# Young's double slit experiment - homemade

I want to do the double slit experiment to demonstrate the particle-wave-duality to people outside of physics. I can do it on paper and try to explain the maths and physics behind it, but it is a dull thing for non-physicists without a demonstration. Building the slits and using a laser to show the interference pattern is easy, but how do I realize the 'measuring device', which detects through which slit a photon went?. What kind of apparatus does it take?

edit: It seems I wasn't clear. What I'm trying to achieve is to display the interference pattern of the wavefunction and then let it collapse due to the measurement of which slit my photon/electron/whatever went through.

• I am not sure about how they measure the photons but if you are doing the experiment with electrons, then you can just shine light on the double slit. the electron will disperse light and hence you will be able to tell which slit the electron came through. Dec 7, 2017 at 14:25
• so I guess the cheapest way to get a handy electron gun would be buying an old tube TV (from a garage sell maybe) and take it out from there? Dec 7, 2017 at 14:38
• Ummmm.... The wavefunction goes through both slits in the normal Young's interference. Dec 7, 2017 at 14:41
• I have only heard of it as a thought experiment. I am not sure if it has been done before. @JonCuster if you are observing the slit, then you are forcing the wavefuntion to collapse and take one slit only. Dec 7, 2017 at 14:48
• If you do this with light, you aren't displaying quantum behavior unless you can detect individual photons. Otherwise you're just testing the classical wave theory of light. Dec 7, 2017 at 19:21

You can get the which path-information via the photons polarization, just put polarization filters (e.g. from 3D cinema glasses) with 90 degrees relative rotation on each slit (let's call the orientations 0 degrees and 90 degrees). You can decide to use this information via a third polarizer in front of the screen: if its orientation is 45 degrees, then you don't have the path information at the screen and you see the interference pattern, but you don't see it for 0 or 90 degrees. See also Quantum Eraser Experiment.

• @t.rathjen: regarding your edit of the question, the polarization measurement of the third polarization filter tells you through which slit the photon went (the one with the polarization filter of the same orientation), thus it is the "measurement of which slit my photon went through" you seem to be looking for. Dec 7, 2017 at 19:58

It is easy to observe the patterns on a screen. Veritassium made a very nice video of Young's original experiment. Even easier if you use a laser.

Or directly, watch the pattern on your own retina by holding a slit (or pair of slits) just in front of one's pupil. Then look at a sodium street lamp or at Christmas led lamps in different colours.

For a permanent record, one can use the sensor of a dslr (digital single-lens reflex) camera. Remove the lens, just use the body. Or one could use the body cap, drill a hole, cover the hole with a double slit and watch the pattern. That is how I made the images in this composite.

The collapsing thing is difficult to show, though. You can close one of the double slits. Or manipulate the polarization of one of them.

• FWIW, you can see single-slit interference with no equipment needed apart from your fingers and a sufficiently bright light source. But I guess it does help if you're a little short-sighted. ;) Dec 7, 2017 at 17:27
• @PM 2Ring. The light from the SE application background suffice :) Dec 7, 2017 at 20:44