Can anyone help out? I was reading about MZ interferometers on Wikipedia http://en.wikipedia.org/wiki/Mach-Zehnder In the wiki article they have a picture of 4 samples of photon paths in a MZ interferometer. They indicate the probability of a finding a photon at detectors (A,B,C) for 3 of the diagrams but the fourth they leave you guessing. I am really bugged. I can't find the answer anywhere. What is the probability of detecting a photon at detector A in the bottom right diagram. I am thinking it must be 50%?