  Today, the nature of light is accounted for by one of the jewels of physics,
quantum electrodynamics (QED), the fundamental theory of light and matter. Yet
owing to its infinite complexity, scientists still debate how its central
concept, the photon, can be reconciled with the perceived existence of light
waves, emerging 200 years ago in the wake of Young's double slit diffraction
experiment. Ever since, the phenomenon of diffraction has been viewed to embody
the wave nature of light, leading to the schizophrenic wave-particle duality.
The latter does not exist in QED which is photon based without the existence of
waves. Here we introduce the new paradigm that diffraction images directly
reflect the fundamental quantum states of light. This is revealed by analysis
of the evolution of modern versions of Young's experiment performed with
differently modified laser light and photon-based detection. In conventional
quantum mechanics, corresponding to first order QED, the fundamental photon
nature of light remains hidden since different quantum states produce only two
basic types of diffraction patterns that may also be explained by coherent and
incoherent wave superposition. The true photon based substructure of light is
shown to clearly emerge through characteristic diffraction images in second
order QED. The degeneracy of the first order images is lifted, the
wave-particle equivalence breaks down, and the patterns directly reveal the
true quantum substructure of light. This allows the replacement of the
conventional concept of wave coherence by a precise order-dependent degree of
coherence that quantifies the interference and diffraction behavior of all
quantum states of light.
