Mind the (Quantum) Context

A new optical experiment provides further proof that quantum mechanics is not hiding some classical framework beneath its veneer of context-dependent observations.

he contrast with classical expectations is one of the most intriguing features of quantum mechanics. It can be illustrated by playing a classic “guessing game” with quantum rules. In this case, the answer to one question will depend on the other questions being asked and the order chosen. This dependence on the context does not conform to our everyday experience, but we may have to live with it, as theorists have shown that we can’t just assume that there exists a hidden answer sheet that is unaffected by the choice of questions. However, designing a generic test of quantum contextuality in the lab has been difficult, so far. Writing in Physical Review Letters, Chong Zu of Tsinghua University in Beijing, China, and his colleagues have shown that context does matter in a three-level photon system generated by a simple but elegant linear-optics setup [1].