  Complementarity or wave-particle duality has been the basis of quantum
mechanics over the last century. Since the Hanbury Brown and Twiss experiments
in 1956, the particle nature of single photons has been intensively studied for
various quantum phenomena such as anticorrelation and Bell inequality
violation. Regarding the fundamental question on quantumness or
nonclassicality, however, no clear answer exists for what quantum entanglement
should be and how to generate it. Here, we experimentally demonstrate the
secrete of quantumness using the wave nature of single photons.
