Information transfer and fidelity in quantum copiers

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Abstract
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 nonorthogonal 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 that optimize information transfer for messages encoded in qubits are found for three situations: (1) when the message is decoded one state at a time; (2) with simple schemes that allow the message to be encoded using block-coding schemes; and (3) when the copier produces independent copies. If the message is decoded one symbol at a time, information is best copied by a Wootters-Zurek copier.

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