Quantum-Secure Communication Networks: A Review of QKD Protocols, Experimental Deployments, and Future Directions

Quantum Key Distribution (QKD) has become a critical technology for secure communication, leveraging quantum mechanics to provide robust protection against both classical and quantum-based threats. This systematic review synthesizes advancements in QKD protocols, experimental implementations, and the integration of quantum communication networks from 2010 to 2025. It highlights the development of foundational protocols like BB84 and E91, and innovative protocols such as Measurement-Device-Independent QKD (MDI-QKD) and Twin-Field QKD (TF-QKD), which address security vulnerabilities and extend secure key distribution to distances over 1,000 km. Experimental achievements in fiber-optic, free-space, and satellite-based QKD systems have demonstrated the feasibility of long-distance secure key exchange, with significant progress in overcoming challenges like photon loss, detector inefficiency, and integration with classical networks. Despite these advances, practical deployment remains hindered by scalability issues, hardware limitations, and the need for standardized components. This review identifies key research gaps and outlines future directions, emphasizing the development of quantum repeaters, photonic integration, machine learning applications for network optimization, and hybrid quantum-classical networks. It underscores the importance of continued innovation in both theoretical and experimental domains to pave the way for globally secure, scalable quantum communication networks.