Past exam of the quantum information course of the University of Cambridge / 2013 / Paper 1 / 1 / a

With both final splitters removed, the path labels map directly into detector labels:In particular $P_{RR}(C^{+},C^{-})=0$, because joint occupation of the two intersecting arms has already annihilated. Each of the other three charged-particle coincidences has unconditional probability $1/4$.