The findings differed by 18 standard deviations from the predictions of Bell’s theorem, strongly indicating that a superposition of temporal order is an intrinsic feature of quantum mechanics.
Yet the experiment is at a stage similar to early entanglement work: numerous loopholes remain. For example, most photons are lost during the procedure (only about 1 percent of those injected emerge on the other side to be measured). It is technically possible that those losses were biased toward a subset of photons that would otherwise display correlations compatible with hidden-variable explanations.
The team also didn’t separate the equipment by sufficient distances to exclude influences traveling slower than light, and indefinite causal-order experiments introduce a few other peculiarities. Still, the study points toward follow-up experiments that could seal these gaps, and past progress shows such loopholes can be closed.
Usually, when reporting on something this strange, all we can do is marvel at how odd reality is compared with our expectations. This instance is different: the physics involved is already known to have many practical uses.
“The [device used in this work] could also be of interest for applications, as it has been shown to outperform causally ordered processes across a wide range 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 authors note.
Put another way, being uncertain about the order of time may actually prove useful.
* I wouldn’t have known this work existed if I hadn’t read an excellent summary of it on the American Physical Society news site.
PRX Quantum, 2026. DOI: 10.1103/5t2y-ddmt (About DOIs).
