Experimental distinction between the quantum and classical field-theoretic predictions for the photoelectric effect. J. F. Clauser in Phys. Rev. D 9:853 (1974). What the paper says?
Our experiment thus resurrects the photoelectric effect as a phenomenon requiring quantization of the electromagnetic field.
This is an early paper studying photon cascades, claiming to show quantization of the light field (an important question at this time of Jaynes' neoclassical theory.
In a first section, Clauser shows that previous experimental attempts were not able to show photon quantization. Then he claims to show that himself, with Cauchy-Schwarz violation in coincidences from photon cascades. The source is Hg atoms excited by electron bombardment, so apparently incoherent pumping, with the cascade $9{}^1P_1\to7{}^3S_1\to6{}^3P_1$.
This is the experimental result (seemingly from Clauser alone!):
Panels (a) and (d) show the cascade, (b) and (c) show no correlation (it should be antibunched; Loudon[1] explains why not [too many atoms]) with the quantum source while (e) and (f) show bunching with the classical source (probably HB effect). The main result is (g) that shows violation of Cauchy-Schwartz inequality and thus that the field is quantized.
Loudon[1] makes a detailed analysis of this experiment, and Al-Hilfy and Loudon[2] also comment on this paper.