Quantum experiment shows events may have no fixed order

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For the first time, a team of physicists in Austria has carried out an experiment that appears to verify the principle of indefinite causal order: an idea that suggests that timelines of events can exist in multiple orders at the same time. Led by Carla Richter at the Vienna Center for Quantum Science and Technology, the researchers hope their result could finally allow physicists to verify a key prediction of quantum theory. The results have been published in PRX Quantum.

Rules of causality

The basic principle of cause and effect underpins everything that happens in the classical world: for an event to occur, it must be triggered by another event in its past. Yet in the quantum world, physicists have long suspected that these rules may not always apply.

Just as quantum particles can exist in superpositions of multiple states which collapse to a single outcome when measured, indefinite causal order suggests something similar may apply to entire sequences of events. Until a measurement is made, multiple orders of cause and effect can exist in superposition.

For two events, A and B, this means that A can occur before B, while B occurs before A at the same time. For researchers, the challenge has been to verify this counterintuitive possibility experimentally. Any convincing test must show that the order is truly indefinite, without relying on assumptions about how the experiment itself operates.

Blurred operation order

To tackle this, Richter's team considered a key theoretical proposal: if causal order is actually definite but hidden, then a hidden variable must exist which determines whether the true order is A before B or B before A in each run.

The question then becomes whether this hidden-variable model can reproduce the observed correlations between measurement choices and their outcomes. This reasoning closely mirrors a Bell test, which is used to verify quantum entanglement by ruling out the possibility of hidden variables—which would predetermine measurement results.

Harnessing these similarities, the researchers developed an experiment based on a quantum switch. In this setup, a single photon is placed in a superposition of two paths using a beam splitter. Along one path, the photon experiences event A before event B; along the other, it experiences B before A. Because the photon travels both paths at once, the order of operations is itself placed in a quantum superposition. The paths are then recombined, allowing interference between the two possible orders.

To implement the Bell-type test, the team also measured the photon's polarization under different settings. The way the polarization evolves depends on the path taken through the setup, so that if a hidden variable determined a definite path with a definite order, there would be limited correlations between these measurement settings.

But if no such hidden variable exists, and the photon truly undergoes indefinite causal order, the combined effect of both paths would lead to stronger correlations. By repeating the experiment many times, the researchers could build up statistics of these correlations.

Indefinite causal order

When they ran the experiment, Richter's team observed correlations that clearly exceeded the limits imposed by any definite causal order. Their measured value lay well beyond the classical bound, providing strong evidence that no hidden-variable model with a fixed order can explain the results.

If the experiment is working as intended, this could represent the first device-independent evidence of indefinite causal order. For now, some loopholes remain in the setup, particularly relating to photon detection and timing—and so some further refinements will be needed to fully confirm the result.

All the same, the work marks an important step towards a definitive test of one of quantum theory's most intriguing predictions. With continued improvements, such experiments could offer new insights into the fundamental ways in which cause and effect operate in the quantum world.

Written for you by our author Sam Jarman, edited by Sadie Harley, and fact-checked and reviewed by Robert Egan—this article is the result of careful human work. We rely on readers like you to keep independent science journalism alive. If this reporting matters to you, please consider a donation (especially monthly). You'll get an ad-free account as a thank-you.

Publication details

Carla M.D. Richter et al, Toward an Experimental Device-Independent Verification of Indefinite Causal Order, PRX Quantum (2026). DOI: 10.1103/5t2y-ddmt

Journal information: PRX Quantum

Key concepts

Quantum correlations, foundations & formalism

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