Quantum eraser experiment
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In physics, the quantum eraser experiment is a double-slit experiment that demonstrates several laws of Quantum Mechanics, including wave-particle duality which seeks to explain certain wave and particle properties of matter, complementarity and the Copenhagen interpretation which outline the idea that, in QM, to gain an accurate measurement of one aspect of certain experimental subjects is to lose precision in its counterpart (much like attempting to view both sides of a coin at once). The experiment also utilizes a special crystal for its ability to produce photon pairs from a single photon and the use of an interferometer to explore the wave-like nature of an object.
[edit] Introduction
The quantum eraser experiment is a double-slit experiment in which particle entanglement and selective polarization is used to determine which slit a particle goes through by measuring the particle's entangled partner. This entangled partner never enters the double slit experiment. As expected, the measurement of 'which way' destroys the interference pattern in the 'double-slit' portion of the experiment. The advantage of the quantum eraser is that you can restore the interference pattern without changing anything in the double-slit experiment by simply destroying the 'which way' information in the entangled particle that was previously used to determine the 'which way' path before the 'which way' information is available for observation. The quantum eraser effectively erases the 'which way' information (and restores interference) without altering the double-slit experiment, and thereby restores the readily visible interference pattern that manifests itself through the constructive and destructive wave interference.
A variation of this experiment, delayed choice quantum eraser, allows the decision whether to measure or destroy the 'which way' information to be delayed until after the entangled particle partner (the one going through the slits) has either interfered with itself or not. This appears to have the bizarre effect of determining the outcome of an event after it has already occurred, which plays along precisely with the aforementioned rules laid down by modern Quantum Mechanics.
[edit] The experiment
First, a photon is shot through a specialized nonlinear beta-barium borate (BBO) crystal. This process, known as spontaneous parametric down conversion (SPDC), converts the single photon into two entangled photons of lower frequency (and thus, lower energy, as visible through E = hf, where h is Planck's constant, f is the frequency, and E is, of course, energy). These entangled photons then proceed to follow separate paths. One photon goes directly to a detector, which sends information of the received photon to a coincidence chamber; the coincidence chamber then waits for news of the other, second photon. Meanwhile, the second photon is faced with the double-slit, whereafter it proceeds to its own detector, which sends information of a received photon to the original coincidence chamber. At this point, the coincidence chamber has been told that both photons of the original pair have been detected, it has been registered as such, and more photons are repeatedly put through this process. As expected, this setup yields the familiar interference pattern according to basic QM, as we have no 'which way' information regarding either photon and it is allowed to continue in an unobserved state.
Now, to gain the desired 'which way' information, a quarter wave plate (QWP) is placed in front of each of the double-slits that the second photon must choose. These crystals will change the polarization of the light. In turn, the now repolarized photon will be measured at the detector, and it can be determined which slit the photon passed through. This 'which way' information about the photon will indeed kill the interference pattern.
The next progression in the setup will attempt to bring back the interference pattern. This is done by placing a polarizer before the detector of the first photon. This will cause the polarization information obtained by that detector to be of no consequence in determining which photon is actually being examined, and thus no conclusions may be drawn and the 'which way' information will be destroyed before it is able to be analyzed and observed (this is the actual 'quantum erasure'). That being done, the interference pattern does, in fact, return.
