A source emits pairs of particles "back to back", 50% into directions a1 and b1, 50 % into directions a2 and b2, where the choice between a1+b1 and a2+b2 is fundamentally undetermined.
Insert Diagram A
suppose that directions a1 and a2 are combined on a 50/50 beam splitter(equal path lengths)
Insert Diagram B
what percentage of the particles emitted to the left will be detected in each detector?