At a Glance
- Physicists have traditionally linked violations of local realism, or “spooky action,” exclusively with the phenomenon of quantum entanglement, which connects particles over vast distances.
- A new experiment, reported in Science Advances, successfully demonstrated a violation of the Bell inequality using photons that were explicitly not entangled before their detection.
- The setup made four photons indistinguishable by their paths, creating nonlocal correlations that could not be explained by classical physics and defied the Bell inequality threshold.
- Researchers concluded that quantum indistinguishability by path identity, not entanglement, was the source of the counterintuitive nonlocal behavior observed in their four-photon interference experiment.
- While the authors acknowledge current experimental limitations, they are confident that their work opens new paths for exploring the fundamental sources of quantum correlation.
For decades, physicists have grappled with one of quantum mechanics’ most bizarre concepts: entanglement. This phenomenon, which Albert Einstein famously dubbed “spooky action at a distance,” links the properties of two particles no matter how far apart they are. This connection, known as nonlocality, appears to violate local realism—the common-sense notion that objects are only influenced by their immediate surroundings and that information cannot travel faster than the speed of light. For years, scientists believed entanglement was the only way to generate this spooky, nonlocal behavior.
A new study challenges that long-held assumption by demonstrating nonlocal effects without using entangled particles. In research published in Science Advances, a team of physicists designed an experiment using a test called the Bell inequality, which acts as a gold standard for detecting violations of local realism. Instead of entangling photons, they sent four photons through a crystal setup where it was impossible to determine which path each particle took to reach the detectors. This created a state of “quantum indistinguishability by path identity.”
The results showed a clear violation of the Bell inequality by more than four standard deviations, a statistically significant outcome that confirms the observed correlations cannot be explained by classical physics. This is the first time such a strong violation has been achieved using unentangled photons. The researchers conclude that the spooky nonlocal correlations arose directly from the photons’ indistinguishability, not from a preexisting entangled link between them.
While groundbreaking, the authors acknowledge that the experiment has limitations that need to be addressed in future work, such as potential loopholes related to experimental design and data selection. However, they are confident that these issues can be resolved with improved technology, much like they were over decades of experiments with entanglement. “Our work establishes a connection between quantum correlation and quantum indistinguishability,” the authors write in a statement to Phys.org, providing new insight into the fundamental weirdness of the quantum world.
References
- Kasal, K. & Phys.org. (2025, August 4). Scientists produce quantum entanglement-like results without entangled particles in new experiment. Phys.Org; Phys.org. https://phys.org/news/2025-08-scientists-quantum-entanglement-results-entangled.html
- Wang, K., Hou, Z., Qian, K., Chen, L., Krenn, M., Aspelmeyer, M., Zeilinger, A., Zhu, S., & Ma, X.-S. (2025). Violation of Bell inequality with unentangled photons. Science Advances, 11(31), eadr1794. https://doi.org/10.1126/sciadv.adr1794
