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arXiv · 2608.03552

Observation of quantum nonclassicality without freedom of choice in a minimal causal network

Ya Xiao·Yan-Xin Rong·Ran He·Yu Meng·Yang Zhang·Xiao-Ye Xu·Yong-Jian Han·Zheng-Hao Liu·Yong-Jian Gu
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Abstract

Quantum causal networks enable tests of nonclassicality beyond Bell nonlocality while relaxing some physically unwarranted assumptions. By relaxing the freedom-of-choice and spacelike-separation assumptions, the unrelated-confounders causal networks provide a simple and robust route to certify quantum nonclassicality. Here, we implement the minimal three-node unrelated-confounders network in an optical experiment using two independent polarization-entangled photon sources and an intervention at the central node, experimentally achieved with a high-fidelity entangling measurement. We employ a causal data-fusion protocol that combines observational and interventional data to significantly improve the protocol's noise tolerance, and observe a violation of the corresponding hybrid causal inequality by more than three standard deviations. Our results provide deeper insights into quantum nonlocality in networks and highlight the UC network as a compact, experimentally accessible platform for device-independent quantum protocols that do not require actively chosen measurement settings.

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Ya Xiao, Yan-Xin Rong, Ran He, Yu Meng, Yang Zhang, Xiao-Ye Xu, Yong-Jian Han, Zheng-Hao Liu, Yong-Jian Gu. 2026-08-04. Observation of quantum nonclassicality without freedom of choice in a minimal causal network. https://arxiv.org/abs/2608.03552

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