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A Do-It-Yourself Quantum Eraser: What polarizers do to photons

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BRIAN MARANAN PINEDA
Polarizing film has an axis (in our diagrams we depict its direction with lines on the film), and the film allows passage of light that is oscillating parallel to the axis. You can think of light 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.

You can see how polarizers work by putting two of them together. As you rotate one of the polarizers, you 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 on an angle (45 degrees is best): adding the third polarizer allows some light to get through, even though you might expect it to be an additional obstacle for the light. See if you can explain why that happens (the answer is at www.sciam.com/ontheweb). The do-it-yourself quantum eraser also relies on a polarizer at 45 degrees changing what the light does.

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