# Communication through quantum-controlled noise

## TL;DR

This paper investigates whether the quantum switch's capacity enhancement is due to indefinite causal order or just coherent control, finding that similar advantages can be achieved without indefinite causal order.

## Contribution

It clarifies the nature of quantum causal activation, showing that coherent control, not indefinite causal order, underpins capacity enhancements.

## Key findings

- Quantum superpositions of processes can match or outperform the quantum switch.
- Advantages are attributable to coherent control, not indefinite causal order.
- Processes with the same resources as the quantum switch can achieve similar capacity enhancements.

## Abstract

In a recent series of works [Ebler et al. Phys. Rev. Lett. 120, 120502 (2018); arXiv:1809.06655v2; arXiv:1810.10457v2], it has been proposed that the quantum superposition of causal order -- the quantum switch -- may offer an enhancement of classical and quantum channel capacity through noisy channels, a phenomena that was coined `causal activation'. In this paper we attempt to clarify the nature of the purported advantage, by comparing the quantum switch to a class of processes that can be interpreted as quantum superposition of processes with the same causal order. We show that some of these processes can match or even outperform the quantum switch at enhancing classical and quantum channel capacity, and argue that they require the same resources as the switch. We conclude, in agreement with Abbott et al. [arXiv:1810.09826v1], that the aforementioned advantages appear to be attributable to the ability to coherently control quantum operations, and not to indefinite causal order per se.