Strategies and networks for state-dependent quantum cloning

State-dependent cloning machines that have so far been considered either de terministically copy a set of states approximately or probabilistically cop y them exactly. In considering the case of two equiprobable pure states, we derive the maximum global fidelity of N approximate clones given M initial exact copies, where N>M. We also consider strategies that interpolate betw een approximate and exact cloning. A tight inequality is obtained that expr esses a trade-off between the global fidelity and success probability. This inequality is found to tend, in the limit N --> infinity, to a known inequ ality that expresses the trade-off between error and inconclusive result pr obabilities for state-discrimination measurements. Quantum-computational ne tworks are also constructed for the kinds of cloning machine we describe. F or this purpose, we introduce two gates: the distinguishability transfer an d state separation gates. Their key properties are described and we show ho w they may be decomposed into basic operations.