Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

arXiv:2505.03341

Electrically Reconfigurable NbOCl$_2$ Metasurface for Quantum Technologies

Omar A. M. Abdelraouf

Published May 6, 2025• 0 citations
arXivHTMLPDF

Abstract

Entangled photon-pair sources are foundational to advancing quantum technologies, including secure communication, quantum sensing, and imaging. For deployment in space-constrained environments such as satellite-based quantum networks or portable devices, compact, reconfigurable, and efficient entanglement sources are essential. Here, we present an electrically tunable entangled photon-pair source utilizing a nanostructured NbOCl$_2$ crystal, engineered for operation in the telecommunication C-band. The inherent non-centrosymmetric lattice symmetry of NbOCl$_2$ enables direct generation of polarization-entangled Bell states without the need for post-selection, leveraging its exceptional second-order nonlinear susceptibility, which surpasses conventional nonlinear materials. By nanopatterning NbOCl$_2$ into a high-quality-factor metasurface, we achieve three orders of magnitude enhancement in photon-pair generation efficiency via resonant excitation of bound states in the continuum resonance, which intensify light-matter interactions. Furthermore, we demonstrate in situ electrical tunability of the photon-pair emission wavelength over a 250 nm range from 1450 nm to 1700 nm by dynamically modulating surrounding liquid crystal layer. Remarkably, the decoupling of photon-pair generation rate and spectral tunability ensures high brightness, above 10,000 coincidences, under active tuning. The air stability and mechanical robustness of NbOCl$_2$ further enhance its practicality for real-world deployment. This work establishes NbOCl$_2$ as a superior material for scalable, on-chip quantum light sources, paving the way for integrated quantum communication systems, adaptive sensors, and portable quantum devices.

DOI
10.48550/arxiv.2505.03341

Related papers

Polarization-entangled photon pair generation from an epsilon-near-zero metasurface

Wenhe Jia, Grégoire Saerens, Ülle‐Linda Talts +5 more

Polarization-entangled photon pair sources are essential for diverse quantum technologies, such as quantum communication, computation, and imaging. However, the generation of complex polarization-entangled quantum states has long been constrained by the available nonlinear susceptibility tensor of natural nonlinear crystals, necessitating a cumbersome and intricate setup for additional coherent superposition or post-selection. In this study, we introduce and experimentally demonstrate a nanoscale polarization-entangled photon pair source utilizing an artificially-engineered metamaterial platform. This platform is based on a plasmonic metasurface that is strongly coupled to an epsilon-near-zero (ENZ) material. By precisely engineering resonances at both pump and signal/idler wavelengths, and leveraging the field enhancement provided by the ENZ effect, the photon pair generation efficiency of the 68-nm-thick metasurface is significantly boosted. More notably, the ENZ metasurface platform facilitates versatile manipulation of the system's anisotropic second-order nonlinear susceptibility tensor, enabling direct control over the polarization states of the photon pairs, which leads to the generation of a polarization-entangled Bell state without the need for additional components. Our approach opens a new avenue for the simultaneous photon pair generation and quantum state engineering in a compact platform.

arXiv:2405.03493•May 6, 2024•5 citations

Entangled Photon Pair Generation from Resonant Bound States in the Continuum

Revista, Zen, PHYSICS, 10

The efficient and controllable generation of entangled photon pairs is a cornerstone for advancing quantum technologies, including quantum computing, communication, and sensing. Traditional methods, primarily relying on spontaneous parametric down-conversion (SPDC) or spontaneous four-wave mixing (SFWM) in bulk nonlinear crystals or waveguides, often suffer from low efficiency, bulky form factors, and limited control over the spectral and spatial properties of the generated photons. This paper explores a transformative approach leveraging resonant Bound States in the Continuum (BICs) within nanophotonic structures to overcome these limitations. BICs are extraordinary optical resonances characterized by theoretically infinite quality (Q) factors, enabling extreme light localization and immense field enhancement within subwavelength volumes. By integrating nonlinear optical materials into BIC-supporting structures, such as photonic crystal slabs or metasurfaces, the local electric field strength experienced by the nonlinear medium can be dramatically increased, boosting the efficiency of entangled photon generation. Furthermore, the sharp, tailor-able spectral response of BICs offers unprecedented control over the joint spectral amplitude of the photon pairs, promising high degrees of spectral purity and entanglement fidelity. This work outlines the theoretical underpinnings, potential device architectures, and anticipated advantages of BIC-enhanced entangled photon sources, paving the way for compact, high-brightness, and spectrally engineered quantum light sources critical for next-generation quantum technologies.

•Nov 28, 2025•0 citations

Tunable entangled photon-pair generation in a liquid crystal

Vitaliy Sultanov, Aljaž Kavčič, E. T. Kokkinakis +3 more

Abstract Liquid crystals, with their ability to self-assemble, strong response to an electric field and integrability into complex systems, are key materials in light-beam manipulation 1 . The recently discovered ferroelectric nematic liquid crystals 2,3 also have considerable second-order optical nonlinearity, making them a potential material for nonlinear optics 4,5 . Their use as sources of quantum light could considerably extend the boundaries of photonic quantum technologies 6 . However, spontaneous parametric down-conversion, the basic source of entangled photons 7 , heralded single photons 8 and squeezed light 9 , has so far not been observed in liquid crystals—or in any liquids or organic materials. Here we implement spontaneous parametric down-conversion in a ferroelectric nematic liquid crystal and demonstrate electric-field tunable broadband generation of entangled photons, with an efficiency comparable to that of the best nonlinear crystals. The emission rate and polarization state of photon pairs is markedly varied by applying a few volts or twisting the molecular orientation along the sample. A liquid-crystal source enables a special type of quasi-phase matching 10 , which is based on the molecular twist structure and is therefore reconfigurable for the desired spectral and polarization properties of photon pairs. Such sources promise to outperform standard nonlinear optical materials in terms of functionality, brightness and the tunability of the generated quantum state. The concepts developed here can be extended to complex topological structures, macroscopic devices and multi-pixel tunable quantum light sources.

arXiv:2401.07362•Nature•Jun 12, 2024•99 citations

A tunable entangled photon-pair source based on a Van der Waals insulator

Xiaodan Lyu, Leevi Kallioniemi, Hao Hong +5 more

Scalable quantum photonic devices drive the development of compact sources of entangled photons, which are pivotal for quantum communication, computing, and cryptography. In this work, we present entangled photon pair generation in rhombohedral boron nitride (r-BN), leveraging its unique optical and structural properties. Unlike conventional hexagonal boron nitride, which suffers from reduced nonlinear response due to centrosymmetric structure in even-layered stacks, r-BN features interlayer ABC stacking and maintains robust in-plane inversion symmetry. These characteristics lead to highly efficient entangled photon generation. Our system demonstrates an entangled photon pair generation rate up to 8667 Hz/(mW·mm) and offers a tunable platform for Bell state generation by simply adjusting the pump polarization, without compromising the entanglement quality or generation efficiency. The polarization entangled state is measured with a fidelity up to 94%. This advancement not only marks a significant step towards ultrathin, scalable quantum devices but also establishes r-BN as a promising candidate for on-chip integrated quantum optical applications. Compact sources of entangled photons are desired for quantum communication, computing, and cryptography. Here, the authors report high entangled photon pair generation rates in rhombohedral boron nitride, showing its potential as a tunable platform for Bell state generation.

Nature Communications•Feb 23, 2025•20 citations

Generation and Engineering of Polarization Entangled Photons from a Lithium Niobate Nonlinear Metasurface

Jinyong Ma, Jihua Zhang, Yuxin Jiang +4 more

Photon-based polarization entanglement in the form of Bell states and qutrits is an essential tool for understanding complex quantum phenomena and implementing various quantum technologies such as free-space quantum communication and quantum imaging. Polarization-entangled photons are conventionally generated in bulky nonlinear crystals through spontaneous parametric down-conversion (SPDC). In this approach, the photon polarization is intrinsically defined by the nonlinear tensor of the material, restricting the versatility of the generated quantum states. The recent developments of dielectric nanostructures with sub-wavelength thickness called metasurfaces [1] enable miniaturization of quantum light sources, while dramatically enhancing the photon-pair generation and allowing the preparation of complex quantum states with spectral [2] and spatial [3] entanglement. However, optically reconfigurable generation of biphoton polarization states with metasurfaces remained unexplored.

Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)•Jun 26, 2023•0 citations

Tunable photon-pair generation in thin layers and liquid crystals

Sultanov, Vitaliy

There is a growing belief that we are currently in the midst of a Second Quantum Revolution. More and more quantum technologies, such as communication, computing, and imaging, are moving from academic exploration to practical industrial applications. These technologies often depend on entangled photons, and their increasing use requires the development of novel quantum light sources that are compact, versatile, and integrable into modern photonic systems. Additionally, the trend toward miniaturization and nano-optics is accelerating, driven by rapid advances in nanofabrication. Therefore, there is an urgent need to create new classes of miniaturized and tunable two-photon sources. This thesis is dedicated to the theoretical investigation and experimental demonstration of miniaturized sources of entangled photons based on spontaneous parametric down-conversion in thin nonlinear films. It focuses not only on traditional crystalline materials but also on the emerging platform of liquid crystals, marking the first-ever demonstration of photon-pair generation in a liquid crystal within this thesis. A significant achievement is the ability to relax phase-matching constraints in ultra-thin nonlinear layers, allowing the full utilization of the nonlinear tensor and enhancing the variety of possible entangled two-photon states. Several aspects of two-photon state engineering with relaxed phase-matching are discussed in the thesis. First, this relaxation facilitates the switching between different polarization two-photon states, with a varying degree of polarization entanglement and the ability to generate non-maximally entangled states. In terms of spectral properties, miniaturized two-photon sources generate exceptionally broad two-photon frequency spectra, leading to a high degree of frequency entanglement and an unprecedented number of frequency modes, which also leads to ultra-narrow time correlations. By considering the effects of Fabry-Perot resonances, it becomes possible to engineer the two-photon spectra and even achieve backward emission of photon pairs. Remarkably, the Fabry-Perot effect can be accurately described by a simple modification of the rigorous quantum theory, providing a powerful predictive tool for photon pair generation in advanced nanostructures, metasurfaces, and nonlinear nano-antennas when properly applied. Another central contribution of this work is the introduction of liquid crystals as tunable nonlinear media for photon pair generation. Controlling the molecular orientation through a static twist or applying an external electric field makes it possible to tune the generated two-photon states dynamically. A theoretical model of this process, verified through direct experimental measurements, predicts quasi-phase-matching-like behavior and enhanced conversion efficiency, which can be utilized to develop exceptionally tunable and integrable sources of photon pairs and squeezed light. Finally, a fundamental challenge addressed in this thesis is mitigating a parasitic photoluminescent background. This study demonstrates a two-order-of-magnitude reduction in photoluminescence-induced contamination of photon pairs generated from thin nonlinear films by switching from the commonly used continuous-wave regime to the pulsed regime and performing time-resolved distillation. It also shows a developed theory that relates measured experimental parameters to the intrinsic purity of the two-photon state. Overall, this thesis advances the quantum micro- and nano-photonics field by providing a deeper understanding and enhanced control over photon pair generation in thin nonlinear films. It bridges theoretical insights with experimental realizations and paves the way toward practical, tunable, and highly integrated quantum light sources.

•Jan 1, 2025•0 citations
Electrically Reconfigurable NbOCl2_22​ Metasurface for Quantum Technologies
Entangled photon-pair sources are foundational to advancing quantum technologies, including secure communication, quantum sensing, and imaging. For deployment in space-constrained environments such as satellite-based quantum networks or portable devices, compact, reconfigurable, and efficient entanglement sources are essential. Here, we present an electrically tunable entangled photon-pair source utilizing a nanostructured NbOCl2_22​ crystal, engineered for operation in the telecommunication C-band. The inherent non-centrosymmetric lattice symmetry of NbOCl2_22​ enables direct generation of polarization-entangled Bell states without the need for post-selection, leveraging its exceptional second-order nonlinear susceptibility, which surpasses conventional nonlinear materials. By nanopatterning NbOCl2_22​ into a high-quality-factor metasurface, we achieve three orders of magnitude enhancement in photon-pair generation efficiency via resonant excitation of bound states in the continuum resonance, which intensify light-matter interactions. Furthermore, we demonstrate in situ electrical tunability of the photon-pair emission wavelength over a 250 nm range from 1450 nm to 1700 nm by dynamically modulating surrounding liquid crystal layer. Remarkably, the decoupling of photon-pair generation rate and spectral tunability ensures high brightness, above 10,000 coincidences, under active tuning. The air stability and mechanical robustness of NbOCl2_22​ further enhance its practicality for real-world deployment. This work establishes NbOCl2_22​ as a superior material for scalable, on-chip quantum light sources, paving the way for integrated quantum communication systems, adaptive sensors, and portable quantum devices.
Polarization-entangled photon pair sources are essential for diverse quantum technologies, such as quantum communication, computation, and imaging. However, the generation of complex polarization-entangled quantum states has long been constrained by the available nonlinear susceptibility tensor of natural nonlinear crystals, necessitating a cumbersome and intricate setup for additional coherent superposition or post-selection. In this study, we introduce and experimentally demonstrate a nanoscale polarization-entangled photon pair source utilizing an artificially-engineered metamaterial platform. This platform is based on a plasmonic metasurface that is strongly coupled to an epsilon-near-zero (ENZ) material. By precisely engineering resonances at both pump and signal/idler wavelengths, and leveraging the field enhancement provided by the ENZ effect, the photon pair generation efficiency of the 68-nm-thick metasurface is significantly boosted. More notably, the ENZ metasurface platform facilitates versatile manipulation of the system's anisotropic second-order nonlinear susceptibility tensor, enabling direct control over the polarization states of the photon pairs, which leads to the generation of a polarization-entangled Bell state without the need for additional components. Our approach opens a new avenue for the simultaneous photon pair generation and quantum state engineering in a compact platform.
The efficient and controllable generation of entangled photon pairs is a cornerstone for advancing quantum technologies, including quantum computing, communication, and sensing. Traditional methods, primarily relying on spontaneous parametric down-conversion (SPDC) or spontaneous four-wave mixing (SFWM) in bulk nonlinear crystals or waveguides, often suffer from low efficiency, bulky form factors, and limited control over the spectral and spatial properties of the generated photons. This paper explores a transformative approach leveraging resonant Bound States in the Continuum (BICs) within nanophotonic structures to overcome these limitations. BICs are extraordinary optical resonances characterized by theoretically infinite quality (Q) factors, enabling extreme light localization and immense field enhancement within subwavelength volumes. By integrating nonlinear optical materials into BIC-supporting structures, such as photonic crystal slabs or metasurfaces, the local electric field strength experienced by the nonlinear medium can be dramatically increased, boosting the efficiency of entangled photon generation. Furthermore, the sharp, tailor-able spectral response of BICs offers unprecedented control over the joint spectral amplitude of the photon pairs, promising high degrees of spectral purity and entanglement fidelity. This work outlines the theoretical underpinnings, potential device architectures, and anticipated advantages of BIC-enhanced entangled photon sources, paving the way for compact, high-brightness, and spectrally engineered quantum light sources critical for next-generation quantum technologies.
Abstract Liquid crystals, with their ability to self-assemble, strong response to an electric field and integrability into complex systems, are key materials in light-beam manipulation 1 . The recently discovered ferroelectric nematic liquid crystals 2,3 also have considerable second-order optical nonlinearity, making them a potential material for nonlinear optics 4,5 . Their use as sources of quantum light could considerably extend the boundaries of photonic quantum technologies 6 . However, spontaneous parametric down-conversion, the basic source of entangled photons 7 , heralded single photons 8 and squeezed light 9 , has so far not been observed in liquid crystals—or in any liquids or organic materials. Here we implement spontaneous parametric down-conversion in a ferroelectric nematic liquid crystal and demonstrate electric-field tunable broadband generation of entangled photons, with an efficiency comparable to that of the best nonlinear crystals. The emission rate and polarization state of photon pairs is markedly varied by applying a few volts or twisting the molecular orientation along the sample. A liquid-crystal source enables a special type of quasi-phase matching 10 , which is based on the molecular twist structure and is therefore reconfigurable for the desired spectral and polarization properties of photon pairs. Such sources promise to outperform standard nonlinear optical materials in terms of functionality, brightness and the tunability of the generated quantum state. The concepts developed here can be extended to complex topological structures, macroscopic devices and multi-pixel tunable quantum light sources.
Scalable quantum photonic devices drive the development of compact sources of entangled photons, which are pivotal for quantum communication, computing, and cryptography. In this work, we present entangled photon pair generation in rhombohedral boron nitride (r-BN), leveraging its unique optical and structural properties. Unlike conventional hexagonal boron nitride, which suffers from reduced nonlinear response due to centrosymmetric structure in even-layered stacks, r-BN features interlayer ABC stacking and maintains robust in-plane inversion symmetry. These characteristics lead to highly efficient entangled photon generation. Our system demonstrates an entangled photon pair generation rate up to 8667 Hz/(mW·mm) and offers a tunable platform for Bell state generation by simply adjusting the pump polarization, without compromising the entanglement quality or generation efficiency. The polarization entangled state is measured with a fidelity up to 94%. This advancement not only marks a significant step towards ultrathin, scalable quantum devices but also establishes r-BN as a promising candidate for on-chip integrated quantum optical applications. Compact sources of entangled photons are desired for quantum communication, computing, and cryptography. Here, the authors report high entangled photon pair generation rates in rhombohedral boron nitride, showing its potential as a tunable platform for Bell state generation.
Photon-based polarization entanglement in the form of Bell states and qutrits is an essential tool for understanding complex quantum phenomena and implementing various quantum technologies such as free-space quantum communication and quantum imaging. Polarization-entangled photons are conventionally generated in bulky nonlinear crystals through spontaneous parametric down-conversion (SPDC). In this approach, the photon polarization is intrinsically defined by the nonlinear tensor of the material, restricting the versatility of the generated quantum states. The recent developments of dielectric nanostructures with sub-wavelength thickness called metasurfaces [1] enable miniaturization of quantum light sources, while dramatically enhancing the photon-pair generation and allowing the preparation of complex quantum states with spectral [2] and spatial [3] entanglement. However, optically reconfigurable generation of biphoton polarization states with metasurfaces remained unexplored.
There is a growing belief that we are currently in the midst of a Second Quantum Revolution. More and more quantum technologies, such as communication, computing, and imaging, are moving from academic exploration to practical industrial applications. These technologies often depend on entangled photons, and their increasing use requires the development of novel quantum light sources that are compact, versatile, and integrable into modern photonic systems. Additionally, the trend toward miniaturization and nano-optics is accelerating, driven by rapid advances in nanofabrication. Therefore, there is an urgent need to create new classes of miniaturized and tunable two-photon sources. This thesis is dedicated to the theoretical investigation and experimental demonstration of miniaturized sources of entangled photons based on spontaneous parametric down-conversion in thin nonlinear films. It focuses not only on traditional crystalline materials but also on the emerging platform of liquid crystals, marking the first-ever demonstration of photon-pair generation in a liquid crystal within this thesis. A significant achievement is the ability to relax phase-matching constraints in ultra-thin nonlinear layers, allowing the full utilization of the nonlinear tensor and enhancing the variety of possible entangled two-photon states. Several aspects of two-photon state engineering with relaxed phase-matching are discussed in the thesis. First, this relaxation facilitates the switching between different polarization two-photon states, with a varying degree of polarization entanglement and the ability to generate non-maximally entangled states. In terms of spectral properties, miniaturized two-photon sources generate exceptionally broad two-photon frequency spectra, leading to a high degree of frequency entanglement and an unprecedented number of frequency modes, which also leads to ultra-narrow time correlations. By considering the effects of Fabry-Perot resonances, it becomes possible to engineer the two-photon spectra and even achieve backward emission of photon pairs. Remarkably, the Fabry-Perot effect can be accurately described by a simple modification of the rigorous quantum theory, providing a powerful predictive tool for photon pair generation in advanced nanostructures, metasurfaces, and nonlinear nano-antennas when properly applied. Another central contribution of this work is the introduction of liquid crystals as tunable nonlinear media for photon pair generation. Controlling the molecular orientation through a static twist or applying an external electric field makes it possible to tune the generated two-photon states dynamically. A theoretical model of this process, verified through direct experimental measurements, predicts quasi-phase-matching-like behavior and enhanced conversion efficiency, which can be utilized to develop exceptionally tunable and integrable sources of photon pairs and squeezed light. Finally, a fundamental challenge addressed in this thesis is mitigating a parasitic photoluminescent background. This study demonstrates a two-order-of-magnitude reduction in photoluminescence-induced contamination of photon pairs generated from thin nonlinear films by switching from the commonly used continuous-wave regime to the pulsed regime and performing time-resolved distillation. It also shows a developed theory that relates measured experimental parameters to the intrinsic purity of the two-photon state. Overall, this thesis advances the quantum micro- and nano-photonics field by providing a deeper understanding and enhanced control over photon pair generation in thin nonlinear films. It bridges theoretical insights with experimental realizations and paves the way toward practical, tunable, and highly integrated quantum light sources.