Editing No-cloning theorem (section)

== Imperfect cloning ==
Even though it is impossible to make perfect copies of an unknown quantum state, it is possible to produce imperfect copies. This can be done by coupling a larger auxiliary system to the system that is to be cloned, and applying a [[unitary transformation]] to the combined system. If the unitary transformation is chosen correctly, several components of the combined system will evolve into approximate copies of the original system. In 1996, V. Buzek and M. Hillery showed that a universal cloning machine can make a clone of an unknown state with the surprisingly high fidelity of 5/6.<ref>{{cite journal |last1=Bužek |first1=V. |last2=Hillery |first2=M. |title=Quantum Copying: Beyond the No-Cloning Theorem |journal=Phys. Rev. A |volume=54 |issue=3 |pages=1844–1852 |year=1996 |doi=10.1103/PhysRevA.54.1844 |pmid=9913670 |arxiv=quant-ph/9607018 |bibcode=1996PhRvA..54.1844B |s2cid=1446565 }}</ref>

Imperfect [[quantum cloning]] can be used as an [[eavesdropping attack]] on  [[quantum cryptography]] protocols, among other uses in quantum information science.