Physicists at the University of Vienna have conducted an experiment demonstrating a superposition of different temporal orders in quantum events, using entangled photons and a Bell inequality equivalent. The results deviate significantly from classical expectations, suggesting indefinite causal order is a fundamental quantum feature. However, several experimental loopholes remain open.
Researchers from the University of Vienna have devised an experiment to test whether quantum mechanics allows for superpositions of causal orders, where the sequence of events—A before B or B before A—becomes probabilistic rather than definite. The setup involves entangled photon pairs. One photon passes through a device that applies two manipulations in a sequence determined by its polarization: either operation A followed by B, or vice versa. The photon's path is then measured, while the second photon's polarization measurement reveals which order the first photon experienced. The team adapted Bell's inequalities—a tool traditionally used to probe quantum entanglement—for indefinite causal order scenarios. Their measurements showed correlations 18 standard deviations beyond what Bell's theorem predicts under classical hidden variable theories, providing strong evidence that temporal order superposition is inherent to quantum mechanics. Despite the promising outcome, the experiment has limitations similar to early entanglement tests. Only about 1 percent of input photons are detected due to losses, potentially allowing hidden variables to survive if losses favor certain subsets. The setup also lacks sufficient spatial separation to exclude sub-light-speed influences, alongside other issues specific to indefinite causal order tests. The researchers believe future refinements can close these loopholes, drawing on precedents from entanglement research that earned Nobel recognition. The authors highlight practical potential: 'The [device used in this work] may also be interesting for applications as it has been shown that it can outperform causally ordered processes at a wide variety of tasks such as channel discrimination, promise problems, communication complexity, noise mitigation, various thermodynamic applications, quantum metrology, quantum key distribution, entanglement generation, and distillation, among others.' The findings appear in PRX Quantum.