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Generering av sammanflätning i optiska fibrer med SPDC i en Sagnac-interferometer

Spyridon Grountas

Published Jan 1, 2026• 0 citations
OpenAlex

Abstract

This thesis presents the characterization of SagnaQ, a fiber-based polarization-entangled photon pair source developed at Quantum Scopes AB. The source generates photon pairs at telecom wavelengths through type-0 Spontaneous Parametric Down-Conversion (SPDC) in a Periodically Poled Lithium Niobate (PPLN) waveguide crystal placed inside a Sagnac interferometer loop. The purely fiber-coupled design moves towards a compact, plug-and-play platform, offering greater stability and ease of use compared to free-space implementations.The generated quantum state was characterized through Quantum State Tomography (QST), polarization correlation measurements, a Clauser-Horne-Shimony-Holt (CHSH) inequality test, and heralded second-order correlation function measurements. The reconstructed density matrix yields a fidelity of $85\,\%$ to the $|\Phi^{-}\rangle$ Bell state, with a purity of $0.759 \pm 0.008$ and a concurrence of $0.715 \pm 0.008$. Polarization correlation visibilities of $0.929 \pm 0.018$ and $0.790 \pm 0.011$ were measured in the HV and DA bases respectively, and the CHSH parameter was found to be $S = 2.3301 \pm 0.0075$, violating the classical bound by more than 44 standard deviations. A heralded second-order correlation function of $g_{H}^{(2)}(0) = 0.087 \pm 0.002$ confirms the single-photon character of the source. Power-dependent measurements demonstrate a linear scaling of coincidence count rates with pump power, with a degradation of state quality observed at higher pump powers.

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This thesis presents the characterization of SagnaQ, a fiber-based polarization-entangled photon pair source developed at Quantum Scopes AB. The source generates photon pairs at telecom wavelengths through type-0 Spontaneous Parametric Down-Conversion (SPDC) in a Periodically Poled Lithium Niobate (PPLN) waveguide crystal placed inside a Sagnac interferometer loop. The purely fiber-coupled design moves towards a compact, plug-and-play platform, offering greater stability and ease of use compared to free-space implementations.The generated quantum state was characterized through Quantum State Tomography (QST), polarization correlation measurements, a Clauser-Horne-Shimony-Holt (CHSH) inequality test, and heralded second-order correlation function measurements. The reconstructed density matrix yields a fidelity of 85 %85\,\%85% to the ∣Φ−⟩|\Phi^{-}\rangle∣Φ−⟩ Bell state, with a purity of 0.759±0.0080.759 \pm 0.0080.759±0.008 and a concurrence of 0.715±0.0080.715 \pm 0.0080.715±0.008. Polarization correlation visibilities of 0.929±0.0180.929 \pm 0.0180.929±0.018 and 0.790±0.0110.790 \pm 0.0110.790±0.011 were measured in the HV and DA bases respectively, and the CHSH parameter was found to be S=2.3301±0.0075S = 2.3301 \pm 0.0075S=2.3301±0.0075, violating the classical bound by more than 44 standard deviations. A heralded second-order correlation function of gH(2)(0)=0.087±0.002g_{H}^{(2)}(0) = 0.087 \pm 0.002gH(2)​(0)=0.087±0.002 confirms the single-photon character of the source. Power-dependent measurements demonstrate a linear scaling of coincidence count rates with pump power, with a degradation of state quality observed at higher pump powers.
We report a source of polarization-entangled photon pairs in the 1550-nm telecommunication band, which is based on non-collinear spontaneous parametric down-conversion in a periodically poled lithium niobate crystal pumped by picosecond pulses. This source is realized utilizing a polarization-based Sagnac interferometer employing a type-0 non-collinear quasi-phase-matching configuration. Polarization entanglement is verified through measurement of the polarization-correlation interference fringes with visibility >96% and by testing the experimental violation of the Clauser-Horne-Shimony-Holt (CHSH) form of Bell's inequality. The CHSH-Bell parameter S is found to be 2.72 ± 0.04, with 18 standard deviations from the statistical uncertainty.
A fully fibered source of entangled photon pairs based on a nonlinear Sagnac interferometer is reported. Operating at telecom wavelengths, the source relies exclusively on standard fiber-optic components and periodically poled lithium niobate (PPLN) waveguides, resulting in a compact, robust, and field-deployable architecture. The generation stage supports both polarization and energy-time entanglement without modification, enabling versatile operation depending on the targeted application. Broadband spontaneous parametric down-conversion allows dense wavelength-division multiplexing over the telecom C and L bands. High normalized brightness (10.3 kpairs/s/nm/mW2^22) is achieved on a standard 100 GHz ITU channel pair, together with high entanglement quality. Polarization and energy-time encodings are characterized through state tomography and two-photon interference measurements, yielding fidelities, purities, and visibilities exceeding 96 % over multiple wavelength channels. The stability and reproducibility of the source are further evaluated through long-duration operation in a network environment. These results demonstrate that the proposed Sagnac source constitutes a practical and scalable building block for future plug-and-play quantum communication and quantum networking platforms.
We demonstrate a compact, practical all-fiber source of non-degenerate telecom-band polarization-entangled photon pairs assembled from off-the-shelf components. The compact fiber-integrated architecture is designed to be affordable, straightforward to assemble and align, and compatible with existing optical-fiber infrastructure. Photon pairs are generated through type-0 spontaneous parametric down-conversion in a pigtailed periodically poled lithium-niobate waveguide embedded in a Sagnac interferometer. At 0.5 mW pump power, quantum-state tomography yields a Bell-state fidelity FΦ−=0.850F_{Φ^-}=0.850FΦ−​=0.850, purity P=0.836P=0.836P=0.836, and concurrence C=0.782C=0.782C=0.782. A separate CHSH measurement gives S=2.3301±0.0075S=2.3301\pm0.0075S=2.3301±0.0075. The measured Bell-state fidelity is sufficient for the reconstructed polarization state to support qubit teleportation above the classical fidelity limit under ideal protocol operations. Analysis of the density matrix shows that approximately 96.7% of the population remains in the desired ∣HH⟩|HH\rangle∣HH⟩-∣VV⟩|VV\rangle∣VV⟩ subspace, with a normalized coherence of approximately 0.86. A residual relative-phase offset indicates that local phase compensation could increase the Bell-state fidelity to approximately 0.8865. We further characterize the brightness--quality trade-off and obtain a corrected pair-rate slope of (0.918±0.040)(0.918\pm0.040)(0.918±0.040) Mcps mW−1^{-1}−1. The broadband emission and fiber interfaces offer a route to multiuser entanglement distribution through off-the-shelf multichannel wavelength-division multiplexers with passbands matched to the signal and idler wavelengths.
A fully fibered source of entangled photon pairs based on a nonlinear Sagnac interferometer is reported. Operating at telecom wavelengths, the source relies exclusively on standard fiber-optic components and periodically poled lithium niobate waveguides, resulting in a compact, robust, and field-deployable architecture. The generation stage supports both polarization and energy-time entanglement without modification, enabling versatile operation depending on the targeted application. Broadband spontaneous parametric down-conversion allows dense wavelength-division multiplexing over the telecom C and L bands. A lumped device brightness of about 104 pairs s−1 nm−1 mW−2 is reported over several standard 100-GHz ITU channel pairs, together with high entanglement quality. Polarization and energy-time encodings are characterized through state tomography and two-photon interference measurements, yielding fidelities, purities, and visibilities exceeding 96% over multiple wavelength channels. The stability and reproducibility of the source are further evaluated through long-duration operation in a network environment. These results demonstrate that the proposed Sagnac source constitutes a practical and scalable building block for future plug-and-play quantum communication and quantum networking platforms.
Combining a Ti-diffusion periodically poled lithium niobate (PPLN) waveguide with a Sagnac interferometer, two opposite directions type-II spontaneous parametric down conversions (SPDC) occur coherently and yield a high brightness, high stability polarization entanglement source. The source produces degenerate photon pairs at 1540.4 nm with a brightness of B = (1.36 ± 0.03) × 106 pairs/(s⋅nm⋅mW). We perform quantum state tomography to reconstruct the density matrix of the output state and obtain a fidelity of F = 0.983 ± 0.001. The high brightness and phase stability of our waveguide source enable a wide range of quantum information experiments operating at a low pump power as well as hold the advantage in mass production which can promote the practical applications of quantum technologies.
Sources of entangled photon pairs are a key component in both fundamental tests of quantum theory and practical applications such as quantum key distribution and quantum computing. In this work, we describe and characterize a source of polarization entangled photon pairs based on two spontaneous parametric down-conversion crystals in a Sagnac interferometer. Our source is compact and produces high-quality entangled states in a very flexible manner. The wavelengths of the photon pairs (around 810 and 1550 nm), the phase between orthogonal components of the entangled state, and the tangle of the state are all independently adjustable. In addition to presenting basic characterization data, we present experimental violations of Clauser-Horne-Shimony-Holt and Leggett inequalities, as well as an instance of the ``beautiful'' Bell inequality, which has not previously been tested experimentally.