It is pointed out that it is impossible to distinguish between photons and classical wave packets by measurement of photoelectric counting statistics for any state of a light beam for which a classical description exists. Therefore, an experiment with a thermal light source recently proposed by Karp as a test for the existence of photons can yield no information on this question, even if it should be able to distinguish between two different statistical models of a light beam. The quantum nature of the optical field can, however, be inferred from other photoelectric measurements, some of which are briefly discussed.
© 1977 Optical Society of America
L. Mandel, "Photoelectric counting measurements as a test for the existence of photons," J. Opt. Soc. Am. 67, 1101-1104 (1977)