What quantum experiment uses 'which-path' information to erase interference patterns?

A quantum eraser is an experiment that removes usable which-path information so that interference can reappear.

In a double-slit setup, interference disappears when some physical property—such as polarization or an entangled partner—records which slit a particle could have taken. The quantum eraser changes the measurement arrangement so that this path information is destroyed or made fundamentally unavailable. When the relevant detection results are grouped appropriately, interference fringes can be recovered.

The effect does not send information backward in time and does not physically force a particle to change an earlier path. In many versions, the raw detection screen remains an interference-free blur; fringes emerge only in correlations between paired measurements. This distinction is a common source of confusion, especially in discussions of the delayed-choice quantum eraser.

The idea was proposed by Marlan Scully and Kai Drühl in 1982. Modern demonstrations commonly use entangled photon pairs produced by spontaneous parametric down-conversion, with one photon carrying path-related information and the other being observed for interference.