In quantum physics, it is impossible to simultaneously measure a particle's position and momentum with precision — so says the Heisenberg uncertainty principle. However, a team from Shanxi University in China has shown that this limitation can be circumvented if one looks not only forward but also backward in time.
The researchers gathered 52 billion rubidium atoms in a small glass chamber. A laser aligned their spins into a single quantum system, after which a second laser measured position and momentum. Instead of the usual approach, the scientists used weak measurements that take into account data both before and after the moment of interest. The experiment lasted 84 minutes continuously.
The result exceeded expectations: the precision of estimating non-commuting variables turned out to be 20 percent higher than the Heisenberg limit. The uncertainty principle was not violated in the process — the probabilities were simply calculated differently, taking into account the system's "past" and "future."
A common analogy here is trying to guess the trajectory of a ball knowing only its position at a single moment. If you add information about where the ball was before and where it will fly afterward, the uncertainty drops sharply. That is exactly how the retrospective scheme worked in the experiment.
This opens the way to more precise quantum sensors and clocks, where retrospective data are already familiar. In the future, the method can be enhanced with entanglement, but for now it remains at the preprint stage.
The results were published in a preprint on arXiv in November 2026 and have not yet undergone peer review. The team notes that the approach combines trajectories and entanglement strategies for quantum-enhanced sensing.
The fundamental limitations of quantum metrology turned out to be not as rigid as was believed, if the very process of measurement is rethought.
