But until recently, there weren’t many ideas about how post-causal physics might work. They say this causal perspective probably has to go if we’re ever to figure out the quantum origin of gravity, space and time. Physicists have long sensed that the usual picture of events unfolding as a sequence of causes and effects doesn’t capture the fundamental nature of things. It’s “really something that could be useful in everyday life,” said Giulia Rubino, a researcher at the University of Bristol who led the first experimental demonstration of the quantum switch in 2017.īut the practical uses of the phenomenon only make the deep implications more acute. Over the last five years, a growing community of quantum physicists has been implementing the quantum switch in tabletop experiments and exploring the advantages that indefinite causal order offers for quantum computing and communication. In this procedure, known as the quantum switch, A’s outcome influences what happens in B, and vice versa the photon experiences both causal orders simultaneously. They then subject one branch of the superposition to process A followed by process B, and subject the other branch to B followed by A. In labs in Austria, China, Australia and elsewhere, physicists observe indefinite causal order by putting a particle of light (called a photon) in a superposition of two states. The possibility follows from the quantum phenomenon known as superposition, where particles maintain all possible realities simultaneously until the moment they’re measured. “It sounds outrageous,” admitted Časlav Brukner, a physicist at the University of Vienna. That is, events can occur in an indefinite causal order, where both “A causes B” and “B causes A” are simultaneously true. Over the last decade, quantum physicists have been exploring the implications of a strange realization: In principle, both versions of the story can happen at once. In another version of events, Bob burns himself and cries out, causing Alice to drop a plate. Alice accidentally drops a plate the sound startles Bob, who burns himself on the stove and cries out. Alice and Bob, the stars of so many thought experiments, are cooking dinner when mishaps ensue.
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