# Communication Enhancement Through Quantum Coherent Control of $N$ Channels in an Indefinite Causal-order Scenario

## TL;DR

This paper explores how quantum coherent control of multiple channels in indefinite causal order can significantly enhance information transmission, surpassing classical limits, with explicit formulas derived for small networks.

## Contribution

It extends quantum control to networks of N channels with arbitrary capacities, providing a formalism and explicit results for the quantum switch output and Holevo limit.

## Key findings

- Transmission for three channels is twice that of two channels with full superposition of causal orders.
- Explicit formulas derived for N=2 and N=3 channels.
- Quantum indefinite causal order enhances information capacity beyond classical limits.

## Abstract

In quantum Shannon theory, transmission of information is enhanced by quantum features. Up to very recently, the trajectories of transmission remained fully classical. Recently, a new paradigm was proposed by playing quantum tricks on two completely depolarizing quantum channels i.e. using coherent control in space or time of the two quantum channels. We extend here this control to the transmission of information through a network of an arbitrary number $N$ of channels with arbitrary individual capacity i.e. information preservation characteristics in the case of indefinite causal order. We propose a formalism to assess information transmission in the most general case of $N$ channels in an indefinite causal order scenario yielding the output of such transmission. Then we explicitly derive the quantum switch output and the associated Holevo limit of the information transmission for $N=2$, $N=3$ as a function of all involved parameters. We find in the case $N=3$ that the transmission of information for three channels is twice of transmission of the two channel case when a full superposition of all possible causal orders is used.