When does noise increase the quantum capacity?

Understand

Superactivation is the property that two channels with zero quantum capacity can be used together to yield positive capacity.

• Here we demonstrate that this effect exists for a wide class of inequivalent channels, none of which can simulate each other.
• We also consider the case where one of two zero capacity channels are applied, but the sender is ignorant of which one is applied.
• We find examples where the greater the entropy of mixing of the channels, the greater the lower bound for the capacity.