Information transfer and fidelity in quantum copiers

We find that very different quantum copying machines are optimal depending on the indicator used to assess their performance. Several quantum copying machine models acting on non-orthogonal input states are investigated, and assessed according to two types of criteria: Transfer of (Shannon) information encoded in the initial states to the copies, and fidelity between the copies and the initial states. Transformations which optimise information transfer for messages encoded in qubits are found. If the message is decoded one symbol at-a-time, information is best copied by a Wootters-Zurek copier.Comment: 14 pages, 3 Figures, REVTeX. Accepted Physical Review A. Corrected minor grammatical error