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arXiv:2507.07756 · Science Advances

Violation of Bell inequality with unentangled photons

Kai Wang, Zhaohua Hou, Kaiyi Qian, Leizhen Chen, Mario Krenn, Markus Aspelmeyer, Anton Zeilinger, Shining N. Zhu, Xiao‐Song Ma

Published Aug 1, 2025• 9 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1126/sciadv.adr1794

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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.
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This thesis presents a study of Bell’s inequality and its experimental tests, with a focus on their implications for the concepts of locality and realism in quantum mechanics. The work reviews the theoretical framework underlying Bell’s inequality, the assumptions of hidden variable theories, and the contrasting predictions of quantum mechanics. It further provides an overview of landmark experimental tests of Bell’s inequality, emphasizing their role in shaping the modern understanding of quantum entanglement and nonlocality. The aim of the study is to offer a clear and pedagogical introduction to the subject, bridging the mathematical formalism with experimental verification of one of the most fundamental results in the foundations of quantum theory.
The interference of quanta lies at the heart of quantum physics. The multipartite generalization of single-quanta interference creates entanglement, the coherent superposition of states shared by several quanta. Entanglement allows non-local correlations between many quanta and hence is a key resource for quantum information technology. Entanglement is typically considered to be essential for creating non-local quantum interference. Here, we show that this is not the case and demonstrate multiphoton non-local quantum interference that does not require entanglement of any intrinsic properties of the photons. We harness the superposition of the physical origin of a four-photon product state, which leads to constructive and destructive interference with the photons' mere existence. With the intrinsic indistinguishability in the generation process of photons, we realize four-photon frustrated quantum interference. This allows us to observe the following noteworthy difference to quantum entanglement: We control the non-local multipartite quantum interference with a photon that we never detect, which does not require quantum entanglement. These non-local properties pave the way for the studies of foundations of quantum physics and potential applications in quantum technologies.