  It was shown recently that entanglement of identical particles has a feature
called dualism [Phys. Rev. Lett. 110, 140404 (2013)], which is fundamentally
connected with quantum indistinguishability. Here we report an experiment that
observes the entanglement duality for the first time with two identical
photons, which manifest polarization entanglement when labeled by different
paths or path entanglement when labeled by polarization states. By adjusting
the mismatch in frequency or arrival time of the entangled photons, we tune the
photon indistinguishability from quantum to classical limit and observe that
the entanglement duality disappears under emergence of classical
distinguishability, confirming it as a characteristic feature of quantum
indistinguishable particles.
