The process-matrix framework describes quantum correlations without presupposing a global causal order, yet its standard formulation implicitly relies on background structure through a fixed Choi-Jamiolkowski identification of local input-output spaces. We analyze how such background assumptions can be treated operationally relative to a fixed device-independent interface defined by a causal game. We impose local-frame covariance, requiring invariance under independent actions of a physical symmetry group $G$ on each laboratory, thereby excluding symmetry-breaking background resources. Covariance induces a representation-theoretic decomposition into symmetry sectors and symmetry-invariant multiplicity subsystems, introducing physical degrees of freedom that lie outside the declared device-independent interface. We then analyze causal-inequality signatures at the level of interface-observable statistics and identify when symmetry-induced, interface-inaccessible degrees of freedom undermine device-independent certification. A certification is called background-free if it arises from a locally covariant implementation and does not rely on hidden control mediated by interface-excluded degrees of freedom. We prove that background-free certifications cannot yield device-independent violations of bipartite causal inequalities in the multiplicity-free regime or when all multiplicity subsystems are classical-classical (CC). Such violations necessarily require non-CC multiplicity, with a concrete sufficient route provided by input-output embeddability of an effective process-matrix structure into non-CC blocks. These results delineate which device-independent causal signatures remain certifiable once both symmetry-breaking background structure and interface-level hidden control are excluded.The process-matrix framework describes quantum correlations without presupposing a global causal order, yet its standard formulation implicitly relies on background structure through a fixed Choi-Jamiolkowski identification of local input-output spaces. We analyze how such background assumptions can be treated operationally relative to a fixed device-independent interface defined by a causal game. We impose local-frame covariance, requiring invariance under independent actions of a physical symmetry group $G$ on each laboratory, thereby excluding symmetry-breaking background resources. Covariance induces a representation-theoretic decomposition into symmetry sectors and symmetry-invariant multiplicity subsystems, introducing physical degrees of freedom that lie outside the declared device-independent interface. We then analyze causal-inequality signatures at the level of interface-observable statistics and identify when symmetry-induced, interface-inaccessible degrees of freedom undermine device-independent certification. A certification is called background-free if it arises from a locally covariant implementation and does not rely on hidden control mediated by interface-excluded degrees of freedom. We prove that background-free certifications cannot yield device-independent violations of bipartite causal inequalities in the multiplicity-free regime or when all multiplicity subsystems are classical-classical (CC). Such violations necessarily require non-CC multiplicity, with a concrete sufficient route provided by input-output embeddability of an effective process-matrix structure into non-CC blocks. These results delineate which device-independent causal signatures remain certifiable once both symmetry-breaking background structure and interface-level hidden control are excluded.

arXiv:2508.17075Aug 23, 20250 citations

Causality in quantum mechanics

David Thomas Pegg

Physics Letters AOct 3, 200514 citations

Experimental violation of a Bell-like causal inequality in a photonic quantum switch

Dengke Qu, Quan Lin, Lei Xiao +2 more

Indefinite causal order can be realized by a quantum switch, yet its experimental certification has so far relied on assumptions about the devices. A device-independent verification of indefinite causal order would show, from the observed correlations alone and without relying on a detailed model of the devices, that they are incompatible with any classical description based on a fixed causal order. Here, we implement a photonic quantum switch in a device-independent scenario and test a Bell-like causal inequality using entangled photons. One photon serves as the control qubit of the quantum switch, while the other is measured by a distant party with rapidly and independently chosen settings. We observe a violation of the causal inequality by 24 standard deviations. These results provide experimental evidence for indefinite causal order, albeit in the presence of remaining loopholes, and mark an important step toward a fully loophole-free device-independent verification of causal indefiniteness in quantum processes.Indefinite causal order can be realized by a quantum switch, yet its experimental certification has so far relied on assumptions about the devices. A device-independent verification of indefinite causal order would show, from the observed correlations alone and without relying on a detailed model of the devices, that they are incompatible with any classical description based on a fixed causal order. Here, we implement a photonic quantum switch in a device-independent scenario and test a Bell-like causal inequality using entangled photons. One photon serves as the control qubit of the quantum switch, while the other is measured by a distant party with rapidly and independently chosen settings. We observe a violation of the causal inequality by 24 standard deviations. These results provide experimental evidence for indefinite causal order, albeit in the presence of remaining loopholes, and mark an important step toward a fully loophole-free device-independent verification of causal indefiniteness in quantum processes.

Science AdvancesAug 7, 20260 citations

Assumption-free derivation of the Bell criteria of contextuality/nonlocality

Ehtibar N. Dzhafarov

Traditional Bell criteria of contextuality/nonlocality can be derived without any falsifiable assumptions, such as context-independent mapping (or local causality), free choice, or no-fine-tuning. This is achieved by deriving Bell criteria for systems with disturbance, based on the generalized definition of contextuality in the Contextuality-by-Default approach, and then specializing these criteria to systems with no disturbance. Context-independent mapping and no-fine-tuning are derived rather than assumed, and free choice follows from context-independent mapping. KEYWORDS: Bell criteria; contextuality; context-independent mapping; free choice; local causality; no-fine-tuning; nonlocality.Traditional Bell criteria of contextuality/nonlocality can be derived without any falsifiable assumptions, such as context-independent mapping (or local causality), free choice, or no-fine-tuning. This is achieved by deriving Bell criteria for systems with disturbance, based on the generalized definition of contextuality in the Contextuality-by-Default approach, and then specializing these criteria to systems with no disturbance. Context-independent mapping and no-fine-tuning are derived rather than assumed, and free choice follows from context-independent mapping. KEYWORDS: Bell criteria; contextuality; context-independent mapping; free choice; local causality; no-fine-tuning; nonlocality.

Aug 12, 20210 citations

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on each laboratory, thereby excluding symmetry-breaking background resources. Covariance induces a representation-theoretic decomposition into symmetry sectors and symmetry-invariant multiplicity subsystems, introducing physical degrees of freedom that lie outside the declared device-independent interface. We then analyze causal-inequality signatures at the level of interface-observable statistics and identify when symmetry-induced, interface-inaccessible degrees of freedom undermine device-independent certification. A certification is called background-free if it arises from a locally covariant implementation and does not rely on hidden control mediated by interface-excluded degrees of freedom. We prove that background-free certifications cannot yield device-independent violations of bipartite causal inequalities in the multiplicity-free regime or when all multiplicity subsystems are classical-classical (CC). Such violations necessarily require non-CC multiplicity, with a concrete sufficient route provided by input-output embeddability of an effective process-matrix structure into non-CC blocks. These results delineate which device-independent causal signatures remain certifiable once both symmetry-breaking background structure and interface-level hidden control are excluded.