Real-Space Analysis of Two-Photon Polarization States in Type-II SPDC for High-Purity Polarization Entanglement

• URL: http://arxiv.org/abs/2608.26499v1
• Date: Thu, 27 Aug 2026 00:46:29 GMT
• Title: Real-Space Analysis of Two-Photon Polarization States in Type-II SPDC for High-Purity Polarization Entanglement
• Abstract summary: Spontaneous parametric down-conversion (SPDC) is one of the most widely used sources of polarization-entangled photon pairs.<n>Understanding the generated biphoton state is essential for realizing high-brightness and high-purity entangled-photon sources.<n>In this study, we calculate the real-space distribution of the two-photon polarization state from the biphoton wavefunction of Type-II SPDC generated in a beta-barium borate (BBO) crystal.
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• License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/
• Abstract: Spontaneous parametric down-conversion (SPDC) is one of the most widely used sources of polarization-entangled photon pairs, and understanding the generated biphoton state is essential for realizing high-brightness and high-purity entangled-photon sources. In particular, Type-II SPDC produces a biphoton wavefunction with a complex coupling between the spatial and polarization degrees of freedom owing to birefringence. In this study, we calculate the real-space distribution of the two-photon polarization state from the biphoton wavefunction of Type-II SPDC generated in a \b{eta}-barium borate (BBO) crystal. By identifying the spatial correlation direction between polarization and real-space coordinates, we designed aperture shapes that restrict the collection along this correlation direction. Through both numerical simulations and experiments, we demonstrate that such correlation-aligned apertures simultaneously achieve higher entanglement purity and improved photon collection efficiency. These results establish a practical design principle for optimizing aperture geometries based on the real-space biphoton wavefunction, providing a new approach to realizing high-purity and high-brightness SPDC entangled-photon sources.

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