There are 8 pages of historical developments and experimental discussions in the paper you cite but only two sentences
are used to define what a “single photon” is. I don't question the accuracy of the experiments or that they are able to make good predictions. I question the assumptions that they begin with and the logic behind them. Can you cite something more basic?
I believe that the Marshall and Santos papers I cited do a better job of looking at fundamentals. Although they do not offer a more accurate theory they have the advantage that they reject non-locality. Will you comment on their argument that when the zero point field is used to describe the photon it is actually a classical model?