Final answer:
When two photons in-phase reflect off a mirror, the path length difference created by their position swap does not affect their phasing if the difference is less than the coherence length of the laser light and if there are no additional phase shifts introduced by the mirror.
Step-by-step explanation:
The question's subject matter concerns the behavior of light waves, specifically with regard to phase relationships and the effects of reflection on these phases.
When two adjacent photons with their wave phases locked strike a 45-degree mirror and are reflected, the alteration of their left-right positions results in a path length difference. However, if the photons are truly locked in phase, or coherent, this path length difference may not necessarily disrupt their phasing, especially if the difference is much smaller than the wavelength of the light.
Furthermore, the inherent properties of reflection, especially off a mirror, do not introduce a phase shift between the two photons. As long as the path difference is less than the coherence length of the laser light (which is usually quite large for laser beams), the phasing between the photons should remain intact post reflection.