Quantum contextuality requiring counterfactual gain

Yuki Sagawa, Jonte R. Hance, Holger Friedrich Hofmann +1 more

Quantum contextuality, where measurement outcomes depend on the measurement context, implies a failure of classical realism in quantum systems. As recently shown, the transition between measurement contexts can be mapped onto the path that a quantum particle takes through an interferometer. Here, we investigate the relation between contextuality and the counterfactual gain observed in the output ports of such an interferometer when one of the paths is blocked. It is shown that experimental evidence of contextuality can only be obtained when counterfactual gain is observed for a specific combination of blocked path and output port. Using a silicon photonic integrated circuit, we experimentally observe the counterfactual gain for a selection of input states and evaluate the associated evidence for contextuality. The results confirm that contextuality can only be observed in the presence of counterfactual gain.

Zhenghao Liu

Alisson Tezzin, Bárbara Amaral, Jonte R. Hance

Ontological models, as used in the generalized contextuality literature, play a central role in current research on quantum foundations, providing a framework for defining classicality, constructing classical analogs of key quantum phenomena, and examining the ontology of quantum states. In this work, we show that a counterfactual account of Lüders' rule—which we argue is naturally implied by the mathematical structure of the rule itself—renders such models inherently incompatible with the quantum formalism. This incompatibility arises because the counterfactual update requires ontological models to update their states according to conditional probability, which in turn renders predictions of sequential measurements order-independent. This implies that ontological models, even contextual ones, must either act differently than what we would expect given (this, typically implicitly assumed account of) quantum state update rule, or cannot model quantum behavior.

Jochen Szangolies

Tezzin A, Amaral B, Hance JR

Ontological models, as used in the generalised contextuality literature, play a central role in current research on quantum foundations, providing a framework for defining classicality, constructing classical analogues of key quantum phenomena, and examining the ontology of quantum states. In this work, we show that a counterfactual account of L"uders’ rule — which we argue is naturally implied by the mathematical structure of the rule itself — renders such models inherently incompatible with the quantum formalism. This incompatibility arises because the counterfactual update requires ontological models to update their states according to conditional probability, which in turn which in turn renders predictions of sequential measurements order-independent. This implies that ontological models, even contextual ones, must either act differently to what we would expect given (this, typically implicitly-assumed account of) quantum state update rule, or cannot model quantum behaviour.

Brian R. La Cour, Andrei Yu. Khrennikov