Optimality of private quantum channels

Abstract

We addressed the question of optimality of private quantum channels. We have shown that the Shannon entropy of the classical key necessary to securely transfer the quantum information is lower bounded by the entropy exchange of the private quantum channel and the von Neumann entropy of the ciphertext state ϱ(0). Based on these bounds we have shown that decomposition of private quantum channels into orthogonal unitaries (if they exist) optimizes the entropy. For non-ancillary single-qubit PQC we have derived the optimal entropy for the arbitrary set of plaintexts. In particular, we have shown that except when the (closure of the) set of plaintexts contains all states, one bit key is sufficient. We characterized and analysed all the possible single-qubit private quantum channels for an arbitrary set of plaintexts. For the set of plaintexts consisting of all qubit states we have characterized all possible approximate private quantum channels and we have derived the relation between the security parameter and the corresponding minimal entropy.