"The Principles of Quantum Mechanics"
"Suppose we have a beam of light consisting of a large number of photons split up into two components of equal intensity.
On the assumptin that the intensity of a beam is connected with the probable number of photons in it, we should have half the total number of photons going into each component. If the two components are now made to interfere, we should require a photon in one component to be able to interfere with one in the other.
Sometimes these two photons would have to annihilate one another, and other times they would have to produce four photons. This would contradict the conservation of energy...
Each photon interferes only with itself. A fraction of a Photon is never observed.
The superposition that occurs in quantum mechanics is of an essentially different nature from any occuring in the classical theory."
P.A.M. DIRAC 1927 in "The Principles of Quantum Mechanics"