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Troubleshooting the Experiment

This sidebar is part of a package that supplements our story on quantum erasure in the May issue of Scientific American

Here are several things you can check if you are having trouble getting this experiment to work and produce results similar to those in the photos in the main article.

  1. Can't see good interference, even with just the laser and the wire.

    Your observation screen might be too close to the wire. If the screen were just after the wire, you would just see the shadow of the wire and no interference. Try to arrange your setup so that the screen is at least 2 to 3 meters from the wire.

    The wire might be too thick. The spacing of the light and dark fringes is smaller for a thicker wire, varying as one over the thickness of the wire; therefore, if the wire is too thick the fringes may be spaced too close together for you to actually see them. If you just use a hair instead of a wire, you should see very clear interference fringes (although because the hair is so thin, it will be difficult to carry out the rest of the experiment with the polarizers).

    One final problem may be that the output spot of the laser is too big, or is irregularly shaped, thereby obscuring the pattern. The tinfoil with the pinhole should fix this problem.

  2. With the adjacent horizontal and vertical labeling polarizers in place, you still observe some interference on the screen.

    This can occur if the two polarizers are not completely perpendicular, i.e., they do not definitively label which path the photon took, resulting in a small amount of residual interference, i.e., barely visible fringes. The fix for this is simply to prepare another set of polarizers, trying to be extra careful that the axes are truly perpendicular.

    Another possibility is that the mating line between the horizontal and vertical polarizers is not directly behind the wire. For example, if that mating line is slightly to one side of the wire, there is the possibility for photons with just one polarization to go both ways around the wire and still reach the screen.

    Also, it's important that the labeling polarizers be fairly close to the wire (i.e., within a few centimeters). If instead you put the polarizers close to the screen, by the time a photon has reached them it is already displaying interference, i.e., there's no way of labeling which path the photon took.

  3. Everything seems to work as expected, except that the diagonal polarizer does not recover interference fringes.

    Assuming the erasing polarizer is really diagonal, the problem here probably resides in the laser polarization. We give two examples.

    First, consider the case where the initial polarization of the laser is horizontal. Then immediately after the horizontal-vertical labeling polarizers, we only have the photons which passed on one side of the wire. Since the other contributing path doesn't even exist, there's no way that we can recover interference by installing an extra diagonal analyzing polarizer. This problem can be easily remedied by simply rotating the laser about its axis, until the intensities getting through the horizontal and vertical polarizers are approximately the same.

    As a second, more subtle case, consider the situation if the light from the laser is initially unpolarized (see polarization box in main article). In this case we do have equal intensities passing through the horizontal and vertical labeling polarizers; nevertheless, in this case it is still impossible to ever perform quantum erasure and recover interference. Why? You could think of unpolarized light as being comprised of photons, half of which are horizontal and half of which are vertically polarized (but we don't know which is which). But we just showed how incident horizontally polarized photons won't show the erasure effect, since any of the photons that make it through the labeling polarizers will have only gone on the one side of the wire. The same argument applies to incident vertically polarized photons. If instead we have incident diagonally polarized photons, a photon that was transmitted by the horizontal-vertical labeling polarizer could still have come from either side of the wire. Therefore, the trick if your laser happens to be unpolarized is to put a diagonal polarizer after it; any light coming through this polarizer is then diagonally polarized.




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?
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