SciAm’s “Quantum Eraser” Home Experiment
Rachel Hillmer and Paul Kwiat have an article in the May 2007 Scientific American called “A Do-It-Yourself Quantum Eraser” (starts on p.90). This is a variant on one of the classic quantum mechanical experiments, the two-slit experiment. That’s the one where in trying to determine whether light has a wave or a particle nature, one comes to the answer that it has both.
Hillmer and Kwiat’s home experiment requires a laser pointer, a staple or paper clip, some metal foil, some stands to hold the laser pointer and other bits in place, and polarizing film ($7 on special from one source). And, of course, a room that is as dark as you can manage.
A pinhole in foil is used to restrict the laser pointer’s output. A rubber band or piece of tape can be used to keep the laser pointer on during the experiment. A straightened-out staple or paper clip (“the thinner the better”) is placed in front of the laser pointer such that it is in the middle of the beam. (Depending on the make of your laser pointer, you may need to use a piece of the polarizing film just in front of it to obtain a polarized beam. See the instructions in the article.) In the dark and beyond that, one should see an interference pattern on a wall or piece of paper. That’s a series of vertical bars, and it indicates that the laser beam is acting as a wave as it passes around the metal obstacle.
H&K discuss the construction of what they call a “path labeler” from two pieces of polarizing film abutted such that their planes of polarization are perpendicular to each other. The metal obstacle goes exactly at the join between those pieces. This means that laser light going around one side of the metal will be polarized in one direction, and going around the other side it will be polarized at a 90 degree angle to that. The effect should be that there are no longer the bars of the interference pattern seen at the screen. In this setup, one can learn about which path a particular photon took, and the laser beam acts as a stream of individual particles.
Up until now, this has all been essentially replicating the two-slit experiment, but in your home. Now comes the “quantum eraser” part. Take another piece of polarizing film and put it in the light path after the metal obstacle and path labeler, where its plane of polarization is at a 45 degree angle to one of the parts of the path labeler, let’s say by turning it clockwise. The interference pattern will again be seen. This is described as the result of the path becoming indeterminate again since the polarization cue to which path a photon took no longer exists. The polarizer has acted as a “quantum eraser” of information. There is also a description of an “antidiagonal” quantum eraser, made by using a piece of polarizing film rotated 45 degrees counterclockwise. One can construct a piece that applies both “erasers” at once, yielding an interference pattern that display bars and gaps at the top and bottom, where a bar on the top is matched with a gap on the bottom.
This experimental setup got my attention. It is simple, cheap, portable, and produces clear results that bring quantum weirdness right into your living room. The author biographical information with the article reveals that Rachel Hillmer is an undergraduate student at the University of Illinois, and Paul Kwiat is Bardeen Chair of Physics there. My hat’s off to both of them.
Addition: Consider the effect of adding a pellicle or beam-splitter following the metal obstacle/path labeler but before the quantum ereaser. I need to set this up to try out; a simple beam-splitter could be just a piece of flat optical glass, like a microscope slide.
