We analyze the possibility of copying (that is, cloning) arbitrary states 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 of its output does not depend on the input. We also examine a machine which combines a unitary transformation and a selective measurement to produce good copies of states in the neighborhood of a particular state. We discuss the problem of measurement of the output states. © 1996 The American Physical Society.

• Received 5 February 1996

DOI:https://doi.org/10.1103/PhysRevA.54.1844