## Abstract

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.

Original language | English |
---|---|

Title of host publication | 10th Conference on the Theory of Quantum Computation, Communication and Cryptography, TQC 2015 |

Publisher | Schloss Dagstuhl- Leibniz-Zentrum fur Informatik GmbH, Dagstuhl Publishing |

Pages | 64-72 |

Number of pages | 9 |

Volume | 44 |

ISBN (Electronic) | 9783939897965 |

DOIs | |

Publication status | Published - 1 Nov 2015 |

Event | 10th Conference on the Theory of Quantum Computation, Communication and Cryptography, TQC 2015 - Brussels, Belgium Duration: 20 May 2015 → 22 May 2015 |

### Conference

Conference | 10th Conference on the Theory of Quantum Computation, Communication and Cryptography, TQC 2015 |
---|---|

Country | Belgium |

City | Brussels |

Period | 20/05/15 → 22/05/15 |

## Keywords

- Capacity
- Private information
- Quantum channels