I know how to perform the double slit experiment in high school: a dark room, a source of light like a consumer electronics laser, a black board, and a card board with double slits.

Is there way to extend this experiment to the one where we observe from which slit the photons pass (and causes them to not create the interference pattern anymore) without sophisticated measurement devices? Something that we can perform at home or at a typical high school?

I have searched a bit but haven't found the answer. Does observing which slit the photons pass through require sophisticated measurement devices?

  • $\begingroup$ I am looking to demonstrate the experiment to kids. thank you for the help, I can more focus on searching for DSE with electrons. (I assume it would be very difficult to do DSE with things largest then electrons.) $\endgroup$
    – Kaveh
    Commented Jul 5, 2023 at 1:39
  • $\begingroup$ Friendly reminder: Please don't use comments to post answers. For one reason, wrong or misleading comments can't be downvoted, and responses to them clutter the question. $\endgroup$
    – rob
    Commented Jul 5, 2023 at 2:48
  • $\begingroup$ If you observe a photon then it is destroyed ..... but it is possible to do the DSE with particles that can be observed! $\endgroup$ Commented Jul 5, 2023 at 15:06
  • $\begingroup$ If you're audience is children .... the most important point of the DSE is that it shows light behaves like a wave ! $\endgroup$ Commented Jul 5, 2023 at 15:08

1 Answer 1


You can "tag" photons by covering your two slits with perpendicular polarizers. You can get polarizing plastic from any experimental supply house, or you can steal the glasses the next time you see a 3D movie. One 3D movie technology uses circular polarization, so you even have access to all three polarization bases for light: horizontal-vertical, diagonal-diagonal, and left-right circular, or HV, DD, LR.

I would make the slits by painting a microscope slide black and then slicing the paint with parallel razor blades (or spend a few dollars to get such a slide where a manufacturer has quality-controlled that the slits have the same width and are properly parallel). You can attach the polarizing films to this slide with any old adhesive, so long as any edge funkiness is in the gap between the slits. Beware that, the wider your gap, the smaller the angles in your double-slit pattern and the further your wall will need to be.

Now you can use a second polarizer downstream of your slits to decide what light to analyze. If your upstream polarizers are in the HV basis, then putting your downstream polarizer horizontal or vertical only admits light from one slit, and you'll see the single-slit pattern. But if you analyze the light in a diagonal basis, you "un-tag" the photons, and you'll get the double-slit pattern. This is related to the "quantum eraser" experiment, but lets you see how the interference patterns in the original bases add up to give the pattern when all of the light is allowed through.

  • $\begingroup$ The OP is asking can we "observe" the photon as it passing thru the slit ..... NO we cannot observe it while passing thru the slit(s). Using polarizers to effectively block the slit is pretty much the same as just taping a piece of paper/foil over the slit to observe the single pattern. As described above you are assuming that a laser is used as the light source (i.e. polarized) your experiment above won't work for a incandescent source. $\endgroup$ Commented Jul 5, 2023 at 15:04
  • $\begingroup$ The question asks whether we can "extend this experiment to the one where we observe from which slit the photons pass (and causes them to not create the interference pattern anymore)." Orthogonal polarizers create distinguishable photons, which no longer interfere. A laser is a nice light source because it is bright and monochromatic; the double slit experiment works fine with sunlight, and the polarization can be managed entirely be the polarizers I mentioned. $\endgroup$
    – rob
    Commented Jul 5, 2023 at 16:34
  • $\begingroup$ pdfs.semanticscholar.org/8144/… You should include the step where the laser polarizations needs to be aligned with the experiment. $\endgroup$ Commented Jul 5, 2023 at 19:53
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    $\begingroup$ @Kaveh I'm pretty sure I learned about this homemade quantum eraser from an article in Scientific American. This 2007 page feels familiar, but not all of the links and images load correctly on my current browser. Not sure whether that is some adblocker on my end or regular link rot — but your favorite library can get you the print version. $\endgroup$
    – rob
    Commented Jul 6, 2023 at 4:19
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    $\begingroup$ I have a recollection of seeing an apparatus at a conference about fifteen years ago that could do an optical double-slit experiment in the single-photon limit. If the vendor wasn't Pasco, it was a similar company; the price point was low four figures. I don't instantly see the apparatus in the Pasco catalog, but maybe it wasn't a good seller and has gone away. It's not a great demo for students who aren't driving the experiment themselves, because the single-photon avalanche photodiode needed to scan its way across the inside of a sealed darkbox. $\endgroup$
    – rob
    Commented Jul 6, 2023 at 4:25

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