Experimental Test of Quantum Contextuality in Neutron Interferometry

We performed an experimental test of the Kochen-Specker theorem based on an inequality derived from the Peres-Mermin proof, using spin-path (momentum) entanglement in a single neutron system. Following the strategy proposed by Cabello et al. [Phys. Rev. Lett. 100, 130404 (2008)], a Bell-like state was generated, and three expectation values were determined. The observed violation 2: 291 +/- 0.008 not less than or equal to 1 clearly shows that quantum mechanical predictions cannot be reproduced by noncontextual hidden-variable theories.