Onset of superactivation of quantum capacity

TL;DR

This paper investigates superactivation of quantum capacity in the finite-blocklength regime, demonstrating that as few as 17 channel uses can enable quantum communication impossible with individual channels.

Contribution

It introduces a finite-blocklength definition of superactivation and provides numerical methods to certify superactivation in this regime.

Findings

Superactivation occurs with as few as 17 channel uses.

Finite-blocklength superactivation can enable quantum communication.

Demonstrates potential for experimental realization.

Abstract

Superactivation of quantum capacity is the phenomenon whereby two quantum channels, each with zero quantum capacity, can exhibit a strictly positive capacity when used in tandem. In this work, we explore superactivation in the previously unexplored non-asymptotic regime of finitely many channel uses. We give a definition of finite-blocklength superactivation and propose numerical methods that can certify it. Then, focusing on the 50% erasure and positive-partial-transpose channels considered in the original work on superactivation, we show that as few as 17 uses of the joint channel already enable qubit transmission with a fidelity unattainable by any number of uses of either channel alone, demonstrating a strong finite-blocklength form of superactivation and opening the door to experimental demonstration.