"pretty Strong" converse for the quantum capacity of degradable channels

Ciara Morgan, Andreas Winter

Research output: Contribution to journalArticlepeer-review

28 Scopus citations


We exhibit a possible road toward a strong converse for the quantum capacity of degradable channels. In particular, we show that all degradable channels obey what we call a "pretty strong" converse: when the code rate increases above the quantum capacity, the fidelity makes a discontinuous jump from 1 to at most 1/√2, asymptotically. A similar result can be shown for the private (classical) capacity. Furthermore, we can show that if the strong converse holds for symmetric channels (which have quantum capacity zero), then degradable channels obey the strong converse. The above-mentioned asymptotic jump of the fidelity at the quantum capacity then decreases from 1 to 0.

Original languageEnglish
Article number6663606
Pages (from-to)317-333
Number of pages17
JournalIEEE Transactions on Information Theory
Issue number1
StatePublished - Jan 2014
Externally publishedYes


  • Quantum information
  • channel coding
  • error-rate trade-off
  • private classical information
  • smooth entropies
  • strong converse


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