Question about the Mach-Zehnder interferometer experiment Let's consider a Mach–Zehnder interferometer with a significant difference in length between the two physical paths. The optical paths are set up such that the two beams interfere 100% destructively on one of the two detectors. What happens when we turn on the light source ? According to classical mechanics there is a transient situation during which only one of the two beams arrives at the last beam splitter, so before having interference between the two beams half of the photons goes to both the detectors. After this situation we have 0% and 100% detections. According to quantum mechanics each photon interferes only with itself, so my question is: how can quantum mechanics explain this experiment? What is the difference among photons emitted at different times? I would say there's no difference, because the preparation is the same.