In this paper, we will celebrate the International Year of Light telling a history about the concept of photon in the second half of the twentieth century. One can think that the concept of photon was established in physics just after the 1930s. However, we will highlight that after the 1950s theoretical and experimental developments contributed to revisit the canonical concept of photon of the old quantum theory. In quantum optics, the photon is not represented by the schizophrenic image of individual particle, but rather by a mathematical tool that describes an excitation of a quantum state. This paper dedicates to reveal the ways that led to a sophisticated concept of the photon, passing through the HBT effect, Glauber's coherent states, and ending with the experiments that brought to the fore the quantum nature of light.
Keywords:
photon; nature of light; wave-particle duality; HBT effect; coherent states; quantum optics; history of physics