Physics
Delayed-choice quantum eraser
The screen never changes; the sorting does. Detectors C and D each show fringes with a contrast near one, and adding them back together gives a contrast near zero - the interference was in the coincidence subset, not on the screen.
What happens
A pair is made: the signal photon goes to the screen, the idler wanders through beam splitters into one of four detectors. The screen never changes - every signal photon arrives the same way whatever happens to its twin. What changes is which subset you look at: detectors A and B reveal which slit was taken and show no fringes at all, C and D have that information destroyed and show fringes shifted half a period from each other, and putting C and D back together cancels them again. The filter is a control on the sorting, not a switch on the physics: the counts do not move when it does, only which of them is being drawn.
Equations used in the module
Equations P_±(x) ∝ |ψ₁ ± ψ₂|² P_+(x)+P_−(x) ∝ |ψ₁|²+|ψ₂|²
