Quantum eraser experiment
Glossary · History
In quantum mechanics, the quantum eraser experiment is a double-slit experiment that demonstrates several fundamental aspects of the quantum theory, including quantum entanglement and complementarity.
The experiment has two stages:
- The experimenter marks through which slit each photon went, without disturbing their movement, and demonstrates that the interference pattern is destroyed. This stage shows that it is the existence of the "which-path" information which causes the destruction of the interference pattern.
- The "which-path" information is "erased," whereupon the interference pattern is recovered. It does not matter whether the erasure procedure is done before or after the detection of the photons.
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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.
The experiment 
First, a photon is shot through a specialized nonlinear optical device: a beta barium borate (BBO) crystal. This action leads to what is known as spontaneous parametric down conversion (SPDC), i.e., it converts the single photon into two entangled photons of lower frequency. From then on these entangled photons follow separate paths. One photon goes directly to a detector, which sends information of the received photon to a coincidence counter, a device that notes the nearly simultaneous reception of a photon in each of two detectors so that it can count how many pairs of entangled photons have made it through the apparatus and exclude the influence of any photons that enter the apparatus without having become entangled. When the coincidence counter is signaled of the arrival of the partner photon it increments its count. A timer is set up so that it signals a stepper motor to move the second detector on a regular basis so that it can scan across the range of positions where interference fringes could be detected. Meanwhile, the second entangled photon is faced with the double-slit, whereupon it proceeds by two paths to the second detector, which sends information of a received photon to the coincidence counter. At this point, the coincidence counter has been told that both entangled photons of the original pair have been detected and that fact is added to its record along with the position currently held by the second detector. After a predetermined amount of time has passed, the detector will be moved by the tractor to examine another location. This apparatus will eventually yield the familiar interference pattern, because nothing has interfered with the disturbance that propagates through two paths after meeting the two slits and getting split up.
Next, in an attempt to determine which path the photon took through the double slits, a quarter wave plate (QWP) is placed in front of each of the double-slits that the second photon must pass through (see Illustration 1). These crystals will change the polarization of the light, one producing "clockwise" circular polarization and the other producing its contrary, thus "marking" through which slit and polarizer pair the photon has traveled. Subsequently, the newly polarized photon will be measured at the detector. Giving photons that go through one slit a "clockwise" polarization and giving photons that go the other way a "counter- clockwise" polarization will destroy the interference pattern.
The next progression in the setup will attempt to bring back the interference pattern by placing a polarizer before the detector of the entangled photons that took the other path out of the beta barium borate crystal (see Illustration 2). Because pairs of photons are entangled, giving one a diagonal polarization (rotating its plane of vibration 45 degrees) will cause a complementary polarization of its entangled pair member. So from this point on, the photons heading down toward the double slits will meet the two circular polarizers after having been rotated. And when photons enter either circular polarizer "half way off" from their original orientation, the result will be that on each sub-path half will be given one kind of circular polarization and half will receive the other polarization. The end result is that half the photons emerging from each circular polarizer will be "clockwise" and half will be "counter-clockwise." It will then be impossible to look at the polarization of a photon and know by which path it has come. Each component of an original wave-function will interfere with itself. And at this stage the interference fringes will reappear.
See also 
- Walborn et al. quantum eraser Another schematic representation of the experiment, one that follows more closely the version given in the original article.
- Delayed choice quantum eraser
- Wheeler's delayed choice experiment
- Marlan Scully
- A more technical analysis of the quantum eraser experiment
- A Scientific American article: A Do-It-Yourself Quantum Eraser - note: SciAm online subscribers only
- but here they let you see the images for free.
- The original paper on which this article is based.
- Demystifying the Delayed Choice Experiments