Hello,
Basically if D clicks the photon is detected in the vertical path (at the moment of the click) so we say its state is V just after detection.
The alternative is that a detector placed horizontaly click and then if it clicked we would say teh photon is detected at that moment in state H.
The point is that the state before the clicks can be alpha H + beta V and afterthe clicks it has collapsed to either H or V.
Hope this clarifies
Best
Nicolas
