Quantum Mechanical Mayhem

[Sunshine]

If you'd like to have your brain shaken and stirred watch this video, don't be insulted by the cartoonish presentation. I have read about this experiment and, the absolutely unexplainable behaviour of some bits. To say it is fascinating is an understatement.

 

Edited October 29, 2019 by Sunshine

[JamesF]

54 minutes ago, Sunshine said:

If you'd like tom have your brain shaken and stirred watch this video, don't be insulted by the cartoonish presentation. I have read about this experiment and, the absolutely unexplainable behaviour of some bits, to say it is fascinating is an understatement.

Thank you for posting that.  I was aware of the double slit experiment as covered in the first part, but the bit with the entangled photons in the second half was new and a bit mind-warping

If I understand correctly, what it is effectively saying is that the photon arriving at sensor D0 has information about what the observer will observe before the observation is made, even though it appears to be the mere act of observation that decides the result of the test.  That's utterly bonkers

James

[Sunshine]

8 minutes ago, JamesF said:

Thank you for posting that.  I was aware of the double slit experiment as covered in the first part, but the bit with the entangled photons in the second half was new and a bit mind-warping

If I understand correctly, what it is effectively saying is that the photon arriving at sensor D0 has information about what the observer will observe before the observation is made, even though it appears to be the mere act of observation that decides the result of the test.  That's utterly bonkers

James

That’s exactly what it means, yes it is beyond human understanding, as yet.

[Knighty2112]

And magic is real, proved by science!   

[Craney]

All makes sense after a few pints........

[vlaiv]

No magic going on here

It can be interpreted as magic, but in reality it is not. Well, all quantum weirdness can be said to be magic. I'm referring to the part about observer ....

Video was made to imply that act of observation has something to do with it - but it does not. At least no consciousness is involved.

Let's examine regular dual slit experiment and see what is going on. First we need to drop the notion of particle being a little ball. It is not "marble", or mass point or anything. It is in fact a wave. Not simple wave, but rather complex wave. We "see" it as being a particle because it interacts once and localized. But when it moves in space, in between of interactions - it is wave (it is wave even when doing interaction in fact - we only think of it as particle because measurement ends up with quantum of something).

Wave part is rather straight forward, no need to explain that. What about when we "look". Well, act of looking brings in something interesting to the table - decoherence. It is a consequence of entanglement of particle with the environment. Maybe best summarized would be like this:

No measurement case: state of electron going to double slit is superposition of "electron went thru slit one" state and "electron went thru slit two" state. This superposition interferes with itself and produces interference pattern.

Measurement case: state will be somewhat different in this case, it will be: superposition of "electron went thru slit one and was entangled with the environment and path was recorded" with "electron went thru other slit, was not recorded and there was no entanglement with the environment, but we using logic conclude that we know which slit it went thru as 'it must be other case'".  This superposition will not interfere with itself because "state that is entangled with the environment in effect produced "disturbed" wave that is no longer capable of interfering with "regular" wave" (I put quotations because it is somewhat layman explanation of what is going on - almost conceptually right but wrong terminology - easier to understand this way). As electron becomes entangled with the environment - properties of electron and environment become correlated, and environment is rather messy - thermal chaotic motion everywhere, so electron also gets "messy" and is not "pure" wave that can easily interfere with itself.

Similar thing happens with delayed choice quantum eraser experiment, except we have added layer of complexity. We now have additional entangled photon being produced after the slit - and that photon lands first on detector D0. Now it may appear that what happens after (at D1-D4) determines where entangled photon will land at D0 and that there is some "going back in time, and influencing past events".

What really happens is that we have single wave function propagating in all paths and probability of detection of photons at certain places is altered whenever complete wave function either does or does no decohere to the environment.

Photon hitting detector D0 will hit it somewhere - and that single hit can't be ascribed neither to interference pattern nor dual lines pattern with 100% certainty. It has certain probability of belonging to either one or the other distribution - this is important thing because we like to conclude that when we do correlation between hits at D0 and D1/2 we get interference patterns at both, and when we correlate hits at D0 with D3/D4 we get dual lines - it must be the case that each photon was exactly in one group or the other group - but this is based only on behavior of ensemble of particles - no reason to think that each photon was distinctly in one group or the other. It is behavior of wave function in a given setup that produces this correlation in hit position probabilities and not photons being distinctly in one group or the other.

No influence of the observer, no future events causing past ones - just a regular wavefunction behaving as it normally does. It is just our wrong conclusions that paint a picture of something magical going on here. There is something magical going on here and that is wavefunction and weirdness of quantum world, so let's focus on that one and stop inventing significance where there is none.

[markse68]

Hi Vlav, can you explain the way anti-reflective coating principle works in similar way? The way the photons don’t reflect as they would cancel if they did? I think it’s related and always had me perplexed 🤔 

[vlaiv]

17 minutes ago, markse68 said:

Hi Vlav, can you explain the way anti-reflective coating principle works in similar way? The way the photons don’t reflect as they would cancel if they did? I think it’s related and always had me perplexed 🤔 

First thing to understand in explaining that is relation between wavefunction and what we detect as photon. There is relationship between the two that is best described as: wavefunction has the information how likely is that we will detect photon at a certain place (in fact wavefunction describing quantum mechanical system carries this sort of information for all things that we can measure - what is the likelihood of measuring certain value - be that energy, position, spin, ...). This is why we have interference pattern in the first place -  there are positions where wavefunction is such that there is high probability that we will detect photon, and places where it gives low probability that we will detect photons. And if we place detector - photons will be detected with these probabilities over time and pattern will form.

By changing the "shape" of wave function we can to some extent "direct" where we want "influence" to go. One way of shaping wave function is to let it interfere with itself - it has a wave like nature so there are peak and trough in it (with light their distribution depends on wavelength / frequency of the light), and if we split wavefunction and then later "combine", but let the path of wavefunction to be of different length between two points - we can "adjust" how it aligns with itself. We can either have peak align with peak, or peak align with trough (one will amplify probability, other will cancel to lower probability) - in fact due to phase difference we can have anything in between probability wise (from low to high).

Now imagine you have a thin layer of transparent material on top of lens. It's thickness is order of wavelength of light (or multiple of). Wave will reflect somewhat from first surface of this boundary layer, and it will reflect from second surface of boundary layer - now we have a split of wave into two components. Depending on thickness of that layer - one component will travel larger distance than the other.

Path that one wave traveled when passing arrow and then reflecting of first surface and going back to arrow will be twice the distance of arrow to first surface (marked as red). Path that other wave (it is in fact same wave function) traveled from arrow head to second surface and then back to arrow - will be two times the distance between arrow and first surface + thickness of the layer.

If thickness of the layer is such that adding that distance to path traveled by orange wave makes it 180 degrees out of phase with red wave (depends on wavelength of light) they will perfectly cancel out because peaks will be aligned to troughs. If they are not perfectly out of phase - probability will not be 0, but will be rather small instead.

In fact - you can't get 100% anti reflective coating for polychromatic light (one containing different wavelengths in continuum), because layer thickness does this for only precisely one wavelength / frequency (and its harmonics). If you have light come in at an angle - distance traveled will be changed and you will loose perfect out of phase alignment. You will also loose perfect out of phase alignment if frequency of the light is not exactly as it should be.

This is why there is multi coating - layers of different thickness impact different wavelengths of light. Multicoating just means that there are different layers applied - each one will "work" on different wave length and not all wavelengths will be covered, but even if there is small offset from perfect out of phase - there will be significant reduction in reflection.

Btw - this is the same principle used in interference filters - layers of certain thickness are layered on top of each other in such way that they block certain wavelengths of light - doing the opposite, instead of creating destructive interference on reflected wave - they create destructive interference on forward going waves, lowering the probability that photon will pass thru the filter.

There are other things at play here that I don't have enough knowledge about to go into detail - like why is glass reflecting about 4% of light on air/glass boundary, and if different materials have that percentage higher or lower and such, but I'm certain that there is explanation for that as well

 

[markse68]

Thanks Vlav, I get the wave cancellation bit but it was the “ so it doesn’t reflect in first place” leading to increased transmission that I don’t get. But I think what you’re saying is the wave function cancels so the probability reduces of the photon appearing after the “reflection” and increased probability of it being transmitted? 🤔

[vlaiv]

25 minutes ago, markse68 said:

Thanks Vlav, I get the wave cancellation bit but it was the “ so it doesn’t reflect in first place” leading to increased transmission that I don’t get. But I think what you’re saying is the wave function cancels so the probability reduces of the photon appearing after the “reflection” and increased probability of it being transmitted? 🤔

Yes indeed - it follows both from energy conservation and the way how we calculate actual probability from wave function (those are related).

Total probability related to wavefunction needs to be added to one "to make sense" - as with normal probability - you can't have probability of all events sum to nothing else but one - "something will surely happen" but we can't have something happen with higher probability than certainty

Once we do that we get proper probabilities confirmed by experiments (there is "deep insight" into nature right there - why do probabilities in quantum realm work exactly the way one would expect for mathematical probabilities?), and yes that is related to why more photons go thru then reflect - if you make minimal probability of those reflected photons, and energy is conserved - all those photons must "go" thru - transmission is increased.

It is again a bit "wrong" to think that more photons are somehow "forced" to go thru - when we do that we think of little balls whizzing around, but it is the wave function - or more specifically disturbances in quantum field that move around and things that we think of when mentioning photons is just interaction of that field with other fields (and their wave fucntions / disturbances).

There is still one unresolved "mystery" about all of that - how does wave function "choose" where to interact (or what mechanism leads to interaction in one place and not other). Accepted view is that it "just happens" on random. But I think it needs to be more than that. It's a slippery slope as it quickly leads to hidden variable territory

 

[Sunshine]

I knew Vlaiv would chime in with some lengthy but clarifying answers, good stuff. I was rubbing my hands together giggling maniacally waiting for his response lol.

Edited October 29, 2019 by Sunshine