Interplays between classical and quantum entanglement-assisted communication scenarios

Prepare and measure scenarios, in their many forms, can be seen as basic building blocks of communication tasks. As such, they can be used to analyze a diversity of classical and quantum protocols -- of which dense coding and random access codes are key examples -- in a unified manner. In particular, the use of entanglement as a resource in prepare and measure scenarios have only recently started to be systematically investigated, and many crucial questions remain open. In this work, we explore such scenarios and provide answers to some seminal questions. More specifically, we show that, in scenarios where entanglement is a free resource, quantum messages are equivalent to classical ones with twice the capacity. We also prove that, in such scenarios, it is always advantageous for the parties to share entangled states of dimension greater than the transmitted message. Finally, we show that unsteerable states cannot provide advantages in classical communication tasks -- tasks where classical messages are transmitted --, thus proving that not all entangled states are useful resources in these scenarios and establishing an interesting link between quantum steering and nonclassicality in prepare and measure scenarios.Comment: 7+6 pages, 2+0 figure