Entangled photon sources (EPSs) are essential building blocks for scalable quantum communication and quantum key distribution (QKD). We present a stable, highly nondegenerate EPS based on type-0 spontaneous parametric down-conversion (SPDC) in a crossed-crystal configuration, generating photon pairs at 680~nm and 1550~nm when pumped by a 473~nm laser. This wavelength combination, reported here for the first time, simultaneously benefits from the peak detection efficiency of the most of the Si-SPADs in the visible/near-infrared spectral range and the low-loss fiber transmission of the telecom C-band. This configuration provides the most favorable balance between performance and cost for detection using Si-SPADs and InGaAs detectors. The source exhibits a measured spectral bandwidth of 300~GHz, corresponding to a spectral brightness of up to 1.9×103~pairs~s−1~mW−1~GHz−1. Heralding efficiencies reach 18~\% (signal) and 34~\% (idler) with Si-SPAD and superconducting nanowire single-photon detectors (SNSPD) detection. The entangled state achieves visibilities of (97.3±1.0)% in the H/V basis and (94.9±1.6)% in the D/A basis, yielding a fidelity of ≥(96.1±1.3)%. These results establish the presented EPS as a practical wavelength-hybrid platform for fiber-based QKD and emerging long-haul quantum network architectures.
@article{arxiv.2512.11630,
title = {Highly Nondegenerate Entangled Photon Source for Fiber-Based Quantum Key Distribution},
author = {Vasile-Laurentiu Dosan and Alek Lagarrigue and Alessandro Zannotti and Tushar Parab and Yannick Folwill and Fabian Steinlechner and Oliver de Vries},
journal= {arXiv preprint arXiv:2512.11630},
year = {2025}
}