  We present an experimental state-independent violation of an inequality for
noncontextual theories on single particles. We show that 20 different
single-photon states violate an inequality which involves correlations between
results of sequential compatible measurements by at least 419 standard
deviations. Our results show that, for any physical system, even for a single
system, and independent of its state, there is a universal set of tests whose
results do not admit a noncontextual interpretation. This sheds new light on
the role of quantum mechanics in quantum information processing.
