 Despite a century of development in quantum theory, the interpretation of a quantum state remains a topic of discussion among physicists. The state vector is used to summarize information about a system and corresponds to a probability distribution over possible values of an unknown physical or ontic state. A recent NERGO theorem shows that distinct state vectors with overlapping distributions lead to predictions different from quantum theory which was tested experimentally using trapped ions. The results confirmed quantum theory within experimental error, ruling out certain kinds of models. This article was authored by Daniel Nigg, Thomas Nons, Philip Schindler, and others.