# Loop-Back Quantum Key Distribution (QKD) for Secure and Scalable Multi-Node Quantum Networks
## 5 Conclusions  
We have presented a scalable and secure QKD protocol that originates from a linear topology and extends naturally to multiuser ring configurations. The Loop-Back QKD protocol was first designed for two-party communication, where it provides strong resilience against IR attacks while maintaining high efficiency. The single-pulse variant offers a straightforward and effective key distribution mechanism, whereas the multi-pulse variant enhances security by reducing the effective QBER. Our analysis indicates that Loop-Back QKD with multi-pulses can tolerate higher QBER thresholds than several well-known QKD protocols, including BB84 and SARG04. This suggests that it may offer enhanced robustness in high-noise environments, although further investigation is needed to establish the security bounds. However, this increased tolerance comes at the cost of a reduced key rate efficiency, as the secret key rate scales as \(O(1/2^{n+1})\), making the protocol less efficient compared to traditional QKD schemes.  
Additionally, the hybrid LB-BB84 configuration enhances the detection of MitM attacks by enabling bidirectional key distillation, leveraging the strengths of both protocols to improve security against active adversaries. These linear configurations demonstrate significant security advantages over traditional point-to-point QKD schemes.  
Building on this, Loop-Back QKD extends naturally to ring topologies, enabling secure multiuser communication. The sequential turn system ensures that each node participates in a structured manner, optimizing network efficiency while maintaining security. By leveraging random pulse direction, the protocol mitigates MitM attacks and provides robust error correction and key reconciliation.  
This makes Loop-Back QKD particularly well suited for secure communication in local quantum networks, where multiple offices or facilities are interconnected in a metropolitan-scale infrastructure.  
Unlike other multiuser QKD schemes that rely on trusted relays or entanglement-based setups, Loop-Back QKD ensures quantum integrity across the entire network without requiring intermediate nodes to reconstruct secret keys. This significantly reduces potential vulnerabilities in large-scale quantum communication systems and makes it a practical candidate for real-world deployment.  
While the progressive establishment of correlations allows for multiuser key distillation, practical implementations will require careful error correction strategies to maintain high fidelity in key reconciliation. Analyzing the optimal balance between transmission rounds and error thresholds remains an open question for future work.  
Furthermore, Loop-Back QKD naturally mitigates PNS attacks by eliminating basis revelation and strengthens security against IR attacks, which pose practical threats to QKD systems. The protocol's ability to verify channel integrity through unpolarized pulses further enhances its robustness against adversarial interference.  
Future work will focus on the experimental implementation of the protocol, validating its feasibility in real-world network environments. This includes developing a physical setup, assessing the impact of practical noise sources, and optimizing key generation rates under realistic conditions.  
Additionally, integrating Loop-Back QKD with existing quantum communication frameworks will be essential to evaluate its interoperability and scalability in metropolitan-scale networks.  
Conceptualization, L.A.L.-P.; Methodology, J.M.L.-R.; Validation, L.A.L.-P.; Formal analysis, L.A.L.-P.; Writing--review & editing, J.M.L.-R.; Supervision, J.M.L.-R. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Data Availability Statement: The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.