  The correlations between two qubits belonging to a three-qubit system can
violate the Clauser-Horne-Shimony-Holt-Bell inequality beyond Cirel'son's bound
[A. Cabello, Phys. Rev. Lett. 88, 060403 (2002)]. We experimentally demonstrate
such a violation by 7 standard deviations by using a three-photon
polarization-entangled Greenberger-Horne-Zeilinger state produced by Type-II
spontaneous parametric down-conversion. In addition, using part of our results,
we obtain a violation of the Mermin inequality by 39 standard deviations.
