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I want to do a double-slit interference pattern for a lobby display.

I've done double slits many times in classrooms and labs. They're small and not highly visible. I'd like to make the pattern large and easily visible from across the lobby, even during the day. I want the pattern to be real and live, produced with EM or particles, a genuine QM effect, not faked with sound waves. A phosphorescent or pixelled backscreen that reacts to the incoming particle/waves is acceptable. The laser or particle beam needed be visible as long as the patter on the screen is highly visible.

The best possible demo would involve particles crossing one at a time constantly, each leaving a trace on the screen that lingers for a time, so that someone looking at the display can see BOTH the individual dots appearing live AND the accumulated interference pattern. But of course, in such a setup, the screen would have to somehow provide the energy to convert single photon/electron strike into a visible trace.

Jerry Guern
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    "not faked with sound waves" I'm curious what you think is "faked" about the acoustic version. – dmckee --- ex-moderator kitten Oct 02 '19 at 21:53
  • @dmckee Because I'm trying to create a live demo of a QM effect, not of its acoustic analog. I wasn't denying the existence of sound. – Jerry Guern Oct 02 '19 at 21:56
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    But showing a visible light diffraction pattern doesn't demonstrate anything inherently quantum mechanical at all. It's just a wave phenomena. It was done and explained long before quantum mechanics was a thing. To show the quantum effect you have to differentiate individual interaction sites (i.e. exhibit both wave and particle behaviors in a single run), which precludes intensities that are can be seen with the naked eye. – dmckee --- ex-moderator kitten Oct 02 '19 at 22:01
  • 17th century physicist Christiaan Huygens provided the explanation. See https://en.wikipedia.org/wiki/Huygens–Fresnel_principle – my2cts Oct 02 '19 at 22:05
  • @dmckee The best possible demo would be electrons (or other particle) crossing one at a time constantly, each leaving a trace on the screen that lingers for a time, so that you can see both the interference pattern and the individual hits. Of course, to make the dot from a single electron show, the screen would have to be actively reactive somehow. – Jerry Guern Oct 02 '19 at 22:19
  • @dmckee BTW, you can actually make the photons trickle through one at a time, and you'll measure each arriving at a single point. But yes, your backscreen would have to provide the energy to convert a single photon into a visible mark. – Jerry Guern Oct 02 '19 at 22:29
  • You also can't blow smoke or wave a card to make the beam visibly present. So even though your demo is honest, your viewer can't easily establish that for themselves. I applaud your desire here, but I don't think I've even seen a lobby demonstration from which a visitor could convince themselve of the salient reality of QM. It's a hard problem. – dmckee --- ex-moderator kitten Oct 02 '19 at 23:04
  • Budget? Single particles or photons won't be detectable by the eye; you'll need a sensitive photodetector / sensor in a light tight box to record the single photon strikes, some software that keeps the pixel lit for 10s of seconds, and a projector or huge display screen. It won't be low cost to rent. A small table top demonstrator for a darkened classroom is budget friendly (for the most part). Trying to make a phosphorescent screen is going to take a quart of paint for U$150. – Rob Oct 02 '19 at 23:43
  • Related. But if you wanted to have the same setup on some amplifying detector that's sensitive to single visible photons, rather than just a bright laser pattern on the wall, then your detector would have to be housed in a dark box. A detector for single photons operating in a sunlit lobby would die after a few seconds. – rob Oct 03 '19 at 03:59

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Here are 2 basic documents on the double slit from Harvard and MIT. The first having a nice demo based on single photons but could be easily converted to a cheaper camera and many photons (same results). And the second gives the "classical" interference math which gives a good approximation of the image geometries based on lamda and other variables that effect the pattern size.

https://dash.harvard.edu/bitstream/handle/1/27413728/single_photon_paper.pdf?sequence=4&isAllowed=y

http://web.mit.edu/8.02t/www/802TEAL3D/visualizations/coursenotes/modules/guide14.pdf

It would be much simpler to forget about use of single photons, it has been proven that the interference pattern results (or is identical) in both situations. You can use a laser diode and have a bright visible image, just exercise some caution. If you want to get into the QM nature of it you could use the polarizers as Harvard did to erase the pattern and then make it reappear.

Also the Huygens principle is an historical (1700s?) explanation that does not stand up to the modern results of the single photon experiments. A more modern explanation is based upon the "photon wave function" and Feynman path theory. It says that photons must travel in multiples of there wavelength (similar to a laser cavity) and that certain paths are allowed (bright areas, many photons) and others are not (dark areas, no photons).

PhysicsDave
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If you don't demand that you are doing a "two slit" experiment, then there are inexpensive diffraction gratings that will give you first maximum angles of ten degrees or more with visible light.

Which means projection distances less than a meter will work, and that in turn means that an inexpensive diode laser is more than good enough.

Such grating can be obtained from any supplier of classroom deomnstration equipment, and the usual limit on how cheap they go is that they want to sell them to you in packs rather than singlely.

dmckee --- ex-moderator kitten
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  • Yeah, that's the thing, there are a host of easy ways I could fake it. But it's gotta be a genuine "two slit" setup, a live demo of QM wave-particle duality. – Jerry Guern Oct 02 '19 at 22:00
  • There is nothing more or less QM about using two slits as opposed to many. You are still diffracting (exhibiting a wave property). What you do to exhibit the particle property is independent of that. – dmckee --- ex-moderator kitten Oct 02 '19 at 22:02
  • If you want to demonstrate two slit interference with particles other than photons you will have to use electrons. Search for "desk top electron diffraction demonstration" . "class room" will also work. – my2cts Oct 02 '19 at 22:13
  • @my2cts I've done the desktop experiment many times. My question is about how to make it large and highly visible. – Jerry Guern Oct 02 '19 at 22:20
  • Can't you just scale up an experiment like this. – my2cts Oct 02 '19 at 22:27
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Whatever people see in your demo setup will not be the photons themselves, but something representing the photons. So, you might as well just use a TV screen, displaying the output of an imaging photomultiplier. If you want the setup to be visible to onlookers while it's operating, you might be able to put a single-wavelength filter over the photomultiplier tube so it only sees light from your laser. An attached computer will allow you to accumulate photon detections over any desired period of time and display the results, or show individual detections as they come in.

S. McGrew
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