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

• URL: http://arxiv.org/abs/2608.03552v1
• Date: Tue, 04 Aug 2026 12:23:29 GMT
• Title: Observation of quantum nonclassicality without freedom of choice in a minimal causal network
• Abstract summary: Quantum causal networks enable tests of nonclassicality beyond Bell nonlocality.<n>We implement the minimal three-node unrelated-confounders network in an optical experiment.<n>We highlight the UC network as a compact, experimentally accessible platform for device-independent quantum protocols.
• Score: 27.101944462785948
• License: http://arxiv.org/licenses/nonexclusive-distrib/1.0/
• 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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