In this chapter we build on the remarkable new paradigm, introduced in the previous Chap. II-2, which was first put forward in [1]. It states that diffraction patterns, long associated with wave interference, are instead direct signatures of the quantum states of light. We here specifically demonstrate this paradigm by discussing modern quantum versions of Young’s double slit experiment (see Fig. I-1.6). This experiment which to this day underlies our mental picture of the phenomenon of diffraction, is discussed within the more fundamental and complete framework of quantum optics, which may be viewed as the photon-only part of QED.