{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,6,6]],"date-time":"2026-06-06T08:03:38Z","timestamp":1780733018678,"version":"3.54.1"},"reference-count":34,"publisher":"Verein zur Forderung des Open Access Publizierens in den Quantenwissenschaften","license":[{"start":{"date-parts":[[2022,8,23]],"date-time":"2022-08-23T00:00:00Z","timestamp":1661212800000},"content-version":"unspecified","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":["quantum-journal.org"],"crossmark-restriction":false},"short-container-title":["Quantum"],"abstract":"<jats:p>Open-air microwave quantum communication and metrology protocols must be able to transfer quantum resources from a cryostat, where they are created, to an environment dominated by thermal noise. Indeed, the states carrying such quantum resources are generated in a cryostat characterized by a temperature <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\"><mml:msub><mml:mi>T<\/mml:mi><mml:mtext>in<\/mml:mtext><\/mml:msub><mml:mo>&amp;#x2243;<\/mml:mo><mml:mn>50<\/mml:mn><\/mml:math> mK and an intrinsic impedance <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\"><mml:msub><mml:mi>Z<\/mml:mi><mml:mtext>in<\/mml:mtext><\/mml:msub><mml:mo>=<\/mml:mo><mml:mn>50<\/mml:mn><mml:mspace width=\"thinmathspace\"\/><mml:mi mathvariant=\"normal\">&amp;#x03A9;<\/mml:mi><\/mml:math>. Then, an antenna-like device is required to transfer them with minimal losses into open air, characterized by an intrinsic impedance of <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\"><mml:msub><mml:mi>Z<\/mml:mi><mml:mtext>out<\/mml:mtext><\/mml:msub><mml:mo>=<\/mml:mo><mml:mn>377<\/mml:mn><mml:mspace width=\"thinmathspace\"\/><mml:mi mathvariant=\"normal\">&amp;#x03A9;<\/mml:mi><\/mml:math> and a temperature <mml:math xmlns:mml=\"http:\/\/www.w3.org\/1998\/Math\/MathML\"><mml:msub><mml:mi>T<\/mml:mi><mml:mtext>out<\/mml:mtext><\/mml:msub><mml:mo>&amp;#x2243;<\/mml:mo><mml:mn>300<\/mml:mn><\/mml:math> K. This device accomplishes a smooth impedance matching between the cryostat and the open air. Here, we study the transmission of two-mode squeezed thermal states, developing a technique to design the optimal shape of a coplanar antenna to preserve the entanglement. Based on a numerical optimization procedure, we find the optimal shape of the impedance, and we propose a functional ansatz to qualitatively describe this shape. Additionally, this study reveals that the reflectivity of the antenna is very sensitive to this shape, so that small changes dramatically affect the outcoming entanglement, which could have been a limitation in previous experiments employing commercial antennae. This work is relevant in the fields of microwave quantum sensing and quantum metrology with special application to the development of the quantum radar, as well as any open-air microwave quantum communication protocol.<\/jats:p>","DOI":"10.22331\/q-2022-08-23-783","type":"journal-article","created":{"date-parts":[[2022,8,23]],"date-time":"2022-08-23T13:48:36Z","timestamp":1661262516000},"page":"783","update-policy":"https:\/\/doi.org\/10.22331\/q-crossmark-policy-page","source":"Crossref","is-referenced-by-count":9,"title":["Coplanar Antenna Design for Microwave Entangled Signals Propagating in Open Air"],"prefix":"10.22331","volume":"6","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-9645-9681","authenticated-orcid":false,"given":"Tasio","family":"Gonzalez-Raya","sequence":"first","affiliation":[{"name":"Department of Physical Chemistry, University of the Basque Country UPV\/EHU, Apartado 644, 48080 Bilbao, Spain"},{"name":"EHU Quantum Center, University of the Basque Country UPV\/EHU"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-1615-9035","authenticated-orcid":false,"given":"Mikel","family":"Sanz","sequence":"additional","affiliation":[{"name":"Department of Physical Chemistry, University of the Basque Country UPV\/EHU, Apartado 644, 48080 Bilbao, Spain"},{"name":"EHU Quantum Center, University of the Basque Country UPV\/EHU"},{"name":"Basque Center for Applied Mathematics (BCAM), Alameda de Mazarredo 14, 48009 Bilbao, Basque Country, Spain"},{"name":"IKERBASQUE, Basque Foundation for Science, Plaza Euskadi 5, 48009 Bilbao, Spain"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"9598","published-online":{"date-parts":[[2022,8,23]]},"reference":[{"key":"0","doi-asserted-by":"publisher","unstructured":"J. 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