The delayed choice experiment wants to point our another strange thing about quantum mechanics, which is that in the experiment the particle seems to know what is going to happen.
This experiment shows again that the quantum world does not act like our classical world. There are various different types of delayed choice experiments, but they all shows that the particle seems to have the ability to see into the future. Here I will take a fairly easy one.
In the experiment you have a photon emitter and the a mirror which lets though half of the light shone on it and reflects half of it(a so called half-silvered mirror). That is it splits the beam up into two paths. Later with the use of other mirrors, these two paths recombine to create the normal interference patter(instead of two slits there’s two paths) so now it seems like the photon has traveled down two paths at once to create the interference pattern.
Then if we place a photon detector along one path to see which path the photon goes. This will, as with the slit experiment, destroy the interference pattern since this measurement collpases the photons wave function, gives it a certain position and now it can only go one path and not create a interference pattern. But then wait until the photon have gone through the mirror to switch it on then the photon has already made a choice to travel down two paths to create an interference patter or just one. But with the detector on the pattern will disappear, all the time. One can wonder, how could the particle know if the photon detector where going to be switched on or not? Whether to take two paths at once or just travel down one of them?