) are especially interesting, as they enable space-division multiplexing, increasing data transmission capacity of the channel. In this work, we present a flexible system for efficient and scalable quantum communication through MCF, using phase-only spatial light modulation (SLM). Firstly, we utilise SLM to enhance the coupling efficiency of near-infrared polarization-entangled photon pairs into a 19-core MCF. Secondly, we present a novel SLM-based system which corrects random polarization transformations incurred over propagation through fiber, for all MCF cores in parallel. The quality of the distributed entanglement is certified by high polarization visibility and violation of the Clauser–Horne–Shimony–Holt (CHSH) inequality for every core pair. Our results show potential for implementation in real-world quantum communication networks, as well as enabling further advanced quantum experiments.
Quantum Key Distribution (QKD) is a crucial step in quantum cryptography protocols, as a secure distribution of the secret key is determinant in the performance of the system. Some QKD protocols like E91 require the distribution of entangled photon pairs between the emitter and the receiver in order to guarantee a safe transfer of information. However, the distribution of entangled quantum states is a remarkably challenging task, due to the fragility of this physical phenomenon. In this project, a new method for the distribution of high-dimensional entangled photon pairs through an optical fiber is presented. This method is based on the spatial modulation of the output beam of an Entangled Photon Source (EPS) in order to increase the coupling efficiency into a multi-core optical fiber (MCF), which otherwise is highly inefficient. In order to perform the spatial modulation, a LCoS phase-only Spatial Light Modulator (SLM) has been used. The presented method is designed for the modulation of a two-dimensional entangled photon beam, both entangled spatially and in polarization. So, the aim is not only to improve the overall single photon efficiency, but also the maximize as much as possible the coupling of entangled photon pairs, always maintaining the spatial correlations and the polarization entanglement. The system has been tested for a 810nmnmnm (NIR) central wavelength EPS, coupled into a 1m long MCF. The obtained results for the single photon efficiency are not as good as initially expected, due to limitations found in the setup along the course of the experiment. Nevertheless, the spatial correlation maintenance and the efficiency in the coupling of entangled photon pairs compared to the obtained single photon efficiency have resulted to be remarkable. The overall conclusion of this project is that the presented method has the potential to achieve much higher coupling efficiencies with some changes in the setup, always maintaining the entanglement. The versatility of the computed phase masks for the spatial modulation allows to adapt the shape of the beam to the used fiber, and also opens a door for other possible future applications. This applications can range from the implementation of the Hong-Ou-Mandel effect in optical fibers to the modulation of high-dimensional entangled photon pairs in other degrees of freedom, like frequency, OAM or time.
The development and wide application of quantum technologies highly depend on the capacity of the communication channels distributing entanglement. Space-division multiplexing (SDM) enhanced channel capacities in classical telecommunication and bears the potential to transfer the idea to quantum communication using current infrastructure. Here, we demonstrate an SDM of polarization-entangled photons over a 411m long 19-core multicore fiber distributing polarization-entangled photon pairs through up to 12 channels simultaneously. The quality of the multiplexed transfer is evidenced by high polarization visibility and CHSH Bell inequality violation for each pair of opposite cores. Our distribution scheme shows high stability over 24 hours without any active polarization stabilization and can be effortlessly adapted to a higher number of channels. This technique increases the quantum-channel capacity and allows the reliable implementation of quantum networks of multiple users based on a single entangled-photon pair source.
The development and wide application of quantum technologies highly depend on the capacity of the communication channels distributing entanglement.Space-division multiplexing (SDM) enhances data channel transmission capacity in classical telecommunication and bears the potential to transfer the idea to quantum communication using current infrastructure.Here, we demonstrate a SDM of polarizationentangled photons over a 411 m long 19-core multicore fiber distributing polarization-entangled photon pairs through up to 12 channels simultaneously.The quality of the multiplexed transfer is evidenced by high polarization visibility and Clauser-Horne-Shimony-Holt (CHSH) Bell inequality violation for each pair of opposite cores.Our distribution scheme shows high stability over 24 h without any active polarization stabilization and can be effortlessly adapted to a higher number of channels.This technique increases the quantum-channel capacity and allows the reliable implementation of quantum networks of multiple users based on a single entangled photon pair source.
Spatial-division multiplexing is a key technology for increasing the capacity of 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 verify the stability of both the relative phase and polarization between the cores over10 km\mathbf{1 0 ~ k m}10 kmof a four-core fiber. We also characterize the polarization mode dispersion of the fiber and find its principal states of polarization to mitigate the effects of random polarization-dependent delay across the fiber cores.
Abstract Entanglement distribution is at the heart of most quantum communication protocols. Inevitable loss of photons along quantum channels is a major obstacle for distributing entangled photons over long distances, as the no-cloning theorem forbids the information to simply be amplified along the way as is done in classical communication. It is therefore desirable for every successfully transmitted photon pair to carry as much entanglement as possible. Spontaneous parametric down-conversion (SPDC) creates photons entangled in multiple high-dimensional degrees of freedom simultaneously, often referred to as hyper-entanglement. In this work, we use a multicore fiber (MCF) to show that energy-time and polarization degrees of freedom can simultaneously be transmitted in multiple fiber cores, even maintaining path entanglement across the cores. We verify a fidelity to the ideal Bell state of at least 95% in all degrees of freedom. Furthermore, because the entangled photons are created with a center wavelength of 1560 nm, our approach can readily be integrated into modern telecommunication infrastructure, thus paving the way for high-rate quantum key distribution and many other entanglement-based quantum communication protocols.