Researchers report on the observation and characterization of a quantum process that lacks a predefined causal order.

No abstract

In a recent series of works [1][2][3], it has been shown that the quantum superposition of causal order -the quantum switch -offers an enhancement of classical and quantum channel capacity through noisy channels, a phenomenon that was coined 'causal activation'. In this paper, we attempt to clarify the nature of the 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. [4], that the aforementioned advantages appear to be attributable to the ability to coherently control quantum operations, and not to indefinite causal order per se.