Comments on: Violation of Bell inequality with unentangled photons

Mario Krenn

Antoni Wójcik, Jan W. Wojcik

Wang et al. [Science Advances, 1 Aug 2025 Vol 11, Issue 31] recently reported an experiment that they interpret as demonstrating a violation of Bell's inequality using unentangled photons. Such a claim is highly controversial, since it is well established that product states cannot surpass the bounds set by local hidden variable models. In this manuscript, we analyze the essential features of the experiment through simplified, idealized scenarios. Our analysis shows that the apparent violation of Bell's inequality originates from two key elements: postselection and an unconventional normalization procedure. These steps produce quantities that formally mimic Bell-type correlations but lack the operational meaning required in a genuine Bell test. We therefore argue that the reported violation does not demonstrate nonlocality without entanglement, but rather reflects a misinterpretation of otherwise valid and interesting multiphoton interference effects.

Tamoghna Das, Marcin Karczewski, Antonio Mandarino +3 more

S. N. Coppersmith

Kai Wang, Zhaohua Hou, Kaiyi Qian +6 more

Violation of local realism via Bell inequality-a profound and counterintuitive manifestation of quantum theory that conflicts with the prediction of local realism-is viewed to be intimately linked with quantum entanglement. Experimental demonstrations of such a phenomenon using quantum entangled states are among the landmark experiments of modern physics and paved the way for quantum technology. Here, we report the violation of the Bell inequality that cannot be described by quantum entanglement in the system but arises from quantum indistinguishability by path identity, shown by the multiphoton frustrated interference. By analyzing the measurement of four-photon frustrated interference within the standard Bell-test formalism, we find a violation of Bell inequality by more than four SDs. Our work establishes a connection between quantum correlation and quantum indistinguishability, providing insights into the fundamental origin of the counterintuitive characteristics observed in quantum physics.

Stasja Stanisic