Traditionally, quantum theory assumes the existence of a fixed background causal structure. But if the laws of quantum mechanics are applied to the causal relations, then one could imagine situations in which the causal order of events is not always fixed, but is subject to quantum uncertainty. Such indefinite causal structures could make new quantum information processing tasks possible and provide methodological tools in quantum theories of gravity. Here, I review recent theoretical progress in this emerging area.
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I thank F. Costa, O. Oreshkov and J. Pienaar for discussions. This work was supported by the Austrian Science Fund (FWF) through FoQuS and individual project 24621, the European Commission Project RAQUEL, FQXi, and the John Templeton Foundation.
The author declares no competing financial interests.
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Brukner, Č. Quantum causality. Nature Phys 10, 259–263 (2014). https://doi.org/10.1038/nphys2930
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