Delayed choice quantum eraser
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A delayed choice quantum eraser is a combination between a quantum eraser experiment and Wheeler's delayed choice experiment. This experiment has actually been performed and published by Yoon-Ho Kim, R. Yu, S.P. Kulik, Y.H. Shih, and Marlan O. Scully [1] Phys.Rev.Lett. 84 1-5 (2000). This experiment was designed to investigate a very peculiar result of the well known double slit experiment of quantum mechanics, the dual wave particle nature of light, and in fact all matter.
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[edit] Introduction
In the double slit experiment, a photon passes through one of two slits in a double slit apparatus, and then registers on a detector. The detector shows where the photon hit it, like an image projected on a screen. If many photons individually pass through the double slit apparatus, and nothing observes which slit a given photon travels through, an interference pattern emerges on the detector. The interference pattern indicates that the light beam is in fact made up of waves. However, if someone observes which of the two slits each photon passes through, a different result will be obtained. In this case, each photon hits the detector after going through only one slit and a single concentration of hits in the middle of the detection field. This result is consistent with light behaving as individual particles, like tiny bullets. It is counterintuitive that a different outcome results based on whether or not the photon is observed after it goes through the slit but before it hits the detector.
In a quantum eraser experiment, one arranges to detect which one of the slits the photon passes through, but also construct the experiment in such a way that this information can be "erased" after the fact. It turns out that if one observes which slit the photon passes through, the "no interference" or particle behavior will result, which is what quantum mechanics predicts, but if the quantum information is "erased" regarding which slit the photon passed through, the photons revert to behaving like waves.
However, Kim, et al. have shown that it is possible to delay the choice to erase the quantum information until after the photon has actually hit the target. But, again, if the information is "erased," the photons revert to behaving like waves, even if the information is erased after the photons have hit the detector.
[edit] The experiment
The experimental setup, described in much more detail at [2], is as follows. First, generate a photon and pass it through a double slit apparatus. After the photon goes through slit A or B, a special crystal (one at each slit) uses spontaneous parametric down conversion (SPDC) to convert the photon into two identical entangled photons with 1/2 the frequency of the original photon. One of these photons continues to the target detector, while the other entangled photon is deflected by a prism to bounce off a mirror some distance away. Now, if the second photon (coming from slit A or slit B) is observed, it is known which slit the original photon went through, so the photon behaves like a particle. If the second photon's paths from slit A and B are combined, the which-way path is not observed, and the first photon behaves like a wave. The experimenter can choose to observe or not observe the which-way information by erasing (or detecting) information about the second photon's path.
The results from Kim, et al. have shown that, in fact, observing the second photon's path will determine the particle or wavelike behavior of the first photon at the detector, even if the second photon is not observed until after the first photon arrives at the detector. In other words, the delayed choice to observe or not observe the second photon will change the outcome of an event in the past.
[edit] Discussion
In terms of the conventional way of viewing the physical universe, this result seems to be a paradox. This experiment demonstrates the possibility of observing both particle-like and wave-like behavior of a photon using quantum entanglement. Furthermore, the behavior of the photon at the primary detector can be changed even after the registration of the event by the detector.
In their paper, Kim, et al. [3] explain that the concept of complementarity is one of the most basic principles of quantum mechanics. According to the Heisenberg Uncertainty Principle, it is not possible to measure both precise position and momentum of a quantum particle at the same time. In other words, position and momentum are complementary. In 1927, Niels Bohr described complementarity as “wave-like” and “particle-like” behavior of a quantum particle. this has come to be known as the wave-particle duality of quantum mechanics. The double-slit experiment is a good example of this concept. Feynman believed that this was the basic mystery of quantum mechanics. The actual mechanisms that enforce complementarity vary from one experimental situation to another. In the double-slit experiment, the common wisdom is that the Heisenberg Uncertainty Principle makes it impossible to determine which slit the photon passes through without at the same time disturbing it enough to destroy the interference pattern. However, in 1982, Scully and Druhl found a way around the position-momentum uncertainty obstacle and proposed a quantum eraser to obtain which-path or particle-like information without introducing large uncontrolled phase factors to disturb the interference. They found that the interference pattern disappears when which-path information is obtained, even if this information was obtained without directly observing the original photon. Even more surprising was that, if you somehow "erase" the which-path information, the interference pattern reappears! And, perhaps most provocative of all, you can delay the "choice" to "erase" or "observe" the which-path information and still restore the interference pattern, even after the original photon has been "observed" at the primary detector!
How can this be? It would seem that the "choice" to observe or erase the which-path information can change the position where the photon is recorded on the detector, even after it should have already been recorded.
One explanation of this paradox would be that this is a kind of time travel. In other words, the delayed "choice" to "erase" or "observe" the which-path information of the original photon can change the outcome of an event in the past. Another explanation would be that in fact both outcomes occur. The universe itself exists in a superposition of states in which either the original photon goes through slit A or slit B and in which the which-path information is either "observed" or "erased". This is described in detail in the Everett many-worlds interpretation of quantum mechanics.
