Quantum copying: Beyond the no-cloning theorem

TLDR

The study investigates the possibility of cloning arbitrary spin‑½ quantum states. The authors analyze a universal quantum‑copying machine and a unitary‑plus‑measurement device that produce approximate copies of spin‑½ states, and discuss how to measure the outputs. They demonstrate a universal quantum‑copying machine that produces approximate copies of spin‑½ states with quality independent of the input. © 1996 The American Physical Society.

Abstract

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. \textcopyright{} 1996 The American Physical Society.