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So I know what happens in a double slit experiment and that if you put an observer, it changes the outcome of the experiment. It has to do with Heisenberg's uncertainty, but how does his uncertainty principle apply to light? And if I were to recreate this experiment, how would I show that it changes based on observation if I was using a laser?

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  • $\begingroup$ The double slit experiment is not dependent on HUP. HUP would only come into play if you were interested in measuring both the position and momentum of the particles going through the slit. $\endgroup$ – Aaron Stevens May 23 at 18:15
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Light is an electromagnetic wave. The uncertainty principle for light applies to electric and magnetic fields, i.e. if you observe electric field at some point, the uncertainty in magnetic field becomes very large. Although in experiments, an equivalent uncertainty relation is used: between energy of electromagnetic waves and their phase.

If you understand quantum mechanics, then I suggest "Sakurai, Advanced Quantum Mechanics". Tell me if you would like a more detailed answer.

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You do two things. First of all you make the light dim enough so that, statistically, there is only one photon in the experiment at a time, and you make sure that your detector can detect single photons. What you then see is a scatter of single-photon detections which buiilds up over time into an interference pattern. This shows that it's not some result of lots of photons: each photon 'knows' about both slits, because there is only one photon in the experiment at a time.

Now you need a way to work out which path the photons took and show that establishing this destroys the interference pattern. The way to do this is polarisers over the slits: if you have a horizontal & vertical polariser, then, if you place (say) a horizontal polariser over both slits, then this doesn't tell you which slit things went through. But if you have a horizontal over one, and a vertical over the other, it does, and the interference pattern goes away: you just get the sum of the two single-slit patterns.

To make things even more cool you can then put, after the slits, a polarizer at $45^\circ$: this is a 'quantum eraser': it removes the information you gained from adding the vertical & horizontal polarisers. And the interference pattern then comes back.

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