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 $1.9\times 10^3$~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 $(97.3\pm 1.0)\,\%$ in the H/V basis and $(94.9\pm1.6)\,\%$ in the D/A basis, yielding a fidelity of $\geq(96.1\pm1.3)\,\%$. These results establish the presented EPS as a practical wavelength-hybrid platform for fiber-based QKD and emerging long-haul quantum network architectures.