Realization and characterization of a 2-photon 4-qubit linear cluster state

We report on the experimental realization of a 4-qubit linear cluster state via two photons entangled both in polarization and linear momentum. This state was investigated by performing tomographic measurements and by evaluating an entanglement witness. By use of this state we carried out a novel nonlocality proof, the so-called ``stronger two observer all versus nothing'' test of quantum nonlocality.