Detector-optimized wavelength-hybrid entangled photon source for scalable fiber quantum key distribution

Practical deployment of entanglement-based quantum key distribution (QKD) networks requires photon sources that simultaneously address fiber transmission loss, detector performance, and system cost–constraints that are individually well-understood but collectively unresolved in current source designs. Here we present a detector-optimized, wavelength-hybrid entangled photon source (EPS) that resolves this trilemma by design and show its performance in a QKD system. The source uses type-0 spontaneous parametric down-conversion in a crossed-crystal periodically poled KTiOPO ₄ (ppKTP) configuration pumped at 473 nm, generating polarization-entangled photon pairs at 680 nm and 1550 nm. This highly nondegenerate wavelength combination is engineered to route the 680 nm photon to a high-efficiency, low-cost Si-SPAD detector while transmitting the 1550 nm photon through standard telecom fiber with minimum attenuation. The source achieves a spectral brightness of (1.9 × 10 3 )s −1 mW −1 GHz −1 pairs over a 300 GHz phase-matching bandwidth, with polarization entanglement visibilities of (97.3 ± 1.0)% and (94.9 ± 1.6)% in the H/V and D/A bases, corresponding to a state fidelity of ≥ (96.1 ± 1.3)%. These results establish the wavelength-hybrid approach as a scalable and cost-effective building block for deployed entanglement-based QKD nodes and future quantum network infrastructure.