I am new here and also physics.
i tried to understand the experiment more. In the following youtube video. the demonstration indicated that an entangled photon arrived at D0 while its partner arrived at, lets say D2 or D3 later on. I dont know why an already entangled single photon can leave interference data on D0. Isn't A PAIR of it can be form interference wave? I think i missing some very basic physic knowledge.