Experiments with single photons

The birth of the light quanta- “licht quanten ” in their original version- is rightfully associated with the article [1] published by Albert Einstein in 1905, “An heuristic point of view about the production and transformation of light”. Interestingly, several points expressed in a very collegial style in this article were exposed again in a more direct, “einsteinian ” style, in a conference [2] that Einstein gave in Salzburg on september 21, 1909. This conference, entitled “The evolution of our conceptions about the nature and the constitution of radiation ” reveals, and to some extend completes, the way of thinking that lead Einstein to the 1905 papers on relativity and radiation. For instance, in 1909 Einstein gives again the list of open problems in the radiation theory, which briefly alluded to in the 1905 article. These problems were: 1. why does the appearance of a photochemical reaction depends only on the colour of light, and not on its intensity? 2. why is short wavelength radiation generally more active chemically than long wavelength radiation? 3. why is the kinetic energy of cathode rays (electrons) produced by the photoelectric effect independant on the light intensity