Ontological models cannot adequately represent state update for sequential measurement of incompatible observables

Abstract: Ontological models (as used in the generalized contextuality literature) play a central role in current research on quantum foundations, serving as a framework for defining classicality, constructing classical analogues of key quantum phenomena, and even examining the ontology of quantum states. In this work, we analyse the quantum state update rule in these models and argue that incompatibility -- or, equivalently, order-dependent theoretical predictions of sequential measurements -- is sufficient to stop them from being able to adequately represent a scenario. This is because, as we argue, the quantum state update rule 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 to what we would expect given the quantum state update rule, or cannot model quantum behaviour. Building on this, we argue that classical wave theory is equally incompatible with ontological models, challenging the notion that generalized contextuality serves as a signature of classicality.