Superadditivity of communication capacity using entangled inputs

TL;DR

This paper demonstrates that entangled inputs can increase the classical communication capacity of quantum channels, challenging the long-held additivity conjecture and revealing new possibilities for quantum information transmission.

Contribution

The authors provide the first explicit counter-example showing that entanglement can enhance quantum channel capacity, disproving the additivity conjecture.

Findings

Entanglement can increase the classical capacity of quantum channels.

The additivity conjecture is false, as shown by the counter-example.

Further research is needed to identify other scenarios where entanglement boosts capacity.

Abstract

The design of error-correcting codes used in modern communications relies on information theory to quantify the capacity of a noisy channel to send information [1]. This capacity can be expressed using the mutual information between input and output for a single use of the channel: although correlations between subsequent input bits are used to correct errors, they cannot increase the capacity. For quantum channels, it has been an open question whether entangled input states can increase the capacity to send classical information [2]. The additivity conjecture [3,4] states that entanglement does not help, making practical computations of the capacity possible. While additivity is widely believed to be true, there is no proof. Here we show that additivity is false, by constructing a random counter-example. Our results show that the most basic question of classical capacity of a quantum channel remains open, with further work needed to determine in which other situations entanglement can boost capacity.