Maximal Elements of Quantum Communication

Teiko Heinosaari and Oskari Kerppo

Quantum Information and Computation, Faculty of Information Technology, University of Jyväskylä, 40014, Finland

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Abstract

A prepare-and-measure scenario is naturally described by a communication matrix that collects all conditional outcome probabilities of the scenario into a row-stochastic matrix. The set of all possible communication matrices is partially ordered via the possibility to transform one matrix to another by pre- and post-processings. By considering maximal elements in this preorder for a subset of matrices implementable in a given theory, it becomes possible to identify communication matrices of maximum utility, i.e., matrices that are not majorized by any other matrices in the theory. The identity matrix of an appropriate size is the greatest element in classical theories, while the maximal elements in quantum theory have remained unknown. We completely characterize the maximal elements in quantum theory, thereby revealing the essential structure of the set of quantum communication matrices. In particular, we show that the identity matrix is the only maximal element in quantum theory but, as opposed to a classical theory, it is not the greatest element. Quantum theory can hence be seen to be distinct from classical theory by the existence of incompatible communication matrices.

This article studies the structure of operational theories. A prepare-and-measure scenario can be described by collecting all conditional outcome probabilities into a row-stochastic matrix. We call such matrices communication matrices, and each prepare-and-measure scenario is described by such a matrix. In all operational theories the set of implementable communication matrices is partially ordered via the possibility to transform one matrix to another by pre- and post-processings. By studying this binary relation between communication matrices we are able to identify maximal elements in both classical and quantum theory. Namely, we find that the identity matrix of appropriate size is a the greatest element in classical theory, while in quantum theory the identity matrix is maximal. This result allows us to conclude that there exists incompatible communication matrices in quantum theory, where compatibility is defined as the existence of a common greater element in the implementable set. Thus our result is a first step in the study of compatibility of prepare-and-measure scenarios.

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Cited by

[1] Teiko Heinosaari and Mark Hillery, "Can a qudit carry more information than a dit?", Contemporary Physics 65 1, 2 (2024).

[2] Joonwoo Bae, Kieran Flatt, Teiko Heinosaari, Oskari Kerppo, Karthik Mohan, Andrés Muñoz-Moller, and Ashutosh Rai, "Random exclusion codes: Quantum advantages of single-shot communication", Physical Review Research 8 1, 013171 (2026).

[3] Samgeeth Puliyil, Leevi Leppäjärvi, and Mário Ziman, "Semi-device-independent channel identification with communication matrices", arXiv:2511.14273, (2025).

The above citations are from Crossref's cited-by service (last updated successfully 2026-08-19 14:50:59) and SAO/NASA ADS (last updated successfully 2026-08-19 14:51:06). The list may be incomplete as not all publishers provide suitable and complete citation data.