Came across the below experiment, on Wikipedia, but don't understand how it was performed... It was shown experimentally that: in a double-slit system where only one slit was open at any time, interference was nonetheless observed provided the path difference was such that the detected photon could have come from either slit. The experimental conditions were such that the photon density in the system was much less than unity. Question: if only one slit is open -- how is the path difference created? how could the photon have come from either slit? Reference is: Sillitto, R.M. and Wykes, Catherine (1972). "An interference experiment with light beams modulated in anti-phase by an electro-optic shutter". Physics Letters A 39 (4): 333–334. Bibcode 1972PhLA...39..333S. doi:10.1016/0375-9601(72)91015-8.