On Norms of Completely Positive Maps
King and Ruskai asked whether the norm of a completely positive map acting between Schatten classes of operators is equal to that of its restriction to the real subspace of self-adjoint operators. Proofs have been promptly supplied by Watrous and Audenaert. Here we provide one more proof, in fact of a slightly more general fact, under the (slightly weaker) assumption of 2-positivity. The argument is elementary and self-contained.
KeywordsNorms of completely positive maps Schatten norms 2-positivity
Mathematics Subject Classification (2000)46L07 46N50 47L07 47L20 81P68
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