Phys. Rev. Lett. 101, 180504 (2008) [4 pages]Optimal Cloning of Unitary TransformationReceived 7 April 2008; published 30 October 2008 After proving a general no-cloning theorem for black boxes, we derive the optimal universal cloning of unitary transformations, from one to two copies. The optimal cloner is realized by quantum channels with memory, and greatly outperforms the optimal measure-and-reprepare cloning strategy. Applications are outlined, including two-way quantum cryptographic protocols. © 2008 The American Physical Society URL:
http://link.aps.org/doi/10.1103/PhysRevLett.101.180504
DOI:
10.1103/PhysRevLett.101.180504
PACS:
03.67.Ac, 03.65.Ta, 03.67.Dd
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