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International Conference on Transparent Optical Networks

Efficient entangled quantum states generation in silica nanofibers for QKD-based quantum cryptography protocols

Abderrahim Azzoune, Oussama Laouedj, Hamza Gouasmia, Hocine Medjadba, Ayoub Boudjelida, Abderrahmane Bourbia

Published Jul 6, 2025• 4 citations
Publisher

Abstract

Entangled quantum states generation is crucial for quantum cryptography, particularly in quantum key distribution (QKD). While optical fibers enable low-loss photon transmission, standard silica fibers exhibit weak nonlinearities for spontaneous parametric processes. This paper proposes an optimized scheme for entangled quantum states generation in subwavelength silica nanofibers, leveraging effective second-order nonlinearities χ(2)induced by surface inversion symmetry breaking. Unlike bulk silica, tapered silica nanofibers exhibit significant second-order responses via surface dipole and bulk quadrupole nonlinearities, enabling spontaneous parametric down-conversion (SPDC). Our key innovation is a second-order nonlinear coating (PMMA/DR1 encapsulated with Teflon), enhancing $\chi _{eff}^{(2)}$ and boosting the spectral density efficiency of entangled quantum states generation by a factor of 103. This approach optimizes phase matching while maintaining optical stability and minimizing scattering losses. Additionally, the proposed source inherently avoids spontaneous Raman scattering (SRS) noise, which is a major limitation in fiber-based SPDC systems, ensuring high-purity entangled quantum states generation. Our results demonstrate that silica nanofibers, with tailored nonlinear coatings, offer a scalable platform for integrated quantum light sources in fiber-based quantum networks.

DOI
10.1109/icton67126.2025.11125365

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Entangled quantum states generation is crucial for quantum cryptography, particularly in quantum key distribution (QKD). While optical fibers enable low-loss photon transmission, standard silica fibers exhibit weak nonlinearities for spontaneous parametric processes. This paper proposes an optimized scheme for entangled quantum states generation in subwavelength silica nanofibers, leveraging effective second-order nonlinearities χ(2)induced by surface inversion symmetry breaking. Unlike bulk silica, tapered silica nanofibers exhibit significant second-order responses via surface dipole and bulk quadrupole nonlinearities, enabling spontaneous parametric down-conversion (SPDC). Our key innovation is a second-order nonlinear coating (PMMA/DR1 encapsulated with Teflon), enhancing χeff(2)\chi _{eff}^{(2)}χeff(2)​ and boosting the spectral density efficiency of entangled quantum states generation by a factor of 103. This approach optimizes phase matching while maintaining optical stability and minimizing scattering losses. Additionally, the proposed source inherently avoids spontaneous Raman scattering (SRS) noise, which is a major limitation in fiber-based SPDC systems, ensuring high-purity entangled quantum states generation. Our results demonstrate that silica nanofibers, with tailored nonlinear coatings, offer a scalable platform for integrated quantum light sources in fiber-based quantum networks.
We report on the use of PMMA/DR1 coating to enhance the efficiency of photon pair generation in silica nanofibers. The coating improves the second-order nonlinear susceptibility of the nanofibers, leading to improved photon pair generation efficiency. We investigate the effect of varying the nonlinear optical properties of the composite material, and we characterize the photon pair generation efficiency of the coated silica nanofibers. Our modelling results show a significant enhancement in photon pair generation efficiency by a factor of 1000 compared to a bare silica nanofiber.
In this paper, we present a design of an all-fiber source of correlated photon pairs based on standard telecommunications tapered fibers. We examine the generation of correlated photon pairs using parametric process χ ( 2 ) in silica tapered optical fibers. This nonlinear process is ensured thanks to surface dipole and bulk multipole nonlinearities. The process of photon creation is modeled by taking into account the vector aspect of the propagation of the optical field in a silica nanofiber. The phase matching is provided by propagating the pump field in one spatial mode while generating a photon pair in another spatial mode. The generation efficiency of photon pairs depends on diameter uniformity of the nanofiber after the manufacturing process. We size this nanofiber for a good optimization of photon pair generation efficiency, and we report that the tolerance in diameter uniformity is Δ d = 2 n m for a generation rate of photon pairs estimated to N ph ≈ 22 000 p a i r s / s , for 1 W power pump and a nanofiber length of 1.1 mm. Deposits on the nanofiber can be used in order to relax the manufacturing constraints on diameter to maximize the generation rate of photon pairs. As an example, the use of polytetrafluoroethylene on the nanofiber applied as a cladding whose thickness is infinite makes it possible to relax the constraints on the nanofiber diameter. For the same Δ d = 2 n m , a generation rate of photon pairs estimated to N ph ≈ 78 000 p a i r s / s for 1 W power pump and a nanofiber length of 2.4 mm is predicted.
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.
Entangled photon sources (EPSs) are essential building blocks for scalable quantum communication and quantum key distribution (QKD). We present a stable, highly nondegenerate EPS based on type-0 spontaneous parametric down-conversion (SPDC) in a crossed-crystal configuration, generating photon pairs at 680~nm and 1550~nm when pumped by a 473~nm laser. This wavelength combination, reported here for the first time, simultaneously benefits from the peak detection efficiency of the most of the Si-SPADs in the visible/near-infrared spectral range and the low-loss fiber transmission of the telecom C-band. This configuration provides the most favorable balance between performance and cost for detection using Si-SPADs and InGaAs detectors. The source exhibits a measured spectral bandwidth of 300~GHz, corresponding to a spectral brightness of up to 1.9×1031.9\times 10^31.9×103~pairs~s−1^{-1}−1~mW−1^{-1}−1~GHz−1^{-1}−1. Heralding efficiencies reach 18~\% (signal) and 34~\% (idler) with Si-SPAD and superconducting nanowire single-photon detectors (SNSPD) detection. The entangled state achieves visibilities of (97.3±1.0) %(97.3\pm 1.0)\,\%(97.3±1.0)% in the H/V basis and (94.9±1.6) %(94.9\pm1.6)\,\%(94.9±1.6)% in the D/A basis, yielding a fidelity of ≥(96.1±1.3) %\geq(96.1\pm1.3)\,\%≥(96.1±1.3)%. These results establish the presented EPS as a practical wavelength-hybrid platform for fiber-based QKD and emerging long-haul quantum network architectures.
Entanglement-based quantum key distribution (QKD) protocols require robust and stable photon-pair sources in terms of high heralding efficiencies or photon-pair generation rates even under harsh environmental conditions, e.g., when operated in the field. Here we report on a type-0 spontaneous parametric down-conversion photon source in periodically poled crystals based on simultaneous operation of second harmonic generation and spontaneous parametric down-conversion (SPDC) in a double-pass configuration within the same nonlinear crystal, further simplifying the hardware requirements of SPDC photon sources. Our source is flexible, tunable, alignment-free, relatively broadband, and completely fiber coupled. Its rack-compatible and modular setup allows a straightforward plug-and-play integration of coding modules, e.g., interferometers to enable various QKD protocols such as phase or phase-time coding. We compare the double-pass configuration with our single-pass type-II and type-0 SPDC source configurations. To evaluate the conversion efficiencies of our modules, we use data postprocessing to remove artifacts from detector after-pulsing and dead times of the detectors.
Spontaneous Parametric Down-Conversion (SPDC) is a quantum optical process where a photon spontaneously splits into a pair of lower energy entangled single-photons. Here, we demonstrate that the quantum effect of SPDC can be substantially boosted at room temperature by utilizing nonlinear plasmonic metasurfaces. The presented plasmonic metasurface design is composed of silver nanostripes grown on a bulk lithium niobate (LiNbO3) crystal terminated by a silver substrate. The quantum-classical correspondence principle is used to compute the SPDC entangled photon pair generation rate by the classical sum frequency generation (SFG) nonlinear optical process. The generated signal and idler frequencies are matched to the metasurface fundamental and higher order resonances leading to substantially boosted SFG efficiency and, consequently, significantly enhanced SPDC generation rate. The quantum SPDC process is at the heart of many quantum optical emerging apparatuses, since efficient single-photon generation at room temperature is required in a plethora of applications in quantum communications, cryptography, and sensing.