English: Schematic setup of a Bell test, where the polarization of photons is used. The source of photons, in the center, emits pairs of photons, where one of the photons flies to Alice, the other one to Bob. As these pairs are in an entangled quantum state, the measurement results are strongly correlated. In the picture, the measurement in the H/V basis is shown. To verify entanglement, measurements in other bases are necessary. From the data one can determine whether the corresponding Bell inequality is violated.