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
