QUANTUM COPYING - A NETWORK

We present a network consisting of quantum gates that produces two imp erfect copies of an arbitrary qubit. The quality of the copies does no t depend on the input qubit. We also show that for a restricted class of inputs it is possible to use a very similar network to produce thre e copies instead of two. For qubits in this class, the copy quality is again independent of the input and is the-same as the quality of the copies produced by the two-copy network. [S1050-2947(97)09510-3].