Ultrabright Fiber‐Coupled Polarization‐Entangled Photon Source with Spectral Brightness Surpassing 2.0 MHz mW⁻¹ nm⁻¹

This study presents an ultrabright polarization‐entangled photon source that is optimally coupled into single‐mode fibers (SMFs). This study theoretically and experimentally examines the characteristics of spontaneous parametric down‐conversion (SPDC) photons, including their spectrum, bandwidth, emission angle, and intensity, as functions of crystal length, temperature and beam waist condition. Notably, this study measures the collinear spatial modes of photon‐pairs and collection optics under various beam waist conditions and analyzes them using a collinear Gaussian approximation model. By employing a simple mode‐matching optical setup, it optimizes the SMF coupling and heralding efficiencies of the photon‐pairs. Consequently, it achieves a spectral brightness exceeding 2.0 MHz mW −1 nm −1 from a fiber‐coupled entangled photon source, utilizing a 30 mm ppKTP crystal inside a polarization Sagnac interferometer. This represents the highest spectral brightness of SPDC photons generated using a continuous‐wave (CW) laser pumped bulk crystal to date. Polarization entanglement is verified by a quantum state tomography and a polarization‐correlation measurement. The fidelity of the entangled state is measured to be 97.8% and the Bell‐Clauser‐Horne‐Shimony‐Holt value S = 2.782 ± 0.04. The results obtained here provide practical insights for designing high‐performance SPDC sources for satellite‐based communication and long‐distance optical links with extremely high‐photon loss.