For over a decade, physicists have suspected that causality—the principle that events must follow a definite temporal sequence—might not hold at the quantum scale. A team of physicists in Austria has now carried out an experiment that appears to verify the principle of indefinite causal order, led by Carla Richter at the Vienna Center for Quantum Science and Technology. The results offer the clearest evidence yet that the arrow of time can become a loop at the quantum level.
The experiment, published in PRX Quantum, tackles a deceptively simple question: do two quantum events always occur in a definite order, or can they exist in a superposition of both temporal sequences simultaneously? 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.
The team's approach borrowed a proven strategy from quantum physics. Rather than attempting to directly demonstrate indefinite causal order, they asked whether a hidden variable could determine a definite temporal order underneath the apparent superposition. 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. The Vienna researchers developed what is essentially a Bell test for causality.
The Experiment
In the 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 device they used, called a quantum switch, allowed researchers to measure the photon's properties in ways that would differ sharply depending on whether the temporal order was actually definite or genuinely indefinite.
To implement the Bell-type test, the team measured the photon's polarisation under different settings. The way the polarisation 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.
The experiment produced a score of 1.8328 against a hard mathematical boundary of 1.75, landing 18 standard deviations above the limit that classical causality cannot cross. To appreciate the significance: in science, five standard deviations ends debates. Eighteen does not leave room for one.
Significance and Limitations
The result strengthens a line of inquiry that has been building for years. To date, demonstrations of indefinite causal order have all been based on a process called the quantum switch and have relied on device-dependent or semi-device-independent protocols. The Vienna team's approach is different. By using a Bell-like inequality test, they have produced what they argue is the first device-independent evidence of indefinite causal order.
However, the work sits where quantum entanglement research sat decades ago: on the edge of breakthrough but not yet definitive. 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. The measurement parties in the experiment sat less than one metre apart on a single optical table. A fully airtight version requires them to be physically separated by distances large enough to rule out any classical communication between them during the measurement. Detection efficiency through the full apparatus ran at approximately one percent, far below what a loophole-free result demands.
These are not theoretical objections but practical engineering challenges. These are engineering problems, not conceptual ones. The path forward is laid out. It is a matter of building it.
What This Means for Quantum Technologies
Beyond fundamental physics, the research has immediate practical implications. Systems using quantum switches, the devices at the centre of the Vienna experiment, have already demonstrated superior performance at quantum communication tasks, noise reduction, precision measurement beyond standard limits, and quantum key distribution for secure information transfer. Each of those advantages depends on indefinite causal order being a genuine physical resource. The Vienna result is the hardest confirmation yet that it is, and it will accelerate work on every one of those applications.
The experiment demonstrates a broader principle about quantum mechanics: phenomena that seem impossible when viewed through the lens of everyday experience become not just possible but observable when the fundamental assumptions we make about reality are questioned. Time may flow in one direction for objects at human scales. But in the quantum realm, 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.
Read the detailed summary from the American Physical Society or access the full paper in PRX Quantum.