  Quantum mechanics provides a statistical description about nature, and thus
would be incomplete if its statistical predictions could not be accounted for
by some realistic models with hidden variables. There are, however, two
powerful theorems against the hidden-variable theories showing that certain
quantum features cannot be reproduced based on two rationale premises of
locality, Bell's theorem, and noncontextuality, due to Bell, Kochen and Specker
(BKS). Noncontextuality is independent of nonlocality, and the contextuality
manifests itself even in a single object. Here we report an experimental
verification of quantum contextuality by a single spin-1 electron system at
room temperature. Such a three-level system is indivisible and then we close
the compatibility loophole which exists in the experiments performed on
bipartite systems. Our results confirm the quantum contextuality to be the
intrinsic property of single particles.
