Causality is a concept deeply rooted in our understanding of the world and lies at the basis of the very notion of time. It plays an essential role in our cognition — enabling us to make predictions, determine the causes of certain events, and choose the appropriate actions to achieve our goals. But even in quantum mechanics, for which countless measurements and preparations have been rethought, the assumption of pre-existing causal structure has never been challenged — until now.
Giulia Rubino and colleagues have designed an experiment to show that causal order can be genuinely indefinite. By creating wires between a pair of operating gates whose geometry is controlled by a quantum switch — the state of single photon — they realized a superposition of gate orders. From the output, they measured the so-called causal witness, which specifies whether a given process is causally ordered or not. The result brings a new set of questions to the fore — namely, where does causal order come from, and is it a necessary property of nature?
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Li, Y. Indefinite causality. Nature Phys 13, 419 (2017). https://doi.org/10.1038/nphys4134