  We demonstrate hybrid entanglement of photon pairs via the experimental
violation of a Bell inequality with two different degrees of freedom (DOF),
namely the path (linear momentum) of one photon and the polarization of the
other photon. Hybrid entangled photon pairs are created by Spontaneous
Parametric Down Conversion and coherent polarization to path conversion for one
photon. For that photon, path superposition is analyzed, and polarization
superposition for its twin photon. The correlations between these two
measurements give an S-parameter of S=2.653+/-0.027 in a CHSH inequality and
thus violate local realism for two different DOF by more than 24 standard
deviations. This experimentally supports the idea that entanglement is a
fundamental concept which is indifferent to the specific physical realization
of Hilbert space.
