Experimental verification of differences between classical and quantum polarization properties

We have carried out polarization measurements on a two-photon quantum state generated by spontaneous parametric down conversion. Our measurements show that the state is unpolarized in the classical theory, but they also show that the state is not invariant under geometric rotation. Therefore, it is not unpolarized in the quantum theory. This is another example of a rather simple experiment that clearly shows the effects of quantum interference. I t also confirms the theoretical investigation by Klyshko [Phys. Lett. A 163 (1992) 349]. The experiment is explained theoretically and a systematic tr eatment of polarization in quantum theory is outlined. (C) 2001 Elsevier Sc ience B.V. All rights reserved.