>>10906861Preserving the which-path information eliminates interference.
Using entanglement, we can cause interference with one photon and retain the path information with the other.
The act of observing the other photon somehow destroys interference of the first photon though. This happens regardless of distance.
The delayed-choice eraser made this one step crazier by showing that interference can be brought back by destroying the path information after the interfering photon has already reached the detector. Ostensibly this means interference is affected by what happens to the entangled partner in the future, i.e. the deletion travels back in time.
Unfortunately, whether or not interference occurs can only be decided after the entangled partner has reached a detector, so we cannot use this to send information back in time.
