 The following quantum eraser experiment was conducted by a team of physicists at the Brazilian Federal University in Minas, Surice. It starts with a normal double-slit experiment like we saw earlier, but uses counters instead of a fluorescent screen to develop the interference patterns. Here we have a laser that feeds a beta-barium borate crystal to create two entangled linearly polarized photons set off in two directions. In this experiment, we call the one direction P and the other S. The photons that go down path P are called P photons, and that those that go down S are called S photons. We'll label their linearly polarized quantum states X and Y. Because they are entangled, they will travel with probabilities for these states without actually exhibiting them, much like the spinning coins, heads, and tails. But we know that if the P photon is found to be in state X, then we know that the S photon is in state Y, and vice versa. The P photons go directly to a single photon detector labeled D sub P. The detector registers the photon and sends a signal to a coincidence counter. The S photons go through a double-slit, but instead of hitting a fluorescent screen, some enter a movable single photon detector, D sub S. When it detects a photon, it too sends a signal to the coincidence counter. Once the coincidence counter receives this second signal, a count is recorded. The counts are tallied for 400 seconds. Then the detector is moved a millimeter, and the number of counts in a 400 second interval is recorded for the new detector position. This is repeated until the detector has scanned across a region equivalent to the screen in a normal double-slit experiment. The results are displayed by plotting the number of counts as a function of the detector's position. The interference pattern is clearly observed. As we did with the double-slit experiment, we keep in mind the quantum state of the particles, both initial and after the S photon passes through the double-slit. Remember that it is the interaction between the two superposition states on the far side of the double-slit that creates the interference pattern, like we did to provide which way information in the double-slit experiment. We put quarter-wave plates in front of each slit. Measuring the polarization at the detector tells us which slit the photon went through. Given the plus or minus 45 degree shifts created by the quarter-wave plates, the two superposition states cancel each other out. We are left with just the two particle-like probabilities. When the coincidence counts were tallied at each detector location, it was found that indeed the interference pattern was gone. In order to regain an interference pattern, we place a polarizer in the P beam closer to the source crystal than the quarter-wave plates, oriented at plus 45 degrees, the same as plate 1, or minus 45 degrees, the same as plate 2. This changes the P photon's state. The entangled S photon is modified as well, but maintains its linear polarity. Therefore it will still be turned into left or right circular polarity by the wave plates, and therefore still eliminate the interference pattern generating quantum state terms, and therefore still create the blob rather than an interference pattern. But now we will no longer count all the detected S photons. We count only the ones that correspond to P photons that make it through the polarization filter. This will produce an interference-like pattern that reflects what is going on with the P photons. This is called a fringe pattern. When we do a run with the filter at minus 45 degrees, we get the anti-fringe pattern. Superficially it looks like the situation that prevented interference has been erased. That is why this is called the quantum eraser. But in fact we see that nothing has been erased. When we add these two together, we get exactly the blob image we've created ever since we added the which-way information. You may have already noted that having the P photon reach the polarizer before the S photon reaches the double slit is irrelevant. The exact same behavior happens if the S photon passes through the double slit before the P photon hits the filter, or after. Again fringe and anti-fringe patterns are produced. This setup is made to look like interacting with the P photon changes what happens to the entangled S photon in the past. This has been given its own name, Delayed Quantum Eraser, even though nothing has been erased. Many eraser experiments use beam splitters and adjusted path links to turn the blob into fringe and anti-fringe patterns. Either way, I find it very sad that some physicists characterized this experiment as an example of the cause coming after the effect.