  This work overviews the single-particle two-way communication protocol
recently introduced by del Santo and Daki\'c (dSD), and analyses it using the
process matrix formalism. We give a detailed account of the importance and the
operational meaning of the interaction of an agent with the vacuum -- 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.
