This experiment is based on a Scientific American article from April 14, 2007 ( http://www.arturekert.org/sandvox/quantum-eraser.pdf [Broken] ) or ( http://www.angelfire.com/folk/thegrieves/transfer/200705.pdf in renderable text). The article demonstrates how to set up an experiment that illustrates what is known as quantum erasure. The text below borrows heavily from the SciAm article, but I reproduced the experiment myself at home, with my own equipment, and the pictures were taken by me. They have not been faked; they are the actual photographs I took using the procedures described. I’m assuming that the reader has some understanding of the dual wave / particle nature of light, Thomas Young’s famous Double Slit experiment performed in the early 1800s (http://physics.about.com/od/lightoptics/a/doubleslit.htm ), the first double slit experiment using electrons, conducted by Claus Jönsson in 1961 ( http://220.127.116.11/~mvr/ch412/joens.pdf ), and an experiment demonstrating the destruction of the interference pattern using an atom interferometer by Dürr et al in 1998: "Origin of quantum-mechanical complementarity probed by a 'which-way' experiment in an atom interferometer": Nature 395, 33-37 (3 September 1998) | doi:10.1038/25653; (1998) (http://www.atomwave.org/rmparticle/ao%20refs/aifm%20pdfs%20by%20group%20leaders/rempe%20%20pdfs/Rempe%20decoherence%201998.pdf#page=3&zoom=150,0,340 [Broken] ). Quantum erasure involves one of the strangest features of quantum mechanics — the ability to take actions that change our basic interpretation of what happened in past events. Before it is explained what is meant by this and the experiment itself is outlined, one caveat must be emphasized in the interest of truth in advertising. The light patterns that will be seen if the experiment is conducted successfully can be accounted for by considering the light to be a classical wave, with no quantum mechanics involved. So in that respect the experiment is a cheat and falls short of fully demonstrating the quantum nature of the effect. Nevertheless, the individual photons that make up the light wave are indeed doing the full quantum dance with all its weirdness intact, although you could only truly prove that by sending the photons through the apparatus and detecting them one at a time. Such a procedure, unfortunately, remains beyond the means of this experimenter. Still, by observing the patterns in this experiment and by thinking about what they mean in terms of the individual photons, the reader can get a firsthand glimpse into the bizarre quantum world. Polarizing film has an axis, and the film allows passage of light that is oscillating parallel to the axis. Light can be thought of as being like a wave on a rope held between two people; the wave can make the rope move up and down or side to side or at any angle in between. The angle of the oscillation is the polarization of the wave. Polarizing film is like a screen of parallel bars that the rope passes through: it lets through waves polarized parallel to it unhindered, blocks perpendicular ones completely and allows waves on other angles to get through with reduced amplitude. Most important, the wave (if any) that comes out the other side of a polarizer is polarized parallel with the polarizer’s transmission axis. The quantum description of what happens to light going through a polarizing film sounds only slightly different: The light is made up of individual particles called photons, and like a wave, the photons can each have a direction of oscillation. A photon will get through every time when it hits a polarizer with the transmission axis parallel to the photon’s polarization. A perpendicular polarizer blocks the photon every time. At a 45-degree angle, the photon has a 50 percent chance of getting through (the exact probability varies as the angle is varied). Most important, when a photon does go through a polarizer, on the other side it will be polarized parallel with the polarizer’s transmission axis. Light can also be unpolarized, which means the photons making up the light have random polarizations. That is another case in which half the photons will get through a polarizer, and, as always, those that do so become polarized parallel with the polarizer. We can see how polarizers work by putting two of them together. As we rotate one of the polarizers, we can see through them clearly when their axes are aligned, barely at all when they are perpendicular and to some extent at other angles. Photons that make it through the first polarizer are polarized by it, and then their probability of getting through the second one depends on the angle between their polarization and the second polarizer’s axis. An interesting effect happens if two polarizers are perpendicular and a third one is inserted between them at an angle (45 degrees is best): adding the third polarizer allows some light to get through, even though we might expect it to be an additional obstacle for the light. The do-it-yourself quantum eraser also relies on a polarizer at 45 degrees changing what the light does. The figures presented below demonstrate quantum erasure in action. Here is what I needed for the experiment: 1. A very dark room. 2. Polarizing film. Plain gray, high-quality film. I salvaged some from a pair of 3-D glasses that were handed out during the movie “Avatar.” These worked very well. I cut the film into three pieces, two for what is called a “path labeler,” and one for what is called an “analyzer” as explained below. 3. A laser pointer, preferably one that emits non-polarized light. I used a red laser pointer which I got online through eBay. The green laser was too powerful for this experiment. I used a 1.5 inch spring paper clip to hold the laser, which fortuitously pressed down the “on” button and kept the laser turned on during the experiment (see Figure 1). 4. A thin, straight piece of wire, such as from a stripped, unused twist tie. The thinner the better. Straightened staples and pencil leads didn’t work as well. 5. I did not use a piece of tinfoil with a pinhole poked through it over the business end of the laser pointer, as suggested in the SciAm article. I found this caused some unwanted diffraction of the light that went through it. 6. Some stands to hold the laser and polarizers in place. I used some nested boxes that were just the right size (see Figure 1). 7. A two-pronged clamp to hold the polarized film pieces in place. Also as noted above, a 1.5 inch spring paper clip to hold the laser and to keep it turned on. An elastic band or some adhesive tape wrapped around the laser pointer will also do. 8. A screen to display the final patterns. I just projected the beam onto a bare wall about 4.5 feet from the laser and polarized film. SEEING THE INTERFERENCE. The laser is set up so it shines on the wall from about 4.5 feet away. It first produces a circular spot of light on the wall. The wire is then positioned vertically and centered in the light. WHAT HAPPENS: As shown below in Figure 1, an interference pattern is produced, consisting of a row of fringes (bright and dark bands). The interference pattern arises because light passing on the left of the wire is combining, or “interfering,” with light passing on the right-hand side. If a piece of paper is held just after the wire, a lobe of light will appear on each side of the shadow of the wire. The lobes expand and largely overlap by the time they reach the wall. For each individual photon arriving at the wall in the overlap region, it is impossible to tell whether it went on the left or the right side of the wire, and the combination of the two ways it went causes the fringes. Although we are looking at trillions of photons, each of them is interfering only with itself. Figure 1. Apparatus with red laser beam passing through single wire showing interference pattern on wall. LABELING THE PATH. Take two polarizers and rotate one of them so that their axes are perpendicular; you have done this correctly if when you overlap the film temporarily, no light goes through the overlap region. Tape them together side by side with no gap or overlap. Do the taping along the top and bottom so the tape will not block the light. This will be called the path labeler. Position the labeler in the beam so that its join is right behind the wire. Attaching the wire to the labeler might be easiest. Wire and labeler will not be moving for the rest of the experiment. We will say that the left-hand polarizer produces vertically polarized light (V), and the right-hand one horizontally polarized (H). It does not matter if we have these labels reversed. WHAT HAPPENS: Even though the light is again passing on both sides of the wire, the fringes should be gone. If a photon reaches the screen by passing to the left of the wire, it arrives V-polarized; if to the right of the wire, H-polarized. Thus, the labeler has made available the information about which way each photon went, which prevents the interference. Figure 2. Apparatus with H-V path labeler and collapsed interference pattern on wall ERASING THE PATH INFORMATION. Rotate the polarizer (the analyzer) 45 degrees clockwise from V, an orientation we call diagonal (D). WHAT HAPPENS: The fringes reappear! Why? The polarizer is erasing the information about which side each photon used. Now each left-passing V photon has a 50 percent chance of getting through it to the screen, as does each right-passing H photon. In both cases, the photons that get through become D-polarized, so there is no way to tell which way each photon went. Once again, each photon apparently goes both ways at once and interferes with itself. Figure 3. Apparatus with path labeler and analyzer, showing restored interference pattern To sum up: the labeler makes available the information about which way each photon went, which prevents the interference. When the analyzer is introduced, it erases the information about which side each photon used. Now each left-passing V (“vertical”) photon has a 50 percent chance of getting through it to the screen, as does each right-passing H (“horizontal”) photon. In both cases, the photons that get through become D-polarized (“diagonally”), so there is no way to tell which way each photon went.