Single-Photon Quantum Contextuality on a Chip

In classical physics, properties of objects exist independently of the context, i.e., whether and how measurements are performed. Quantum physics showed this assumption to be wrong, and that Nature is indeed "contextual". Contextuality has been observed in the simplest physical systems, such as single particles, and plays fundamental roles in quantum computation advantage. Here, we demonstrate for the first time quantum contextuality in an integrated photonic chip. The chip implements different combinations of measurements on a single photon delocalized on four distinct spatial modes, showing violations of a Clauser-Horne-Shimony-Holt (CHSH)-like noncontextuality inequality. This paves the way to compact and portable devices for contextuality-based quantum-powered protocols.