Free encoding capacity: A universal unit for quantum resources

Abstract

A perfect d-dimensional quantum channel can convey log d-bits of classical information by encoding messages in d-orthogonal quantum states. Alternatively, for every quantum state at the senders end, there exist d-encoding operations which produce d-orthogonal quantum states. Transmitting which via a d-level perfect quantum channel it is possible to communicate log d-bits of classical information. But what if the set of encoding operations is restricted only within a physically constrained class? Here, we consider such a class of encoding operations to be the set of free operations for any quantum resource theory and show that the constrained capacity - namely, the free encoding capacity (FEC) emerged as a unit of the corresponding quantum resource. Moreover, we show that for the pointed resource theories - a resource theory admitting only a single free state - FEC becomes a faithful resource measure also. We also discuss the implications of FEC in the question of resource-theoretic state transformations and the possibility of extending its faithfulness for general quantum resource theories.