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

Interchanging the two wings givesNow detection at $D^{-}$ occurs only together with $C^{+}$, with probability $1/8$. The remaining nonzero coincidence probabilities are $P(C^{+},C^{-})=1/8$ and $P(D^{+},C^{-})=1/2$.