I have read an article were it is claimed that if when two photons are entangled by parametric down-conversion, you don't observe interference in the first photon (after going through a double slit) if you don't collapse the second in such a way that which-way information is destroyed.
The reasoning is that if the second photon is still alive, in principle you can recover this which-way information.
This line of reasoning does not seem very clear to me.
What do you think?