# Double slit detector

Everybody has an animated cartoon of the double slit experiment and how amazing it is. Does anyone have an actual video of it actually being done? In particular, I want to see how they detect which slit the photon goes through without destroying the waves. NO CARTOONS PLEASE.

• This is this Veritasium video about it in which he claims to detect single photons with a "photomultiplier tube". I cannot vouch for it one way or another: youtu.be/GzbKb59my3U?t=145 Aug 18, 2022 at 20:26

Here's Brian Greene demonstrating an interferometer to Stephen Colbert after the LIGO's discovery of gravitational waves. There the question is "which arm of the interferometer did each photon travel down," rather than "which slit did the photon go through," but the idea is the same, as is much of the mathematical treatment. You can see the interference pattern disappear when Greene blocks one arm of the interferometer, removing the "which-arm" ambiguity. (You can also see the interference pattern changing throughout the piece as the floor of the performance stage flexes under the interferometer, well before Colbert shouts "SCIENCE!")

The answers to this question suggest a way you could perform the experiment yourself and take your own video. You should be able to pick up an appropriate laser pointer next time you're at an office supply store, for about \$10.

A manifestly quantum-mechanical double-slit experiment requires turning down the intensity of your light source until the most probably number of photons in your experiment is zero. You'd need a position-sensitive photon detector and data acquisition hardware to determine whether the interference pattern still emerges after you've accumulated enough statistics. It does, because each photon interferes with itself:

I snatched that image, published in 1989 by Tonomura et al., from this answer to a similar question, which includes a link to this Veritaseum video which seems to be exactly what you're after. (Note that a double-slit experiment operating in the single-photon brightness regime would, all on its own, make for pretty unsatisfying video, because "one photon at a time," to most video cameras, is not distinguishable from "complete darkness." What you see in the video is pretty much what you'd see if you came to my lab with a shopping list and we built such an experiment together.)

The tail end of your question ("I want to see how they detect which slit the photon goes through without destroying the waves") implies a misunderstanding: determining which slit the light travels through destroys the interference pattern. A Scientific American article from ten years ago describes a home experiment to that effect, but doing it looks just like in the first link where Brian Greene blocks one interferometer arm and the interference pattern goes away.

There's an excellent video of double-slit interference for electrons here. Your statement "how they detect which slit the photon goes through without destroying the waves" doesn't make sense, though.

• That shows a result and not the experiment set up. Many double slit experiment animations claim that the act of observing which slit it goes through causes the partical to know and magicly pick a slit thus destroying the interference pattern. I think they are just doing bad science or missing a critical concept. I see lots of post of "lots of people have done it" but none of them seem to have cameras. Dec 7, 2016 at 6:04
• The act of observing does nothing to the experiment. It's the act of detecting that interferes. If you place the detector somewhere in the middle of the experiment it will block the photon from making it to the detection screen where it can add to the derivation creating a fringe pattern on the wall. The pattern you see on the detection screen is live and happening in front of you. You can record it if you want to. Dec 8, 2016 at 3:33

There's an actual video of the electron double-slit experiment included in this article: http://physicsworld.com/cws/article/news/2013/mar/14/feynmans-double-slit-experiment-gets-a-makeover