Indefinite causal order: Quantum physics answers the ‘chicken or the egg’ question

Indefinite causal order: Quantum physics answers the ‘chicken or the egg’ question

Indefinite causal order: quantum physics answers the ‘chicken or the egg’ question

Physicists at the University of Queensland, Australia, and the NÉEL Institute, France, have answered an age old ‘chicken or the egg’ question by establishing the effect of indefinite causal order in a recently published study.

What came first: the chicken or the egg?

The study found that quantum physics can allow events to happen without a straightforward causal order. Jacqui Romero, from the Australian Research Council (ARC) Centre of Excellence for Engineered Quantum Systems, said: “The weirdness of quantum mechanics means that events can happen without a set order.” This effect is called ‘indefinite causal order’. This dynamic is not possible for people to experience in their everyday life because actions and events happen one after the other.

What did the study involve?

The title of the paper is ‘Indefinite Causal Order in a Quantum Switch’ and was published in the journal Physical Reviews Letters by the American Physical Society. The study aimed to solve the issue of determining cause and effect by observing indefinite causal order.

To observe the effect of indefinite causal order, the scientists set up a photonic quantum switch in their laboratory. Dr Fabio Costa from the University of Queensland said: “By measuring the polarisation of the photons at the output of the quantum switch, we were able to show the order of transformations on the shape of light was not set.”

About the results of their study, the authors wrote: ‘We confirm our quantum switch has no definite causal order by constructing a causal witness and measuring its value to be 18 standard deviations beyond the definite-order bound.’

How can the concept of indefinite causal order be used in science?

Dr Costa added: “This is just a first proof of principle, but on a larger scale indefinite causal order can have real practical applications, like making computers more efficient or improving communication.”



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