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High-Brightness Entangled Photon Pair Source Based on Integrated Photonics Silicon Nitride Ring Resonator

Mohsen Shokrolahi, A. Allahi, Ali Poursohrabi, Masoud Yousefi

Published Jan 1, 2026• 0 citations
Publisher
DOI
10.2139/ssrn.7195570

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Nature•Mar 19, 2025•42 citations
Developing a high-quality, bright source of polarization-entangled photon pairs on CMOS-compatible integrated platforms, such as silicon or silicon nitride, remains a significant challenge. Here, a novel silicon chip-based nonlinear Sagnac interferometer, which mainly integrates a bidirectionally pumped microring resonator with a polarization-splitting rotator, was proposed to generate polarization-entangled photon pairs. By pumping the system with a pulsed laser at 1550 nm and a power of 78 μW, polarization-entangled photon pairs in the telecom C-band were produced through spontaneous four-wave mixing, achieving a state-of-the-art brightness of 1.6 × 107 pairs s−1 GHz−1 mW−2. Using an off-chip polarization analyzer, the visibility of polarization-correlation interference fringe was measured to be 96.6%. Quantum state tomography revealed a fidelity of 0.958 for the maximally entangled state produced by the source. These exceptional performance metrics, combined with the integrability of the device, make it highly suitable for advanced quantum applications, particularly in multi-user entanglement-based quantum communication networks.
ABSTRACT High‐brightness–integrated quantum light sources represent a fundamental element for large‐scale photonic quantum technologies. Integrated microring resonator (MRR) in highly nonlinear and ultralow‐loss material platforms enable entangled photon‐pair generation, but device performance is affected by fabrication tolerances and can thus not be judiciously controlled. Redundant designs based on parameter variation are impractical for chip‐scale circuits, as the sheer number of parameters would be too complex. To address this, passive photonic circuits must be combined with active control elements to optimize photon‐pair emission and to match emission characteristics among sources. Here, we present a comprehensive study of an actively tunable, interferometrically coupled integrated MRR implemented in silicon nitride (SiN). By manipulating two thermally controlled phase shifters, the ring–bus coupling and the resonator phase can be dynamically adjusted, enabling post‐fabrication control of its quality factor (Q‐factor) between 10 5 and 10 6 to optimize the photon emission bandwidth and the pair generation rate. We directly demonstrate for the first time that the optimal coupling and emission characteristics do not occur at critical coupling and vary across frequency modes due to dispersion effects, in good agreement with our theoretical model. These findings highlight the role of adaptive integrated photon sources for single‐batch fabrication of reconfigurable devices with application‐specific spectral properties.
Abstract Entangled photon pairs from spontaneous parametric down-conversion (SPDC) 1 are central to many quantum applications 2–6 . SPDC is typically performed in non-centrosymmetric systems 7 with an inherent second-order nonlinearity ( χ (2) ) 8–10 . We demonstrate strong narrowband SPDC with an on-chip rate of 0.8 million pairs per second in Si 3 N 4 . Si 3 N 4 is the pre-eminent material for photonic integration and also exhibits the lowest waveguide loss (which is essential for integrated quantum circuits). However, being amorphous, silicon nitride lacks an intrinsic χ (2) , which limits its role in photonic quantum devices. We enabled SPDC in Si 3 N 4 by combining strong light-field enhancement inside a high optical Q -factor microcavity with an optically induced space-charge field. We present narrowband photon pairs with a high spectral brightness. The quantum nature of the down-converted photon pairs is verified through coincidence measurements. This light source, based on Si 3 N 4 integrated photonics technology, unlocks new avenues for quantum systems on a chip.