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Revisiting additivity violation of quantum channels. (English) Zbl 1300.81016

Summary: We prove additivity violation of minimum output entropy of quantum channels by straightforward application of \({\epsilon}\)-net argument and Lévy’s lemma. The additivity conjecture was disproved initially by Hastings. Later, a proof via asymptotic geometric analysis was presented by Aubrun, Szarek and Werner, which uses Dudley’s bound on Gaussian process (or Dvoretzky’s theorem with Schechtman’s improvement). In this paper, we develop another proof along Dvoretzky’s theorem in Milman’s view, showing additivity violation in broader regimes than the existing proofs. Importantly, Dvoretzky’s theorem works well with norms to give strong statements, but these techniques can be extended to functions which have norm-like structures-positive homogeneity and triangle inequality. Then, a connection between Hastings’ method and ours is also discussed. In addition, we make some comments on relations between regularized minimum output entropy and classical capacity of quantum channels.

MSC:

81P45 Quantum information, communication, networks (quantum-theoretic aspects)
94A40 Channel models (including quantum) in information and communication theory
94A17 Measures of information, entropy

References:

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