Information transfer and fidelity in quantum copiers - art. no. 062304

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 a ssessed according to two types of criteria: transfer of (Shannon) informati on encoded in the initial states to the copies, and fidelity between the co pies and the initial states. Transformations that optimize information tran sfer for messages encoded in qubits are found for three situations: (1) whe n 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 th e copier produces independent copies. If the message is decoded one symbol at a time, information is best copied by a Wootters-Zurek copier.