Abstract

It is an unresolved question in quantum mechanics whether quantum states apply to individual quantum systems, or to ensembles of quantum systems. We show by way of a thought experiment that quantum states apply only to ensembles of quantum systems. A further unresolved question is whether quantum systems possess ontic states. If a quantum state is the state of an ensemble, as we claim, the answer to this question is that quantum states are not ontic. However, a notable recent result in quantum foundations shows that if there are any ontic states at all, then the quantum state must be ontic. Collectively, these two results imply that there are no ontic states. We examine the assumptions required for these results, and suggest that the retrospective effect on state preparations by entangling measurements provides good reason for relaxing the assumption of preparation independence at the ontic level.