# What are the specific requirements for a do-it-yourself quantum double-slit experiment?

I was shocked to recently learn that the double-slit experiment is not only possible to do with completely ordinary equipment (with photons of course), but it actually looks rather easy. This is from a MIT tech video:

That time stamp starts around the description of their device. Here are some of the specifications:

• Stand 10-20m away from a light source in an otherwise mostly dark environment
• 1/4 -1/2 mm separation between the two pinholes (which act as the slits for this double-slit experiment), around 4:56 in the video
• It looks like the holes are made about 1/4 mm in diameter, but this isn't explicitly stated
• Obviously observe or resolve the pattern maybe 10 cm behind the slits with ambient light shielded out.

Now, the obvious question is why these parameters work. More importantly, what is the scientific process for selection of these experimental parameters? I understand that a combination of the separation between the slits and the wavelength determines the distance between the peaks of the interference pattern, but what is the condition on the distance between the peaks? If this is a pattern that you can observe by looking into the tube, does that mean that several peaks of the pattern must fit in the diameter of a human iris?

Also, what is really the limit that must be satisfied for the light to be sufficiently coherent? Will this experiment not work if the pinholes are too large? If the source is too close? What is the actual limits from the mathematics of the double-slit experiment that can be applied to a real home experiment?

• Look at altair.org/TwoSlit.html Commented Nov 10, 2011 at 10:35
• @mtrencseni Tried it from 2 separate locations and link doesn't work. Commented Nov 10, 2011 at 14:02
• It's not my site, it works for me. Try the google cache: webcache.googleusercontent.com/… Commented Nov 10, 2011 at 14:13
• Young's slit experiment as described in the videos with ordinary light can be explained classically using interference of waves. It's a good question though how to do the quantum version of it using single particles at home. I wonder about using a cooled CCD capable of single photon detection (maybe an EMCCD) looking at a faint astronomical source through a telescope. If you can show a time-lapse movie of the build-up of the image, one photon at a time, you'd be demonstrating the quantum nature of light.
– JxB
Commented Nov 11, 2011 at 18:28

You can acquire a laser pointer from an office supply store. Then your source will be coherent but you need to spread the beam with a lens or just back up really far away. You can see a pattern generated from just a single hole if I remember correctly.

For another home experiment, look at the link below. It can be done with some polarized lenses from sun glasses or you could order some inexpensive polarizing film online and cut it up.

http://www.users.csbsju.edu/~frioux/polarize/POLAR-sup.pdf

• It's not that the light source needs to be coherent, as each and every photon individually passes through both slits to interfere with itself. The source of light should be far enough away that it is point-like. Otherwise, it will be like having many sources spread out in space and that will destroy the pattern. Still, a laser pointer is a good point like source and a cool thing to put into your toolkit. Commented Feb 28, 2012 at 23:36
• you just redefined spatial coherence. Commented Mar 1, 2012 at 17:00