which-path information

Which-path information is any knowledge — recorded anywhere, even in principle — about which of two or more routes a quantum particle took. In the double-slit experiment it means knowing through which slit the particle passed. The remarkable rule of quantum mechanics is that the moment such information exists, the interference pattern dies and the particle behaves like a plain pellet, regardless of whether anyone bothers to read the information out.

The point is subtle and easy to misstate. Interference is lost not because a clumsy detector physically kicks the particle, but because obtaining which-path information entangles the particle with something else, and that entanglement destroys the delicate phase relationship the two paths must share to interfere. Definite knowledge of the path and a visible interference pattern simply cannot coexist; they are complementary.

This trade-off can be tuned smoothly. Gather partial which-path information and you get partial fringe visibility; the sharper your knowledge of the route, the fainter the stripes, following a precise quantitative bound. The lesson is that interference is not about the particle splitting in space, but about whether the world keeps a record distinguishing the alternatives — if no such record exists anywhere, the alternatives interfere.