If we regard interference and entanglement as the most distinctive features of the quantum world, then “which-way” experiments lie at the heart of quantum physics, vividly illustrating the contrast between classical and quantum conceptions of nature. The idea of “which-way” experiments can be traced back to Einstein’s famous proposal of a double-slit experiment with a movable slit at the 1927 Solvay conference, intended to demonstrate an inconsistency in the orthodox (Copenhagen) interpretation of quantum theory. In response, Bohr analyzed this setup within his own interpretation when addressing the Einstein–Podolsky–Rosen (EPR) paradox. Since then, the conceptual and experimental issues surrounding “which-way” experiments have been explored extensively, both theoretically and experimentally, from many different viewpoints.
