Uniform finite-dimensional approximation of basic capacities of energy-constrained channels
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We consider energy-constrained infinite-dimensional quantum channels from a given system (satisfying a certain condition) to any other systems. We show that dealing with basic capacities of these channels we may assume (accepting arbitrarily small error \(\varepsilon \)) that all channels have the same finite-dimensional input space—the subspace corresponding to the \(m(\varepsilon )\) minimal eigenvalues of the input Hamiltonian. We also show that for the class of energy-limited channels (mapping energy-bounded states to energy-bounded states) the above result is valid with substantially smaller dimension \(m(\varepsilon )\). The uniform finite-dimensional approximation allows us to prove the uniform continuity of the basic capacities on the set of all quantum channels with respect to the strong (pointwise) convergence topology. For all the capacities, we obtain continuity bounds depending only on the input energy bound and the energy-constrained diamond-norm distance between quantum channels (generating the strong convergence on the set of quantum channels).
KeywordsFinite-dimensional subchannel Energy constraint m-Restricted capacities Quantum conditional mutual information Strong convergence of channels Energy-constrained diamond norm
I am grateful to the participants of the workshop “Recent advances in continuous variable quantum information theory”, Barcelona, April 2016, for the stimulating discussion. I am grateful to A. Winter for sending me a preliminary version of the paper  used in this work. I am also grateful to A.S. Holevo and G.G. Amosov for useful comments and to M.M. Wilde for valuable communication concerning capacities of infinite-dimensional channels with energy constraints. Special thanks to Yu.V. Andreev and L.V. Kuzmin for the help with MATLAB. Many thanks are due to unknown reviewers for useful suggestions.
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