Nonadditivity in classicalquantum wiretap channels
Abstract
Due to Csiszar and Koerner, the private capacity of classical wiretap channels has a singleletter characterization in terms of the private information. For quantum wiretap channels, however, it is known that regularization of the private information is necessary to reach the capacity. Here, we study hybrid classicalquantum wiretap channels in order to resolve to what extent quantum effects are needed to witness nonadditivity phenomena in quantum Shannon theory. For wiretap channels with quantum inputs but classical outputs, we prove that the characterization of the capacity in terms of the private information stays singleletter. Hence, entangled input states are of no asymptotic advantage in this setting. For wiretap channels with classical inputs, we show by means of explicit examples that the private information already becomes nonadditive when either one of the two receivers becomes quantum (with the other receiver staying classical). This gives nonadditivity examples that are not caused by entanglement and illustrates that quantum adversaries are strictly different from classical adversaries in the wiretap model.
 Publication:

arXiv eprints
 Pub Date:
 February 2020
 arXiv:
 arXiv:2002.06580
 Bibcode:
 2020arXiv200206580T
 Keywords:

 Quantum Physics;
 Computer Science  Information Theory
 EPrint:
 v2: updates from review process