title: Optimal cloning of unitary transformation.
abstract: 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.
authors.affiliation: QUIT Group, Dipartimento di Fisica "A. Volta" and INFN Sezione di Pavia, via Bassi 6, 27100 Pavia, Italy. chiribella@fisicavolta.unipv.it
authors.forename: Giulio
authors.initials: G
authors.lastname: Chiribella
authors.forename: Giacomo Mauro
authors.initials: GM
authors.lastname: D'Ariano
authors.forename: Paolo
authors.initials: P
authors.lastname: Perinotti