do all which-way attempts/techniques introduce phase difference between the "waves"? In a double slit experiment One hypothesis (or fairly well accepted theory?) is: If we try to find which-way via Quarter Wave Plates (QWPs), we add a phase difference (between the "waves that seems to have traveled" from each slit). The phase difference results in the waves not being coherent which in turn spoils/distorts the interference pattern. Interference pattern is the (neatest and) cleanest when the waves are coherent. Even in the (modified) Mach–Zehnder experiments, attempts to get which-way cause phase difference between the "waves that seem to travel" both the arms simultaneously. Are there any alternate techniques/methods (other than QWPs) to get which-way without causing any phase difference in the "waves that seem to travel" both/numerous paths? Wheeler had as a very hypothetical/thought-experiment, suggested telescopes. But I guess telescopes cannot be used. Also can we ever observe/detect a photon (position, spin etc) without disturbing it?