Superadditivity and boosting coherent information using useless channels

Superadditivity of the coherent information makes it hard to understand and compute the quantum capacity of a quantum channel, a central quantity in quantum information.

Recently, by using a qubit channel in parallel with itself, Leditzky, Leung and Smith [Phys. Rev. Lett. 121, 160501 (2018)] produced simple examples of superadditivity. We extend their results to a larger class of channels with qubit inputs and some channels with qutrit inputs. Some of these exhibit superadditivity in the sense that the coherent information of a channel is boosted when placed in parallel with a zero capacity channel, in particular a symmetric amplitude damping channel. Our superadditivity examples include limits of simple pcubed qubit and qutrit channels [Phys. Rev. A 94, 052331 (2016)].