QUANTUM COPYING - BEYOND THE NO-CLONING THEOREM

We analyze the possibility of copying (that is, cloning) arbitrary sta tes of a quantum-mechanical spin-1/2 system. We show that there exists a ''universal quantum-copying machine'' (i.e., transformation) which approximately copies quantum-mechanical states such that the quality o f its output does not depend on the input, We also examine a machine w hich combines a unitary transformation and a selective measurement to produce good copies of slates in the neighborhood of a particular stat e. We discuss the problem of measurement of the output states.