On 1–qubit channels
نویسنده
چکیده
The entropy HT (ρ) of a state with respect to a channel T and the Holevo capacity of the channel require the solution of difficult variational problems. For a class of 1-qubit channels, which contains all the extremal ones, the problem can be significantly simplified by associating an Hermitian antilinear operator θ to every channel of the considered class. The channel’s concurrence CT can be expressed by θ and turns out to be a flat roof. This allows to write down an explicit expression for HT . Its maximum would give the Holevo (1–shot) capacity.
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