Single photons break a classical rule about measurement order

Quantum contextuality—the fact that measurement outcomes depend on which other measurements are performed alongside them—is tested here using a photonic circuit with sequential linear-optical operations and simple on-off detectors. The setup carefully ensures each observable is implemented by the same physical device regardless of context, closing a key loophole in contextuality tests. Experimental results show a clear violation of the five-measurement KCBS inequality. Beyond foundational interest, the scheme doubles as a diagnostic for photon-number statistics, offering a practical way to certify single-photon sources.