When does noise increase the quantum capacity?

TL;DR

This paper investigates superactivation in quantum channels, demonstrating its prevalence across various channels, including depolarizing channels, and exploring how channel mixing entropy influences quantum capacity.

Contribution

It shows superactivation occurs in a broad class of inequivalent channels and analyzes the impact of channel mixing entropy on capacity bounds.

Findings

Superactivation exists for many inequivalent channels.

Greater entropy of mixing can increase capacity lower bounds.

Superactivation is demonstrated with depolarizing channels.

Abstract

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. Finally, we show that the effect of superactivation is rather generic by providing example of superactivation using the depolarizing channel.