State-independent experimental test of quantum contextuality

Quantum theory put in context Quantum mechanics has had notable success in the almost 90 years since it was first introduced, and its predictions have been confirmed in numerous experiments. Nevertheless, many physicists not content with the axioms of the theory have been searching for an explanation of quantum physical predictions in terms of a classical theory. An intuitive feature of classical models is non-contextuality: the property that any measurement has a value independent of other compatible measurements being carried out at the same time. Theory suggests that non-contextuality is in conflict with quantum mechanics, and experiments undertaken with photons and neutrons seem to support this. However, these tests required the generation of special quantum states and left various loopholes open. Here, Kirchmair et al. perform an experiment with trapped ions that overcomes these problems and cannot be explained in non-contextual terms. Contextuality is therefore a property of nature that does not require the generation of special quantum states or quantum entanglement.