Quantum Entanglement

[The Thing]

Okay, you're probably thinking this is another "can I use quantum entanglement to achieve superluminal-speed communication?". Well, it's not exactly that issue.

 

I understand that an entangled pair of photons behave like there is no space between them and that once one has been observed it has an instantaneous effect on the other. But then, WHY? How is the information of photon A being observed travel to photon B? Does THIS information (that says, "oh, photon A's been observed as something") travel faster than light? Does it perhaps travel in another dimension? Or is this information not needed at all to affect the other photon and I'm just thinking in classical mechanic terms?

 

One more thing: a similar thing I think has been asked by bascule before, but he couldn't get any answers:

 

A photon (or photons) is shot through a double slit, and is then split into two entangled photons by two down converters (1 at each slit). The two photons are then sent to the two ends of the apparatus, which are a light hour apart. Detectors at the end show interference patterns. But, if a path from one of the down converters to one of the detectors is blocked by a screen, then the other detector which is a light hour away, will immediately show a solid line instead of interference. Isn't this a set up for FTL communication?

 

Thanks.

[Edtharan]

From what I understand of this (ie I'm no expert). The reason that you can't use these phenomina to send a message is that you have no control over the result of the check.

 

Say for instance you were using entangled photons that would alwase have polarities at 90 degrees to each other. If you detect the polarity of photon A at one point, and at point B detect the other photon , then they will always have differnet polarities. But you can not control at point A what the polarity at point B will be. All you can tell is that it will be different to what you (at point A) detect it as.

[swansont]

Okay' date=' you're probably thinking this is another "can I use quantum entanglement to achieve superluminal-speed communication?". Well, it's not exactly that issue.

 

I understand that an entangled pair of photons behave like there is no space between them and that once one has been observed it has an instantaneous effect on the other. But then, WHY? How is the information of photon A being observed travel to photon B? Does THIS information (that says, "oh, photon A's been observed as something") travel faster than light? Does it perhaps travel in another dimension? Or is this information not needed at all to affect the other photon and I'm just thinking in classical mechanic terms?[/quote']

 

Entanglement means that all of the information you are getting is encoded in both photons. So measuring one tells you about both. Saying that the measurement has an "instantaneous effect on the other" isn't an accurate reflection of the phenomenon.

 

 

One more thing: a similar thing I think has been asked by bascule before' date=' but he couldn't get any answers:

 

A photon (or photons) is shot through a double slit, and is then split into two entangled photons by two down converters (1 at each slit). The two photons are then sent to the two ends of the apparatus, which are a light hour apart. Detectors at the end show interference patterns. But, if a path from one of the down converters to one of the detectors is blocked by a screen, then the other detector which is a light hour away, will immediately show a solid line instead of interference. Isn't this a set up for FTL communication?

 

Thanks.[/quote']

 

You'd need to desribe the experiment topology in more detail to see why it won't work. Interference patterns take time to make, for one. These types of experiments are often very subtle and it's not often straightforward to see why it won't work for FTL communication.

[5614]

How is the information of photon A being observed travel to photon B? Does THIS information (that says, "oh, photon A's been observed as something") travel faster than light?

You might want to look up; EPR paradox, Bell's Theroem and hidden variables.

 

The people who made the EPR paradox; Einstein, Podolsky and Rosen didn't like the idea of entanglement (using QM's explanation) which showed that the particles could communicate instantly as this violated special relativity, or so they thought. It turns out that as no information can be transferred using this method it does not violate SR (but they did not know this).

 

EPR paradox (assumes there is not instant communication between particles and so) says that there are things called hidden variables. That is each particle knows whether it is say "up" or "down" spin, a common property used in the entanglement of particles. Although you might not know, the particle does. This is incorrect according to QM, which says that the particles do not know, in fact do not have a definitive spin until they are observed.

 

Bell's Theorem says that using hidden variables would violate parts of quantum mechanics, and these parts we know (theoretically and experimentally) to be correct. Bell set up an inequality in which either QM or hidden variable theory is correct. When an experiment testing the inequality was carried out it showed the the inequality was violated, meaning that QM is correct and hidden variable theory cannot be correct.

 

The not-so-simple truth is that the particles do communicate instantly, but a crucial point is that this cannot be used to send data so does not violate relativity.

 

A simple analogy to why this is not FTL is this:

I hold a ball and so do you. One of ours is red and the other blue. We do not know. I look at mine and it's red. I know instantly that yours is blue. However to actually tell anyone, to use the data, to communicate with something or anything else I need to use a "classical" method of communication, be it via radio waves, the Internet or a carrier pigeon. I have, as soon as I looked mine, instantly force yours to be blue - but this cannot be used to transmit information. I could tell you that yours is blue, but like I said, that would require another means to communication, say the phone, which is limited to the speed of light.

 

then the other detector which is a light hour away, will immediately show a solid line instead of interference

Why will it immediately show a solid line? I assume the line is the observation when a photon interacts with the screen, but why would it appear instantly when it should appear when the photon arrives, ie. 1 hour after it goes through the screen.

[J.C.MacSwell]

Entanglement means that all of the information you are getting is encoded in both photons. So measuring one tells you about both. Saying that the measurement has an "instantaneous effect on the other" isn't an accurate reflection of the phenomenon.

 

.

 

The above seems like common sense. But it's a quantum effect.

 

Are you sure it's correct?

 

I thought that a measurement of one forced the other to eliminate some of it's "options", not just reveal something hidden about each.

[Klaynos]

The above seems like common sense. But it's a quantum effect.

 

Are you sure it's correct?

 

I thought that a measurement of one forced the other to eliminate some of it's "options"' date=' not just reveal something hidden about each.[/quote']

 

Yes it's correct, you are eliminating the options, all of them except the one related to the quantum state you measured of the particle you measured...

[swansont]

The above seems like common sense. But it's a quantum effect.

 

Are you sure it's correct?

 

I thought that a measurement of one forced the other to eliminate some of it's "options"' date=' not just reveal something hidden about each.[/quote']

 

Yes, it's quantum, and as Klaynos said, you do remove options. What is not true is the assumption that the other particle was in that particular state all along, which is why e.g. the red vs. blue ball analogy is good, but not perfect.

[5614]

What is not true is the assumption that the other particle was in that particular state all along, which is why e.g. the red vs. blue ball analogy is good, but not perfect.

Yes, I was going to point it out but thought I'd save the complication as it was otherwise a more simple explanation.

 

Although now I'm having debates with myself. If it were a particle then neither of the particles have a state until observed, at which point one takes one and the other takes the other state. But moving onto balls... what swansont is saying is that the balls both have a colour, we might not know but they do - and this is where the analogy breaks down. But that statement sounds very 'hidden variable' to me. Is that statement true or not?

 

The balls are in a superposition, no one in the universe has any way of knowing which one is which... can we not say that they are in a superposition and so neither of them have a colour?

 

I suppose this reminds of Schrödinger's cat. Sure it is only meant to be an example and not taken too literally, but as the cat is in superposition it is alive and dead at the same time... so the ball must be blue and red at the same time, or another way of saying the same thing is that neither of them have a colour.

[Norman Albers]

Here's the idea I get from just getting into this. The clearest statement I've seen said that you'd expect a cosine relationship between two detectors, which is expanded as [math]1- \tfrac{1}{2}\theta^2[/math]. What is measured is a [math]\theta[/math] term. This suggests to me a sine component to be explained.

[Norman Albers]

I got things backward, above. Quantum mechanics predicts dependence of the square of the cosine, while Bell's theorem (which fails) says it should not be stronger than linear dependence on angle. Squaring the cosine expansion still yields a square term.

[Severian]

Let me have a go:

 

How is the information of photon A being observed travel to photon B? Does THIS information (that says' date=' "oh, photon A's been observed as something") travel faster than light? Does it perhaps travel in another dimension? Or is this information not needed at all to affect the other photon and I'm just thinking in classical mechanic terms?

[/quote']

 

It is already there. Think of the 2-photon system not as a 2 separate things but as one big ensemble. In the classic Stern-Gerlach experiment there are two possibilities for the result of the measurement, and making the measurement picks one of the possibilities for the whole ensemble. It doesn't pick a possibility for one photon and transmit it to the second. Indeed, you have to measure both photons to make the experiment, and although you might claim that you measured photon 1 before photon 2's measurement was made, there will exist a frame of reference where photon 2 was measured first.

 

A photon (or photons) is shot through a double slit, and is then split into two entangled photons by two down converters (1 at each slit). The two photons are then sent to the two ends of the apparatus, which are a light hour apart. Detectors at the end show interference patterns. But, if a path from one of the down converters to one of the detectors is blocked by a screen, then the other detector which is a light hour away, will immediately show a solid line instead of interference. Isn't this a set up for FTL communication?

 

This isn't a well defined experiment. An interference pattern is made from lots and lots of photons hitting the screen. If you have one photon, all you will get is a dot (or a line, dpending on the setup). The think which is spread out is the probability distribution of where that dot/line will be, but you can't say anything with just one go - you have to perform the experiment lots of times or use lots of photons (it is better to do it lots of times so you can see the genuine particle/wave duality).

[Norman Albers]

My thesis is that the radiation field contains also a population of fragment levels of disturbance. Can we find further understanding to this problem by saying that what propagates are the two entangled states and their reaction in the background field? If this could change the statistics of polarization between detectors we'd be onto something good.

[Cap'n Refsmmat]

Please, rather than derailing this thread with your own new theory, try to stick with what we already know. The Thing is trying to ask about entangement, not your personal theory of it.

[Norman Albers]

Sorry, I was thinking too loosely and shall try to put such thoughts into SPECULATIVE posts. What I read to date seems to say that coincidence in two detectors decreases more slowly than it should, as a function of relative angle of polarization. Is this what we consider non-locality?

[Ragib]

Just to expand a bit on the ball analogy. Imagine there are two, or even more balls. You haven't seen them yet. You dont know their color, neither do they. In fact, it doesn't until its observed. You know though, these balls can only take a certain value. If you have two balls, with 2 colors, you observe one, it doesn't matter if the other ball is a million light years away, you know its color. You observation doesn't suddenly transmit this information of color to someone else, unless they see the ball as well, which in that case, no information was really transmitted. If you have More than 2 balls, say, 3, they can have 3 colors, each ball its own different color. If you observe one, you still dont know the exact color of the others, you just eliminate one. In Real physical systems, there are alot more than 3 "balls" lol. Sorry, i know i just restated what other people said..

[swansont]

The difficulty in the analogy, as I stated before, is that the ball doesn't have a color initially, and doesn't have one until you measure it. So the obvious question is, "How does the other ball find out what color the first ball is?" It's not a dumb question, and understanding the answer requires QM. It's not a classical phenomenon, so classical analogies will always be of limited usefulness.

[Norman Albers]

 

 

 

 

You'd need to desribe the experiment topology in more detail to see why it won't work. Interference patterns take time to make' date=' for one. These types of experiments are often very subtle and it's not often straightforward to see why it won't work for FTL communication.[/quote']

So this is a statistical experience, right? It is difficult to get to which is why I am seeking more specific experiment details. We have a polarizer in front of a detector, yah? Then, two of these at relative angle theta. I get lost reading comparative words like "more" and "less". Bell's theorem projects a dependence of first order; we observe second order. If theta is less than a radian we are expanding a small quantity and the first order term is larger. Here I get lost.

[Norman Albers]

Trying to see the sense here, if the coincidence rate falls less slowly than it should with increasing theta, is this relatively larger coincidence what we refer to as not locally explainable by separate statistics on the two states?

[bascule]

You'd need to desribe the experiment topology in more detail to see why it won't work. Interference patterns take time to make, for one. These types of experiments are often very subtle and it's not often straightforward to see why it won't work for FTL communication.

 

Here was the original setup I described:

 

 

This experiment takes place in three different locations: A, B, and C. Locations A and C are equidistant from B.

 

At B we have a laser which emits a single photon at a time over a given interval. The photon passes through a beam splitter (BS). BS splits the probability in two such that photons have a 50% chance of exiting through either of the two outputs.

 

Both of BS's outputs are connected to down converters (DC) which generate entangled photons. One of each of the outputs of the down converters are directed at locations A and C.

 

At C we redirect the beams coming from B such that they overlap and interfere, provided that the person at A is not using the which path detector (WPD) to observe which path the photons coming out of BS are taking.

 

The idea is that if someone turns on the WPD, then over the course of a few bazillion photons you see a pattern like this on the "screen" at C:

 

 

Here there's no interference pattern because we're observing which path the photons take through the beam splitter.

 

But if we turn off the WPD, then the probability waves split by BS interfere with each other, and the pattern that's traced out on the screen will look something like this:

 

 

(note: This image assumes that the probability wave will collapse due to outside interference about 50% of the time)

 

I checked out a book called Superluminal Loopholes in Physics which described a setup more or less like this one, then proceeded to do a lot of math I didn't understand to show why it didn't work. Unfortunately being a layman I have no idea what they were actually doing, but bottom line: it won't work.

[Norman Albers]

Thank you, Bascule. I ponder.

[Norman Albers]

Please, rather than derailing this thread with your own new theory, try to stick with what we already know. The Thing is trying to ask about entangement, not your personal theory of it.

Rather than use the term 'fragments' how about if I use the term 'vacuum polarization'?

[emcelhannon]

Entanglement means that all of the information you are getting is encoded in both photons. So measuring one tells you about both. Saying that the measurement has an "instantaneous effect on the other" isn't an accurate reflection of the phenomenon.

 

How is this concept not consistant with a local hidden varriable theory. What did the Aspect experiment prove, if it's not the non-locality of quantum effect, or it's hidden varriables?

 

(I don't know how to insert two quotes in one message)

 

(second quote from Swansont)

"The difficulty in the analogy, as I stated before, is that the ball doesn't have a color initially, and doesn't have one until you measure it. So the obvious question is, "How does the other ball find out what color the first ball is?" It's not a dumb question, and understanding the answer requires QM. It's not a classical phenomenon, so classical analogies will always be of limited usefulness. "

 

So, without analogy, how can the information already be encoded in both particles, and yet not have any state whatsoever except that it's entangled?

[swansont]

How is this concept not consistant with a local hidden varriable theory. What did the Aspect experiment prove, if it's not the non-locality of quantum effect, or it's hidden varriables?

 

The concept is consistent with hidden variables. That's why physicists had to devise experiments that would rule hidden variables out, or not. The Aspect experiment was not simply demonstrating entanglement.

 

(I don't know how to insert two quotes in one message)

 

You have to do it manually, with copy & paste

 

 

(second quote from Swansont)

"The difficulty in the analogy, as I stated before, is that the ball doesn't have a color initially, and doesn't have one until you measure it. So the obvious question is, "How does the other ball find out what color the first ball is?" It's not a dumb question, and understanding the answer requires QM. It's not a classical phenomenon, so classical analogies will always be of limited usefulness. "

 

So, without analogy, how can the information already be encoded in both particles, and yet not have any state whatsoever except that it's entangled?

 

 

"Not have any state whatsoever" is still part of the analogy. If the states are red and blue, and you detect red, then this means the ball isn't red the whole time. Analogies have shortcomings, and there is no real classical analogy for superposition. If my choices are red or blue, and the answer is "neither," then I'm going to say it doesn't have one of the allowed states. In QM, we'd say that it's in a superposition of red and blue. Further, the red and blue states of the two particles are entangled.

 

How can this be? I don't know — that's not really what physics is trying to answer. But that's the way nature behaves, which is the question physics is trying to answer.

[iNow]

(I don't know how to insert two quotes in one message)

You have to wrap the text you are quoting with quote tags, like this:

 

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All of that would look like this:

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Second quote:

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