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Lett."],"published-print":{"date-parts":[[2026,9,1]]},"abstract":"<jats:p>Photonic hyperentanglement enables increased information capacity and\n\t\t\t\t\tenhanced functionality for quantum communication and networking.\n\t\t\t\t\tHowever, synchronization of hyperentangled photon pairs requires\n\t\t\t\t\tmaintaining correlations simultaneously across multiple degrees of\n\t\t\t\t\tfreedom (DOFs). The preservation of polarization and energy-time\n\t\t\t\t\tentanglement in hyperentangled photon pairs is demonstrated using a\n\t\t\t\t\tfree-space optical delay line based on nested Herriott cells. After a\n\t\t\t\t\tdelay of 647.0(6) ns, a two-photon interference visibility of 93.9(3)%\n\t\t\t\t\tis observed in the energy-time DOF, while a CHSH parameter of 2.758(5)\n\t\t\t\t\tis obtained in the polarization DOF. These results confirm that\n\t\t\t\t\tentanglement correlations in both DOFs are preserved after propagation\n\t\t\t\t\tthrough the delay line. They demonstrate that free-space optical delay\n\t\t\t\t\tlines are compatible with complex photonic quantum states and provide\n\t\t\t\t\ta promising route toward delay-based memories for synchronization and\n\t\t\t\t\tmultiplexing in quantum networks.<\/jats:p>","DOI":"10.1364\/ol.606763","type":"journal-article","created":{"date-parts":[[2026,8,4]],"date-time":"2026-08-04T18:00:09Z","timestamp":1785866409000},"page":"4781","update-policy":"https:\/\/doi.org\/10.1364\/crossmark_policy","source":"Crossref","is-referenced-by-count":0,"title":["High-fidelity preservation of photonic hyperentanglement in a free-space optical delay line"],"prefix":"10.1364","volume":"51","author":[{"ORCID":"https:\/\/orcid.org\/0000-0001-9104-0945","authenticated-orcid":true,"given":"Yu","family":"Guo","sequence":"first","affiliation":[{"id":[{"id":"https:\/\/ror.org\/01tgyzw49","id-type":"ROR","asserted-by":"publisher"}]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0001-1655-7319","authenticated-orcid":true,"given":"Arya","family":"Chowdhury","sequence":"additional","affiliation":[{"id":[{"id":"https:\/\/ror.org\/01tgyzw49","id-type":"ROR","asserted-by":"publisher"}]},{"name":"National University of Singapore"},{"name":"Thales Research and Technology"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0005-5907-4797","authenticated-orcid":true,"given":"Pranay","family":"Tiwari","sequence":"additional","affiliation":[{"name":"National University of Singapore"}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0009-0002-5468-413X","authenticated-orcid":true,"given":"Jia Boon","family":"Chin","sequence":"additional","affiliation":[{"id":[{"id":"https:\/\/ror.org\/01tgyzw49","id-type":"ROR","asserted-by":"publisher"}]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-6926-706X","authenticated-orcid":true,"given":"Anindya","family":"Banerji","sequence":"additional","affiliation":[{"id":[{"id":"https:\/\/ror.org\/01tgyzw49","id-type":"ROR","asserted-by":"publisher"}]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-5866-1141","authenticated-orcid":true,"given":"Alexander","family":"Ling","sequence":"additional","affiliation":[{"id":[{"id":"https:\/\/ror.org\/01tgyzw49","id-type":"ROR","asserted-by":"publisher"}]},{"name":"National University of Singapore"}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"285","published-online":{"date-parts":[[2026,8,18]]},"reference":[{"key":"ol-51-17-4781-R1","doi-asserted-by":"publisher","first-page":"46","DOI":"10.1016\/j.scib.2016.11.007","type":"journal-article","volume":"62","author":"Deng","year":"2017","journal-title":"Sci. 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