Conjugate degradable channels: A new class of quantum channels with a tractable quantum capacity formula

Abstract

Conjugate degradable channels are channels whose quantum capacity is calculable. They were defined and studied in [1] where, however, only channels that are both degradable and conjugate degradable were found. In this paper we bring the very first example of conjugate degradable channels that are not degradable. We also identify the physical origin of these channels and show that they belong to the class of optimal universal asymmetric cloners. We thus not only positively answer the question whether conjugate degradable channels form a new class of channels with a single-letter capacity formula but as a side result we also calculate the quantum capacity of the optimal asymmetric 1->1+1 cloning channels.