Introduction
The quantum eraser experiment is a variation of Thomas Young's classic double-slit experiment. It establishes that when a photon is acted upon in a fashion that allows which slit it has passed through to be determined, the photon cannot interfere with itself. When a stream of photons is marked in this way, then the interference fringes characteristic of the Young experiment will not be seen. This experiment displays the capability to create situations in which a photon that has been 'marked' to expose through which slit it has passed can later be 'unmarked'. A photon that has been 'marked' cannot interfere with itself and will not produce fringe patterns, but a photon that has been 'marked' and then 'unmarked' can interfere with itself and will produce the fringes characteristic of Young's experiment.
This experiment involves an apparatus with two main sections. After two entangled photons are created, each is directed into its own section of the apparatus. It then becomes clear that anything done to learn the path of the entangled partner of the photon being examined in the double-slit part of the apparatus will influence the second photon, and vice-versa. The experimental apparatus is so constructed that at some point between the double slits and the detection screen (or between a beam splitter that also creates two paths for photon travel and thus the possibility of interference) a change in the apparatus can be made that either maintains separation of the two paths or else runs them together. If the two paths are kept separate, no interference phenomena will be observed. However, if the two paths are reunited then it becomes impossible to determine by which single path a photon might have arrived after the reunion. (Imagine an Interstate highway that takes a northern path around a city, I-1000 North, and a southern path around the city, I-1000 South. While a car is north of the city it is clear that it has traveled by way of I-1000 North, but after its path merges with traffic from I-1000 South neither it nor any other car can be identified as having gone north or south of the city.)
The advantage of manipulating the entangled partners of the photons in the double-slit part of the experimental apparatus is that experimenters can destroy or restore the interference pattern in the latter without changing anything in that part of the apparatus. Experimenters do so by manipulating the entangled photon, and they can do so before or after its partner has entered or after it has exited the double-slits and other elements of experimental apparatus between the photon emitter and the detection screen. So, under conditions where the double-slit part of the experiment has been set up to prevent the appearance of interference phenomena (because there is definitive "which path" information present), the quantum eraser can be used to effectively erase that information. In doing so, the experimenter restores interference without altering the double-slit part of the experimental apparatus. An event that is remote in space and in time can restore the readily visible interference pattern that manifests itself through the constructive and destructive wave interference. The apparatus currently under discussion does not have any provision for varying its time parameters, however.
A variation of this experiment, delayed choice quantum eraser, allows the decision whether to measure or destroy the "which path" information to be delayed until after the entangled particle partner (the one going through the slits) has either interfered with itself or not. Doing so appears to have the bizarre effect of causing the outcome of an event after the event has already occurred. In other words, something that happens at time t apparently reaches back to some time t - 1 and acts as a determining causal factor at that earlier time.
Read more about this topic: Quantum Eraser
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