Waxigloo is correct The first clear evidence for the quantization of the EM field as opposed to a quantization of matter field interaction or a semi classical model was the Lamb shift The photoelectric effect compton scattering and blackbody radiation were all evidence that a photon model was useful but they didn t strictly require it The first clear evidence for photons as particles was the Kimble experiment in 1977 which showed antibunching in a Hanbury Brown Twiss style measurement Other similar experiments by Aspect and Clauser should also be cited as they were the first to actually create a beam of light containing a single photon within some experimental caveats This stuff should definitely be in the article Here is a reference Phys Rev Lett 39 691 695 1977 Photon Antibunching in Resonance Fluorescence H J Kimble M Dagenais and L Mandel