# Loop-Back Quantum Key Distribution (QKD) for Secure and Scalable Multi-Node Quantum Networks
## 1 Introduction  
QKD protocols are central to quantum cryptography in the quantum era, harnessing the principles of quantum mechanics to establish secure communication channels. These protocols facilitate secure two-party communication by exploiting quantum phenomena to detect eavesdropping attempts, thereby ensuring information-theoretic security. The BB84 protocol [1, 2, 3, 4], one of the most recognized schemes, serves as a foundation for numerous subsequent protocols designed to enhance security and efficiency. The B92 [5, 6] protocol simplifies key exchange by using only two non-orthogonal quantum states, but its practical implementation often requires additional error correction due to noise and errors in detection, which can impact the key exchange process. The six-state protocol (SSP) [7, 8] extends BB84 by employing three mutually unbiased bases, increasing eavesdropper detection at the cost of greater resource consumption. The differential phase shift (DPS) protocol [9, 10] leverages coherent pulse sequences, improvingrobustness against photon-number splitting (PNS) attacks and facilitating integration with fiber-based networks. To optimize QKD for long distances, the Coherent One-Way (COW) protocol [11; 12] uses coherent states with temporal modes, maintaining security through coherence monitoring. The SARG04 protocol [13; 14], a BB84 variant, enhances resilience against PNS attacks by modifying basis selection. These protocols underscore the theoretical security of QKD while adapting to practical implementation constraints. With the expansion of quantum networks, scalable QKD solutions are essential. The trusted-node architecture [15; 16] extends QKD over long distances by relaying quantum signals but introduces vulnerabilities if nodes are compromised. To mitigate this, MDI-QKD [17; 18] eliminates attacks on photon detectors, enhancing security despite requiring complex quantum relay stations. The TF-QKD protocol [19; 20] addresses signal attenuation by exploiting quantum interference between distant sources, significantly improving long-distance communication. However, its implementation demands precise control of quantum states and advanced optical infrastructure. Despite these advancements, challenges in scalability, security, and technological feasibility persist. Future research must refine protocols like MDI-QKD and TF-QKD to ensure the reliable and widespread adoption of QKD in quantum networks. In this work, we introduce the Loop-Back QKD protocol, a novel scheme specifically designed for secure key distribution in multiuser ring topologies. Unlike trusted-node QKD, it preserves quantum integrity throughout the transmission process by ensuring that secret keys are never processed by intermediate nodes. Additionally, it differs from MDI-QKD and TF-QKD as it does not rely on entanglement or complex quantum measurement setups, making it well suited for near-term deployment in local and metropolitan-scale quantum networks. Furthermore, Loop-Back QKD does not require the public revealing of the measurement bases, significantly enhancing its security. The concept of not requiring basis disclosure was introduced earlier by the same authors in [21; 22]. The proposed Loop-Back QKD protocol leverages symmetry principles in its bidirectional transmission and structured turn-based mechanism, ensuring balanced key distribution across network nodes. This inherent symmetry enhances the protocol's scalability and robustness against adversarial attacks, making it well suited for secure multiuser quantum networks. Loop-Back QKD, operating in a ring topology, supports an arbitrary number of participants by assigning sequential transmission turns and allowing each node to randomly select the pulse direction. This structure not only ensures scalability but also enhances security against intercept-resend (IR) attacks, a practical form of MitM attack. Furthermore, by leveraging a multi-pulse transmission scheme, the protocol effectively reduces the QBER, increasing robustness against adversarial interference.