Contractivity of positive and trace-preserving maps under \(L_{p}\) norms. (English) Zbl 1112.15007

Summary: We provide a complete picture of contractivity of trace preserving positive maps with respect to \(p\)-norms. We show that for \(p > 1\) contractivity holds in general if and only if the map is unital. When the domain is restricted to the traceless subspace of Hermitian matrices, then contractivity is shown to hold in the case of qubits for arbitrary \(p \geqslant 1\) and in the case of qutrits if and only if \(p=1, \infty\). In all noncontractive cases best possible bounds on the \(p\)-norms are derived.


15A04 Linear transformations, semilinear transformations
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