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We give a detailed account of the importance and the operational meaning of the interaction of an agent with the vacuum \u2013 in particular its role in the process matrix description. Our analysis shows that the interaction with the vacuum should be treated as an operation, on equal footing with all other interactions. This raises the issue of counting such operations in an operational manner. Motivated by this analysis, we apply the process matrix formalism to capped Fock spaces using the framework of second quantisation, in order to characterise protocols with an indefinite number of identical particles.<\/jats:p>","DOI":"10.22331\/q-2023-04-20-986","type":"journal-article","created":{"date-parts":[[2023,4,20]],"date-time":"2023-04-20T16:42:14Z","timestamp":1682008934000},"page":"986","update-policy":"https:\/\/doi.org\/10.22331\/q-crossmark-policy-page","source":"Crossref","is-referenced-by-count":2,"title":["Operational interpretation of the vacuum and process matrices for identical particles"],"prefix":"10.22331","volume":"7","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-4155-7396","authenticated-orcid":false,"given":"Ricardo","family":"Faleiro","sequence":"first","affiliation":[{"name":"Instituto de Telecomunica\u00e7\u00f5es, Avenida Rovisco Pais 1, 1049-001, Lisboa, Portugal"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-9345-4321","authenticated-orcid":false,"given":"Nikola","family":"Paunkovic","sequence":"additional","affiliation":[{"name":"Instituto de Telecomunica\u00e7\u00f5es, Avenida Rovisco Pais 1, 1049-001, Lisboa, Portugal"},{"name":"Departamento de Matem\u00e1tica, Instituto Superior T\u00e9cnico, Universidade de Lisboa, Avenida Rovisco Pais 1, 1049-001, Lisboa, Portugal"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-6977-4870","authenticated-orcid":false,"given":"Marko","family":"Vojinovic","sequence":"additional","affiliation":[{"name":"Institute of Physics, University of Belgrade, Pregrevica 118, 11080 Belgrade, Serbia"}]}],"member":"9598","published-online":{"date-parts":[[2023,4,20]]},"reference":[{"key":"0","doi-asserted-by":"publisher","unstructured":"O. 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