{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,1,13]],"date-time":"2026-01-13T14:23:39Z","timestamp":1768314219922,"version":"3.49.0"},"reference-count":28,"publisher":"MDPI AG","issue":"4","license":[{"start":{"date-parts":[[2025,3,30]],"date-time":"2025-03-30T00:00:00Z","timestamp":1743292800000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Symmetry"],"abstract":"<jats:p>Quantum key distribution (QKD) is a cornerstone of secure communication in the quantum era, yet most existing protocols are designed for point-to-point transmission, limiting their scalability in networked environments. In this work, we introduce Loop-Back QKD, a novel QKD protocol that supports both two-party linear configurations and scalable multiuser ring topologies. By leveraging a structured turn-based mechanism and bidirectional pulse propagation, the protocol enables efficient key distribution while reducing the quantum bit error rate (QBER) through a multi-pulse approach. Unlike trusted-node QKD networks, Loop-Back QKD eliminates intermediate-node vulnerabilities, as secret keys are never processed by intermediate nodes. Furthermore, unlike Measurement-Device-Independent (MDI-QKD) and Twin-Field QKD (TF-QKD), which require complex entanglement-based setups, Loop-Back QKD relies solely on direct polarization transformations, reducing vulnerability to side-channel attacks and practical implementation challenges. Additionally, our analysis indicates that multi-pulse Loop-Back QKD can tolerate higher QBER thresholds. However, this increased robustness comes at the cost of a lower key rate efficiency compared to standard QKD schemes. This design choice enhances its robustness against real-world adversarial threats, making it a strong candidate for secure multiuser communication in local and metropolitan-scale quantum networks.<\/jats:p>","DOI":"10.3390\/sym17040521","type":"journal-article","created":{"date-parts":[[2025,3,31]],"date-time":"2025-03-31T05:21:04Z","timestamp":1743398464000},"page":"521","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":2,"title":["Loop-Back Quantum Key Distribution (QKD) for Secure and Scalable Multi-Node Quantum Networks"],"prefix":"10.3390","volume":"17","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-5109-2927","authenticated-orcid":false,"given":"Luis Adri\u00e1n","family":"Lizama-Perez","sequence":"first","affiliation":[{"name":"Departamento de Electr\u00f3nica, Universidad T\u00e9cnica Federico Santa Mar\u00eda, Av. Vicu\u00f1a Mackenna 3939, San Joaqu\u00edn, Santiago 8940897, Chile"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-3435-9241","authenticated-orcid":false,"given":"J. M.","family":"L\u00f3pez-Romero","sequence":"additional","affiliation":[{"name":"Cinvestav Quer\u00e9taro, Libramiento Norponiente 2000, Real de Juriquilla, Santiago de Quer\u00e9taro 76230, Quer\u00e9taro, Mexico"}]}],"member":"1968","published-online":{"date-parts":[[2025,3,30]]},"reference":[{"key":"ref_1","unstructured":"Bennett, C.H.G. (1984, January 10\u201312). Brassard \u2018Quantum cryptography: Public key distribution and coin tossing\u2019. Proceedings of the IEEE International Conference on Computers, Systems & Signal Processing, Bangalore, India."},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"189","DOI":"10.1142\/S0219749905000736","article-title":"Deterministic plug-and-play for quantum communication","volume":"3","author":"Lucamarini","year":"2005","journal-title":"Int. J. 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