This comes from the Interference of Photons section in the book The Principles of Quantum Mechanics by P Dirac:
We shall discuss the description which quantum mechanics provides of the interference. Suppose we have a beam of light which is passed through some kind of interferometer, so that it gets split up into two components and the two components are subsequently made to interfere. We may, as in the preceding section, take an incident beam consisting of only a single photon and inquire what will happen to it as it goes through the apparatus. This will present to us the difficulty of the conflict between the wave and corpuscular theories of light in an acute form.
Corresponding to the description that he had in the case of the polarization, we must now describe the photon as going partly into each of the two components into which the incident beam is split. The photon is then, as we may say, in a translational state given by the superposition of the two translational states associated with the two components. We are thus lead to a generalization of the term "translational state" applied to a photon. For a photon to be in a definite translational state it need not be associated with one single beam of light, but may be associated with two or more beams of light which are the components into which one original beam has been split. In the accurate mathematical theory each translational state is associated with one of the wave functions of ordinary wave optics, which wave function may describe either a single beam or two or more beams into which one original beam has been split. Translational states are thus superposable in a similar way to wave functions.
Is this the view held by leading physicists today?
Can we really talk of the incident and exiting photon in the beam splitter being the same photon?
Is Dirac saying that the photon is partly in the two split beams, but no where else in space?