Delayed-choice quantum eraser

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A delayed-choice quantum eraser experiment is an elaboration on the quantum eraser experiment that incorporates concepts considered in John Archibald Wheeler's delayed-choice experiment. The experiment was designed to investigate peculiar consequences of the well-known double-slit experiment in quantum mechanics, as well as the consequences of quantum entanglement. Delayed-choice quantum eraser experiments are designed to investigate the following apparent paradox arising from the traditional double-slit experiment: if, upon observing a photon, one can deduce that it arrived at a detector by following a particular path, then "common sense" (which Wheeler and others challenge) says that it must have entered the double-slit device as a particle, whereas if the photon's path cannot be deduced, then it must have entered the double-slit device as a wave. By this logic, a spontaneous change in the mode of observation while the photon is in transit may force it to retroactively alter its initial "commitment" to exhibiting either wave-like or particle-like behavior. Wheeler pointed out that, if one imagines an experimental setup of interstellar proportions, this reasoning suggests that a last-minute decision made on Earth on how to observe a photon could alter a physical configuration established millions or even billions of years earlier. While delayed-choice experiments might seem to allow decisions (about how to make measurements) to alter events that have already occurred by the time they have been made, this conclusion may only be reached by adopting a non-standard interpretation of quantum mechanics. Under the standard interpretation, a photon in transit is considered to be in a superposition of states only one of which is observed in the measurement.