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

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.

Quantum Physics (quant-ph); Optics (physics.optics)

arXiv:2608.26499 [quant-ph]

(or arXiv:2608.26499v1 [quant-ph] for this version)

https://doi.org/10.48550/arXiv.2608.26499 Focus to learn more arXiv-issued DOI via DataCite (pending registration)

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