Coherent version of the setup with photon propagating along both paths 1 and 2. None of the interfering branch vectors is erased in order to illustrate the fact that there are 3 branch vectors constructively interfering and one branch vector destructively interfering at each of the detectors located at paths 5 and 6. The question is which two branch vectors from the quadruple set do annihilate each other and which two branch vectors remain? Branch vectors in blue come from path 1, while branch vectors in red come from path 2.
