How much information can a physical system fundamentally communicate?

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After a brief motivation of this question, the presentation is divided in two parts. We first introduce the principle of quantum information causality, which states the maximum amount of quantum information that a transmitted quantum system can communicate as a function of its Hilbert space dimension, independently of any quantum physical resources previously shared by the communicating parties. The second part of the talk considers superdense coding within the framework of general probabilistic theories and addresses the question of why in quantum theory, no more than two bits can be communicated by transmission of a single, entangled, qubit. We introduce hyperdense coding in general probabilistic theories: superdense coding in which N

> 2 bits are communicated by transmission of a system  that locally

encodes at most one bit, and present protocols with N arbitrarily large. Our hyperdense coding protocols imply superadditive classical

capacities: two entangled systems can encode N > 2 bits, even though each system locally encodes at most one bit. Our protocols violate either a reversibility condition or tomographic locality.