{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,10,21]],"date-time":"2026-10-21T19:32:00Z","timestamp":1792611120113,"version":"4.5.0"},"reference-count":33,"publisher":"MDPI AG","issue":"12","license":[{"start":{"date-parts":[[2025,12,11]],"date-time":"2025-12-11T00:00:00Z","timestamp":1765411200000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/100009051","name":"Universidad T\u00e9cnica Federico Santa Mar\u00eda","doi-asserted-by":"crossref","award":["PI_LII_24_15"],"award-info":[{"award-number":["PI_LII_24_15"]}],"id":[{"id":"10.13039\/100009051","id-type":"DOI","asserted-by":"crossref"}]},{"name":"Agencia Nacional de Investigaci\u00f3n y Desarrollo (ANID), Chile","award":["11251432"],"award-info":[{"award-number":["11251432"]}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Entropy"],"abstract":"<jats:p>The Loop-Back Quantum Key Distribution (LB-QKD) protocol establishes a bidirectional architecture in which a single photon travels forth and back through the same optical channel. Unlike conventional one-way schemes such as BB84, Alice performs both state preparation and measurement, while Bob acts as a passive polarization modulator and reflector. This design eliminates detectors at Bob\u2019s side, minimizes synchronization requirements, and enables compact, low-power implementations suitable for quantum-mobile and IoT platforms. An extended three-basis configuration {X,Y,Z} is introduced, preserving the simplicity of the two-basis scheme while improving noise tolerance through enhanced orthogonality-based filtering. Analytical modeling shows that the effective protocol error decreases from Eprotocol(2)=e\/2 to Eprotocol(3)=e\/3, achieving a 33% improvement in noise resilience. Despite its slightly lower sifting efficiency (\u03b7=1\/6), the total information gain reaches G=0.26 bits per pulse, maintaining post-sifting throughput comparable to BB84. The protocol doubles the tolerable QBER of conventional QKD, sustaining secure operation up to 22% for two bases and approximately 47.58% for three bases. Its passive, self-verifying architecture enhances resistance to man-in-the-middle, photon-number-splitting, and side-channel attacks, providing a scalable and energy-efficient framework for secure key distribution and authentication in next-generation mobile and distributed quantum networks.<\/jats:p>","DOI":"10.3390\/e27121249","type":"journal-article","created":{"date-parts":[[2025,12,11]],"date-time":"2025-12-11T11:17:27Z","timestamp":1765451847000},"page":"1249","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":2,"title":["Three-Basis Loop-Back QKD: A Passive Architecture for Secure and Scalable Quantum Mobile Networks"],"prefix":"10.3390","volume":"27","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-5109-2927","authenticated-orcid":false,"given":"Luis Adri\u00e1n","family":"Lizama-P\u00e9rez","sequence":"first","affiliation":[{"name":"Departamento de Electr\u00f3nica, Universidad T\u00e9cnica Federico Santa Mar\u00eda, Av. Vicu\u00f1a Mackenna 3939, San Joaqu\u00edn, Santiago 7820436, Chile"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-1234-4583","authenticated-orcid":false,"given":"Patricia","family":"Morales-Calvo","sequence":"additional","affiliation":[{"name":"Departamento de Electr\u00f3nica, Universidad T\u00e9cnica Federico Santa Mar\u00eda, Av. Vicu\u00f1a Mackenna 3939, San Joaqu\u00edn, Santiago 7820436, Chile"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1968","published-online":{"date-parts":[[2025,12,11]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"78","DOI":"10.1145\/1008908.1008920","article-title":"Conjugate coding","volume":"15","author":"Wiesner","year":"1983","journal-title":"ACM Sigact News"},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"023007","DOI":"10.1088\/1367-2630\/ab6872","article-title":"Semi-device-independent quantum money","volume":"22","author":"Horodecki","year":"2020","journal-title":"New J. Phys."},{"key":"ref_3","doi-asserted-by":"crossref","unstructured":"Lizama-Perez, L.A., and L\u00f3pez-Romero, J. (2025). Loop-Back Quantum Key Distribution (QKD) for Secure and Scalable Multi-Node Quantum Networks. Symmetry, 17.","DOI":"10.3390\/sym17040521"},{"key":"ref_4","unstructured":"Bennett, C.H., and Brassard, G. 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