I have a really straightforward (I think) question regarding the effect(s) of obtaining the which-path information of photons that have travelled through a double slit and are hitting a photosensitive screen. In the thought experiment, all the photons arriving at the screen have entangled twins which simultaneously arrive at which-path detectors that are either ON or OFF. The set up is super-simple but I can’t find this particular configuration - so not sure if it would actually work. Main Question or Discussion Point A laser provides a constant stream of photons which pass through a double-slit. The photon stream emerging from each slit then passes through a crystal which splits each photon into coherent entangled pairs. One photon from each pair heads towards a photosensitive screen and the entangled twin travels toward one of two detectors (one for the left slit, one for the right slit). When the detectors are switched to the ON position, the which-path information for each photon (and its entangled twin) can be obtained. In this case, all photons behave as particles and do not interfere with one another. The pattern formed on the photosensitive screen does not exhibit a wave-interference pattern (no light and dark bands). When the detectors are switched to the OFF position the which-path information for each photon (and its entangled twin) is completely lost. In this case, all photons behave as waves and interfere with one another. The pattern formed on the photosensitive screen exhibits light and dark bands (standard wave-interference pattern). So basically, assuming 100% conversion of photons to entangled pairs, can an apparatus such as this be constructed/operate such that someone can, in real time, and only by observing the photosensitive screen, know whether the detectors are in the ON or OFF position?