Optimal Encryption of Quantum Bits

Abstract

We characterize the complete set of protocols that may be used to securely encrypt n quantum bits using secret and random classical bits. In addition to the application of such quantum encryption protocols to quantum data security, our framework allows for generalizations of many classical cryptographic protocols to quantum data. We show that the encrypted state gives no information without the secret classical data, and that 2n random classical bits are the minimum necessary for informationally secure quantum encryption. Moreover, the quantum operations are shown to have a surprising structure in a canonical inner product space. This quantum encryption protocol is a generalization of the classical one time pad concept. A connection is made between quantum encryption and quantum teleportation, and this allows for a new proof of optimality of teleportation.