Gibeen Gu

Quantum key distribution (QKD) is a promising technology for secure communication, particularly for satellite-based global networks, but its implementation is limited by atmospheric turbulence. In this study, we develop a 1.3 km campus-scale free-space optical link as a testbed for entanglement-based QKD and implement a Sagnac-based polarization-entangled photon source using a type-II PPKTP crystal. The photon-pair generation rate is characterized as a function of pump power, showing slopes of 244.8 kHz/mW with a 10 nm bandpass filter, achieving up to 1 MHz pair rate. The coincidence-to-accidental ratio (CAR) is also measured, revealing a trade-off between pair rate and multi-pair-induced degradation. These results establish a robust platform for future entanglement distribution over atmospheric channels and provide a foundation for satellite-based QKD systems.