# Causal order as a resource for quantum communication

## TL;DR

This paper demonstrates that indefinite causal order limits quantum communication but still allows classical information transfer, establishing causal order as a resource in quantum communication theory.

## Contribution

It introduces a framework for quantum communication with uncertain causal order, highlighting causal order as a new resource similar to entanglement.

## Key findings

- Maximal causal uncertainty prevents asymptotic quantum communication
- Classical information transfer remains possible under causal uncertainty
- Provides a foundation for quantum information theory in indefinite causal structures

## Abstract

In theories of communication, it is usually presumed that the involved parties perform actions in a fixed causal order. However, practical and fundamental reasons can induce uncertainties in the causal order. Here we show that a maximal uncertainty in the causal order forbids asymptotic quantum communication, while still enabling the noisy transfer of classical information. Therefore causal order, like shared entanglement, is an additional resource for communication. The result is formulated within an asymptotic setting for processes with no fixed causal order, which sets a basis for a quantum information theory in general quantum causal structures.