Superdense coding in the resource theory of asymmetry

Abstract

We consider a task in which classical information is encoded into a quantum system by an operation restricted by symmetry. The maximum amount of classical information that can be encoded under this restriction, namely the symmetry-restricted classical information capacity, depends on the initial state of the system. Our focus is on whether the capacity of an asymmetric state can be strictly larger than that of any symmetric states, whereas the latter is a strictly positive constant. That is, we ask whether an analog of superdense coding is implementable in the resource theory of asymmetry. We prove that superdense coding is implementable if and only if the unitary representation of the symmetry is non-Abelian and reducible. Thereby we provide an information theoretical classification of symmetries of quantum systems.