Quantum Capacity Can Be Greater Than Private Information for Arbitrarily Many Uses

Authors David Elkouss, Sergii Strelchuk

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David Elkouss
Sergii Strelchuk

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David Elkouss and Sergii Strelchuk. Quantum Capacity Can Be Greater Than Private Information for Arbitrarily Many Uses. In 10th Conference on the Theory of Quantum Computation, Communication and Cryptography (TQC 2015). Leibniz International Proceedings in Informatics (LIPIcs), Volume 44, pp. 64-72, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2015)


The quantum capacity of a quantum channel is always smaller than the capacity of the channel for private communication. However, both quantities are given by the infinite regularization of respectively the coherent and the private information. Here, we construct a family of channels for which the private and coherent information can remain strictly superadditive for unbounded number of uses. We prove this by showing that the coherent information is strictly larger than the private information of a smaller number of uses of the channel. It turns out that even though the quantum capacity is upper bounded by the private capacity, the non-regularized quantities can be interleaved. From an operational point of view, the private capacity can be used for gauging the practical value of quantum channels for secure communication and, consequently, for key distribution. We thus show that in order to evaluate the interest a channel for this task it is necessary to optimize the private information over an unlimited number of uses of the channel.
  • Quantum channels
  • capacity
  • private information


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