After Planck’s introduction of the quantisation hypothesis concerning the energy exchanges between the oscillator and the radiation field, the concept that also electromagnetic radiation could be quantised was introduced by Albert Einstein [1] in 1905, the year in which Einstein produced some of the most important papers in the history of physics, in a paper entitled On a heuristic [2] point of view concerning the creation and conversion of light [3]. In the paper, Einstein clearly recognised Planck’s discovery, which had until then attracted little attention, and introduced the hypothesis that radiation was quantised not only when emitted or absorbed but also when it travels in space [4].