High-brightness fiber-based Sagnac source of entangled photon pairs for multiplexed quantum networks

TL;DR

We address the need for practical, high-quality, fiber-based entangled photon sources for quantum networks. The authors present a fully fibered nonlinear Sagnac source using PPLN waveguides that generates both polarization and energy-time entanglement without changing the generation stage, and supports dense wavelength-division multiplexing across the C and L telecom bands. They report high brightness (10.3 kpairs/s/nm/mW^2) on 100 GHz ITU channel pairs, polarization-tomography fidelities above 96% and purities above 97%, and energy-time visibility around 99%, with long-term operation validated in a deployed network (mean SKR ~1.95 kbps, QBER in X and Z bases ~6.5% and 4.7%). The results establish the source as a mature, robust component for plug-and-play quantum communications and scalable quantum networking.

Abstract

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/mW$^2$) 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.

Figures (7)

• Figure 1: Experimental setup of the fiber-based Sagnac source. EDFA: erbium-doped fiber amplifier; VOA: variable optical attenuator; PC: polarization controller; CIR: circulator; PBS: polarizing beam splitter; NF: notch filter; DWDM: dense wavelength-division multiplexer.
• Figure 2: Comparison of the photon pair spectrum (red) with the spontaneous Raman scattering spectrum (blue). Exploited photon pair spectrum for entanglement distribution over 100 GHz DWDM management plan is represented here (not to scale).
• Figure 3: TOP : Fiber-based energy-time analyzer (Franson interferometer) used for two-photon interference measurements. BOTTOM : Two-photon interference fringes used to extract the raw energy-time visibility for the selected ITU channel pair.
• Figure 4: Polarization tomography module. A motorized set of quarter- and half-wave are used in front of polarizing beamsplitter in order to perform projective measurement of each photons along $|H\rangle$, $|V\rangle$, $|D\rangle$, and $|R\rangle$ quantum states. QWP: quarter waveplate, HWP: half waveplate, PBS: polarizing beamsplitter.
• Figure 5: Raw fidelity and purity as function of the ITU channel pair. The coincidence and singles rates are recorded and most-likely density matrix calculated using dariano2003quantum. The fidelity to the state $\ket{\Phi^+}$ and the state purity are extracted from the density matrix.