Here are some possible sources of polarizing film: has a special offer for Scientific American readers: An 8.5 x 6 inch piece of polarizing film for $7, US shipping included. Offer good until 31 July 2007. sells larger quantities in its main store.

Or you can search Scientifics Online and Science Kit for "polarizer" for more options. Note: You want a "linear polarizer" (a square or round shape is fine) but not "circular polarizer." (Article continues below.)

We found it easiest to use polarizers that were as thin as possible, so that they were easier to cut, and easier to butt against each other when creating the which-path labeler arrangement, and also the "double-erasure" arrangement used in step 7 of the experiment. A small sheet of the film should be sufficient.

To do the experiment, you don't need to know which orientation of your polarizer is horizontal and which vertical. It all works the same, so long as the two sides of the "path labeler" are polarized at right angles to one another, the two "analyzers" are at those same two angles, and the "erasers" are at 45 degrees from each of them.

Nevertheless, there is a way to tell when your polarizer is vertical. It turns out that the glare reflecting off the floor or off of water is predominantly horizontally polarized. Thus, if you look at the glare through your polarizer and you rotate your polarizer, when the glare is maximally extinguished, the polarizer orientation is vertical. Curiously, light scattered in the sky is also somewhat polarized. If you look at a section of blue sky with the sun behind you, the light scattering toward you will be somewhat horizontally polarized. Some bees can see the polarization of the sky and use it to help them navigate.

More to Explore:

View the slideshow of quantum erasure in action

Discuss the experiment in the blog

What You Will Need For the Experiment

What Polarizers Do To Photons

How A Quantum Eraser Works

Notes on Polarizing Film

Troubleshooting the Experiment

More Experiments

Answer to the 3-Polarizer Puzzle Featured in the Print Edition

Whither Waves? More About Interference

Cutting-Edge Experiments: Interfering Soccer Balls

Delayed-Choice Experiments

What Do the Quantum Particles Really Do?

What is Being Erased?