{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,2,15]],"date-time":"2026-02-15T06:02:04Z","timestamp":1771135324765,"version":"3.50.1"},"reference-count":25,"publisher":"MDPI AG","issue":"2","license":[{"start":{"date-parts":[[2026,2,13]],"date-time":"2026-02-13T00:00:00Z","timestamp":1770940800000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/100012818","name":"Comunidad de Madrid","doi-asserted-by":"crossref","id":[{"id":"10.13039\/100012818","id-type":"DOI","asserted-by":"crossref"}]},{"name":"European Union\u2014NextGenerationEU","award":["PRTRC17.I1"],"award-info":[{"award-number":["PRTRC17.I1"]}]},{"name":"Ministerio para la Transformaci\u00f3n Digital y de la Funci\u00f3n P\u00fablica"},{"name":"European Union\u2014NextGenerationEU","award":["PRTR-C16.R1"],"award-info":[{"award-number":["PRTR-C16.R1"]}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Entropy"],"abstract":"<jats:p>A monitor and control framework for quantum-key-distribution (QKD) networks equipped with switching capabilities was developed. On the one hand, this framework provides real-time visibility into operational metrics. Specifically, it extracts essential data, such as the switching capabilities of QKD modules, the number of keys stored in buffer queues of the QKD links, and the respective key generation and consumption rates along these links. On the other hand, this framework allows software-defined networking (SDN) applications to operate on the collected information and address the cryptographic needs of the network. The SDN applications dynamically adapt the configuration of the switched network to align with its changing demands, e.g., prioritizing key availability on critical paths, responding to link failures, or reallocating generation capacity to prevent bottlenecks. This contribution demonstrates that the combination of switched QKD, centralized control, and global optimization strategies enables efficient, policy-driven operation of QKD networks. The cryptographic resources are allocated to maximize performance and resilience while remaining aligned with the specific policies set by network administrators.<\/jats:p>","DOI":"10.3390\/e28020219","type":"journal-article","created":{"date-parts":[[2026,2,13]],"date-time":"2026-02-13T16:09:32Z","timestamp":1770998972000},"page":"219","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["Switching Coordinator: An SDN Application for Flexible QKD Networks"],"prefix":"10.3390","volume":"28","author":[{"ORCID":"https:\/\/orcid.org\/0009-0008-3673-490X","authenticated-orcid":false,"given":"Rub\u00e9n B.","family":"M\u00e9ndez","sequence":"first","affiliation":[{"name":"Center for Computational Simulation, Universidad Polit\u00e9cnica de Madrid, 28660 Madrid, Spain"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Hans H.","family":"Brunner","sequence":"additional","affiliation":[{"name":"Munich Research Center, Huawei Technologies Duesseldorf GmbH, 80992 Munich, Germany"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Juan P.","family":"Brito","sequence":"additional","affiliation":[{"name":"Center for Computational Simulation, Universidad Polit\u00e9cnica de Madrid, 28660 Madrid, Spain"},{"name":"DLSIIS, ETSI Inform\u00e1ticos, Universidad Polit\u00e9cnica de Madrid, 28660 Madrid, Spain"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-0597-2039","authenticated-orcid":false,"given":"Hamid","family":"Taramit","sequence":"additional","affiliation":[{"name":"Center for Computational Simulation, Universidad Polit\u00e9cnica de Madrid, 28660 Madrid, Spain"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Chi-Hang Fred","family":"Fung","sequence":"additional","affiliation":[{"name":"Munich Research Center, Huawei Technologies Duesseldorf GmbH, 80992 Munich, Germany"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-2849-9782","authenticated-orcid":false,"given":"Antonio","family":"Pastor","sequence":"additional","affiliation":[{"name":"Telef\u00f3nica gCTIO, 28050 Madrid, Spain"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Rafael","family":"Cant\u00f3","sequence":"additional","affiliation":[{"name":"Telef\u00f3nica gCTIO, 28050 Madrid, Spain"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-1207-4077","authenticated-orcid":false,"given":"Jes\u00fas","family":"Folgueira","sequence":"additional","affiliation":[{"name":"Center for Computational Simulation, Universidad Polit\u00e9cnica de Madrid, 28660 Madrid, Spain"},{"name":"Telef\u00f3nica gCTIO, 28050 Madrid, Spain"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-8326-2000","authenticated-orcid":false,"given":"Diego R.","family":"L\u00f3pez","sequence":"additional","affiliation":[{"name":"Telef\u00f3nica gCTIO, 28050 Madrid, Spain"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-7928-0409","authenticated-orcid":false,"given":"Momtchil","family":"Peev","sequence":"additional","affiliation":[{"name":"Munich Research Center, Huawei Technologies Duesseldorf GmbH, 80992 Munich, Germany"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Vicente","family":"Martin","sequence":"additional","affiliation":[{"name":"Center for Computational Simulation, Universidad Polit\u00e9cnica de Madrid, 28660 Madrid, Spain"},{"name":"DLSIIS, ETSI Inform\u00e1ticos, Universidad Polit\u00e9cnica de Madrid, 28660 Madrid, Spain"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1968","published-online":{"date-parts":[[2026,2,13]]},"reference":[{"key":"ref_1","unstructured":"Shor, P.W. (1994, January 20\u201322). Algorithms for quantum computation: Discrete logarithms and factoring. Proceedings of the 35th Annual IEEE Symposium on Foundations of Computer Science, Santa Fe, NM, USA."},{"key":"ref_2","doi-asserted-by":"crossref","unstructured":"Martin, V., Martinez-Mateo, J., and Peev, M. (2017). Introduction to quantum key distribution. Wiley Encyclopedia of Electrical and Electronics Engineering, John Wiley & Sons, Ltd.","DOI":"10.1002\/047134608X.W8354"},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"3","DOI":"10.1007\/BF00191318","article-title":"Experimental quantum cryptography","volume":"5","author":"Bennett","year":"1992","journal-title":"J. Cryptol."},{"key":"ref_4","doi-asserted-by":"crossref","unstructured":"Elliott, C. (2005). The DARPA quantum network. Quantum Communications and Cryptography, CRC Press.","DOI":"10.1201\/9781420026603.ch4"},{"key":"ref_5","doi-asserted-by":"crossref","first-page":"075001","DOI":"10.1088\/1367-2630\/11\/7\/075001","article-title":"The SECOQC quantum key distribution network in Vienna","volume":"11","author":"Peev","year":"2009","journal-title":"New J. Phys."},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"10387","DOI":"10.1364\/OE.19.010387","article-title":"Field test of quantum key distribution in the Tokyo QKD network","volume":"19","author":"Sasaki","year":"2011","journal-title":"Opt. Express"},{"key":"ref_7","doi-asserted-by":"crossref","first-page":"214","DOI":"10.1038\/s41586-020-03093-8","article-title":"An integrated space-to-ground quantum communication network over 4,600 kilometres","volume":"589","author":"Chen","year":"2021","journal-title":"Nature"},{"key":"ref_8","doi-asserted-by":"crossref","first-page":"80","DOI":"10.1038\/s41534-024-00873-2","article-title":"MadQCI: A heterogeneous and scalable SDN-QKD network deployed in production facilities","volume":"10","author":"Martin","year":"2024","journal-title":"npj Quantum Inf."},{"key":"ref_9","doi-asserted-by":"crossref","first-page":"453","DOI":"10.3390\/quantum2030032","article-title":"Demonstration of software defined network services utilizing quantum key distribution fully integrated with standard telecommunication network","volume":"2","author":"Lopez","year":"2020","journal-title":"Quantum Rep."},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"38","DOI":"10.1140\/epjqt\/s40507-023-00194-x","article-title":"Demonstration of a switched CV-QKD network","volume":"10","author":"Brunner","year":"2023","journal-title":"EPJ Quantum Technol."},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"5803","DOI":"10.1002\/sec.1737","article-title":"Software-defined networking (SDN): A survey","volume":"9","author":"Benzekki","year":"2016","journal-title":"Secur. Commun. Netw."},{"key":"ref_12","doi-asserted-by":"crossref","first-page":"20","DOI":"10.1109\/MCOM.2019.1800763","article-title":"The engineering of software-defined quantum key distribution networks","volume":"57","author":"Aguado","year":"2019","journal-title":"IEEE Commun. Mag."},{"key":"ref_13","unstructured":"Selentis, A., Makris, N., Papageorgopoulos, A., Konteli, P., Christodoulopoulos, K., Kanellos, G.T., and Syvridis, D. (2026). Evaluating Relayed and Switched Quantum Key Distribution (QKD) Network Architectures. arXiv."},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"104749","DOI":"10.1109\/ACCESS.2019.2930548","article-title":"QKD-Based Quantum Private Query Protocol in the Single-Photon Interference Communication System","volume":"7","author":"Liu","year":"2019","journal-title":"IEEE Access"},{"key":"ref_15","doi-asserted-by":"crossref","unstructured":"Horoschenkoff, P., Henrich, J., B\u00f6hn, R., Khan, I., R\u00f6diger, J., Gunkel, M., Bauch, M., Benda, J., Bl\u00e4cker, P., and Eichhammer, E. (2025). DemoQuanDT: A carrier-grade QKD network. arXiv.","DOI":"10.1364\/JOCN.563470"},{"key":"ref_16","doi-asserted-by":"crossref","unstructured":"Christodoulopoulos, K., Makris, N., Kanellos, G.T., and Syvridis, D. (2024, January 24\u201328). Optimizing key consumption in switched QKD networks. Proceedings of the 2024 Optical Fiber Communications Conference and Exhibition (OFC), San Diego, CA, USA.","DOI":"10.1364\/OFC.2024.W2A.34"},{"key":"ref_17","doi-asserted-by":"crossref","first-page":"139","DOI":"10.1109\/JLT.2019.2949864","article-title":"End-to-End Quantum Secured Inter-Domain 5G Service Orchestration over Dynamically Switched Flex-Grid Optical Networks Enabled by a q-ROADM","volume":"38","author":"Wang","year":"2020","journal-title":"J. Light. Technol."},{"key":"ref_18","doi-asserted-by":"crossref","first-page":"291","DOI":"10.1038\/s41377-024-01633-9","article-title":"Continuous-variable quantum passive optical network","volume":"13","author":"Hajomer","year":"2024","journal-title":"Light Sci. Appl."},{"key":"ref_19","doi-asserted-by":"crossref","unstructured":"Wenning, M., Patri, S.K., M\u00fcller, J., Autenrieth, A., Elbers, J.P., Rydlichowski, P., and Mas-Machuca, C. (2023, January 5\u20139). Towards optimized demand routing in QKD networks. Proceedings of the 2023 Optical Fiber Communications Conference and Exhibition (OFC), San Diego, CA, USA.","DOI":"10.23919\/OFC49934.2023.10116122"},{"key":"ref_20","doi-asserted-by":"crossref","first-page":"69","DOI":"10.1145\/1355734.1355746","article-title":"OpenFlow: Enabling innovation in campus networks","volume":"38","author":"McKeown","year":"2008","journal-title":"SIGCOMM Comput. Commun. Rev."},{"key":"ref_21","doi-asserted-by":"crossref","first-page":"67","DOI":"10.1038\/s41534-021-00400-7","article-title":"Experimental authentication of quantum key distribution with post-quantum cryptography","volume":"7","author":"Wang","year":"2021","journal-title":"npj Quantum Inf."},{"key":"ref_22","unstructured":"Enns, R., Bj\u00f6rklund, M., Bierman, A., and Sch\u00f6nw\u00e4lder, J. (2025, December 14). Network Configuration Protocol (NETCONF). RFC 6241. Available online: https:\/\/datatracker.ietf.org\/doc\/html\/rfc6241."},{"key":"ref_23","unstructured":"Bierman, A., Bj\u00f6rklund, M., and Watsen, K. (2025, December 14). RESTCONF Protocol. RFC 8040. Available online: https:\/\/datatracker.ietf.org\/doc\/html\/rfc8040."},{"key":"ref_24","unstructured":"ETSI Industry Specification Group (ISG) (2022). Quantum Key Distribution (QKD); Control Interface for Software Defined Networks, ETSI. Technical Report ETSI GS QKD 015 V2.1.1."},{"key":"ref_25","doi-asserted-by":"crossref","first-page":"404","DOI":"10.1364\/JOCN.513637","article-title":"Switching in quantum networks: An optimization investigation","volume":"16","author":"Karavias","year":"2024","journal-title":"J. Opt. Commun. 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