Highly Nondegenerate Entangled Photon Source for Fiber-Based Quantum Key Distribution

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 1.9times 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 \(97.3pm 1.0\) \% in the H/V basis and \(94.9pm1.6\) \% in the D/A basis, yielding a fidelity of geq\(96.1pm1.3\) \%. These results establish the presented EPS as a practical wavelength-hybrid platform for fiber-based QKD and emerging long-haul quantum network architectures.