ABSTRACT
In 1905, Albert Einstein showed that the photoelectric effect could be explained by the hypothesis that the energy of a light beam was distributed in discrete bundles, later known as photons. Photon-counting experiments provide a fairly direct measurement of the photon probability distribution for all kinds of light embraced by the quantum theory. The field of quantum optics occupies a central position involving the interaction of atoms with the electromagnetic field. Young's experiment is one example of a phenomenon that can be explained by both classical and quantum theory. In Young's experiment, each photon must be capable of interfering with itself in such a way that its probability of striking the second screen at a particular point is proportional to the calculated intensity at that point. Multiphoton transitions are found whenever electrons bound in atoms or molecules interact with sufficiently intense electromagnetic radiation.
