What exactly does quantum entanglement imply?

[Daniel K]

I've been having issues understanding quantum entanglement and non-locality recently, and certain explanations that I have been told has just made the matter more confusing. The first portion of this thread will be explaining what I know and the second part will contain the questions.

First Part: What I understand from entanglement is that a particle becomes connected with another particle, and this generates a phenomenon in which neither particle can any longer be considered independent, but rather intrinsically intertwined to the corresponding entangled particle. Any affects that one particle experiences will affect the other particle instantaneously.

To explain quantum entanglement, physicists postulated the idea of non-locality. Non-locality states that an object can affect other objects that are not in its immediate surroundings.Second Part: My first question lies within non-locality. It seems to me that non-locality was postulated because physicists could just not explain why can object is affected by another object at a distance where faster than light speeds would have to be necessary. Is this true?

Second question lies within a certain entanglement explanation that I've been told. This explanation goes as follows: "Quantum entanglement does not imply faster than light speeds because nothing is being communicated, rather it's just merely correlation between the particles. If one particle is measured to contain spin up, then the other entangled particle can be sure to contain spin down. Nothing is being communicated; we just know what spin the other particle is going to be." This explanation would make sense to me, although the particles do seem to communicate something. Communication can be seen observed by the other particle always containing an opposite spin. The only to explain this using this explanation is to say that particles always have to be opposite spin to entangle together, although isn't this just the hidden variable theory?

Any feedback would be appreciated!

 

[Simon Bridge]

1. Yes. It's the name for the effect; see:
https://en.wikipedia.org/wiki/Bell's_theorem

2. I've been having trouble finding a decent lay description ... when I had the same issue you did I basically had to do the math. The statistics are different.
You can imagine a situation where there are two boxes, a blue ball in one box and a red one in the other - you don't know which is where but looking in one box tells you right away what is in the other one. That is hidden variables - where it does not match up with the entangled case is in the statistics: in a nutshell, the example is too simple.
This looks promising: https://cs.uwaterloo.ca/~watrous/CPSC519/LectureNotes/20.pdf

 

[Salvador]

also my 2 cents if I may, just because once you see one particle reveals the other doesn't mean communication , because for communication the other end (whoever may be there) needs to also know what you saw at your end of the line.

you could say , well okay make a code sheet and then give it to each of the observers at each end so they don't have to communicate back to each other to confirm but as they see their result they can compare it wih what has been written for that exact result but this would work if one could determine the outcomes of the entangled particles spin states etc, but we can't determine those , we only know the other particles state once the first one is revealed, so it's more of a lottery.

 

[DrChinese]

you could say , well okay make a code sheet and then give it to each of the observers at each end so they don't have to communicate back to each other to confirm but as they see their result they can compare it wih what has been written for that exact result but this would work if one could determine the outcomes of the entangled particles spin states etc,

Just to be clear: there are no such code sheets possible which would agree with the predictions of QM.

 

[entropy1]

My two cents: the measured value by Bob depends on the measured value by Alice, or vice-versa, which is equivalent. So, Alice's measurement basis of her free choice influences the correlation (between Alice and Bob), and similarly for Bob. This is non-locality. The problem is that you can't tell who influences who, the correlation just is! So in this case, local hidden variables are ruled out, though non-local hidden variables are not.

 

[gjonesy]

A good lay description I've heard is: imagine walking into a shoe store taking a box off the shelf and inside you have a LEFT white size 9 nike with blue trim, immediately you know that somewhere regardless of distance space or time, that there is a RIGHT white size 9 nike with blue trim, we know this simply because they are a correlated pair. As for this spooky "action" at a distance that's a bit harder to explain, entangled pairs act as one.

 

[Nugatory]

A good lay description I've heard is: imagine walking into a shoe store taking a box off the shelf and inside you have a LEFT white size 9 nike with blue trim, immediately you know that somewhere regardless of distance space or time, that there is a RIGHT white size 9 nike with blue trim, we know this simply because they are a correlated pair. As for this spooky "action" at a distance that's a bit harder to explain, entangled pairs act as one.

Unfortunately, this is the classic example of local variables at work - we know that a shoe is created with a size, a color, and a handedness footedness and it has these properties even if we don't measure them. Entangled quantum particles don't behave like that..

Suppose you and I somewhere else in the universe are looking at the pairs of shoes as they come by, but we're each only allowed to measure at random one of the three properties: color (white or red), size (9 or 10), left-foot or right-foot. If, for a given pair, you measure the size and get nine and I measure the color and get red, we might conclude when we compare notes that you had a white size nine shoe of unknown footedness while I had a red size nine shoe of unknown footedness; similar logic works for all the other possible pairs of measurements. (If we both randomly measure the same property, the other two will be unknown even after we compare notes).

However, suppose that when we compare notes we discovered that the number of white size nine shoes (using the logic above, where we combine your measurement and mine to infer something about my shoe) that I saw is more than the sum of the number of white left shoes that passed me plus the the number of size nine right shoes? That's the equivalent quantum mechanical prediction; it has been confirmed experimentally and it tells us pretty clearly that the two particles in the entangled pair were not created with definite values of all three attributes.

 

[Daniel K]

And so something is being communicated between the entangled particles, right? From my research I know that we cannot tell what is being communicated or who's doing the communication, but just to clarify, we know that something is being communicated?

 

[Simon Bridge]

That would be the classical interpretation of the results... otherwise, how does the distant particle know which set of statistics to follow?

Technically, what we are saying is that any "hidden variables" style communication would have to be ftl... this potentially allows for causality violations[1] so we don't think this is what happens. Instead we have to face the idea that reality can be non-local. ie action at a distance is real. In this case, two objects can be part of the same quantum system even at very large distances.

[1] Summary of (2012) Nature paper, link to paper at bottom.
http://arstechnica.com/science/2012/10/quantum-entanglement-shows-that-reality-cant-be-local/

 

[bhobba]

Before discussing entanglement you need to understand what it is.

Simply its an extension of the principle of superposition to different systems. Suppose two systems can be in state |a> and |b>. If system 1 is in state |a> and system 2 is in state |b> that is written as |a>|b>. If system 1 is in state |b> and system 2 is in state |a> that is written as |b>|a>. But we now apply the principle of superposition so that c1*|a>|b> + c2*|b>|a> is a possible state, The systems are entangled - neither system 1 or system 2 are in a definite state - its in a peculiar non-classical state the combined systems are in.

If you observe system 1 and get state |a> then you know system 2 is in state |b>, and similarly if you observe system 1 and get |b> you know system 2 is in state |a>. That's all entanglement is - a correlation. That's it, that's all. Let that sink in devoid of the stuff you read about it in the pop sci literature.

Imagine you have two slips of paper a red and a green one and put them in envelopes. Send one to the other side of universe and keep the other. Open the envelope and you see red - you immediately know the other is green, and conversely. Nothing weird or mysterious here. That's all that's going on with entanglement with a twist I will explain.

Now for the QM twist. It turns out the paper analogy is not quite the same as QM. The correlation is a bit different - its still just a correlation - but has statistical properties different to the paper example. Why the difference? The difference is in QM things do not have properties until observed to have them, whereas the slips of paper remain red or green at all times. But what if we insist it's like the slips of paper - then it turns out you need some kind of non local superluminal communication. That's really weird. But there is nothing compelling anyone to insist its like the slips of paper - simply accept QM allows a different kind of correlation and things are no longer mysterious.

Without going into the details there is good reason to not apply the concept of locality to correlated systems (its to do with QFT an the so called cluster decompostion property). You can define it to be applicable but it doesn't sit well with other things. If you do that then you don't run into issues in the first place. Only by allowing it to apply can you say its like the slips of paper. If you say its not an applicable concept then the answer is simple - it can never be like the slips of paper. QM correlations are different from classical ones - big deal.

Thanks
Bill

 

[zonde]

And so something is being communicated between the entangled particles, right? From my research I know that we cannot tell what is being communicated or who's doing the communication, but just to clarify, we know that something is being communicated?

Right, as long as you remember that all knowledge in science is tentative.
Or the longer answer would be that we know there can't be any light speed limit compatible scientific explanation of our loophole free Bell test experiments as far as we can trust that experiments are loophole free.

 

[Daniel K]

If you observe system 1 and get state |a> then you know system 2 is in state |b>, and similarly if you observe system 1 and get |b> you know system 2 is in state |a>. That's all entanglement is - a correlation. That's it, that's all. Let that sink in devoid of the stuff you read about it in the pop sci literature.

The difference is in QM things do not have properties until observed to have them, whereas the slips of paper remain red or green at all times.

That is the explanation in which I am having difficulty understanding. How would system 1 know that system 2 is measured if there is no communication but merely correlation? You said yourself that "QM things do not have properties until you have observed them.." and so how would a certain system know that it has to be opposite to the opposing system, if it has the superposition of both systems prior to measurement?

 

[entropy1]

Imagine you have two slips of paper a red and a green one and put them in envelopes. Send one to the other side of universe and keep the other. Open the envelope and you see red - you immediately know the other is grenn, and conversely. Nothing weird or mysterious here. That's all that's going on with entanglement with a twist

I would rephrase that: suppose you have two envelopes and in envelope A you put a green/red striped sheet of paper, and in envelope B a red/green striped sheet of paper. If you send one to Alice and the other to Bob, and Alice finds out that, after putting the envelope through some process, she has a green sheet, then she know Bob has a red one. But you can view this the other way round: suppose Bob puts his envelope to some process and after opening it finds it contains a green sheet! The he knows Alice must have the red one! Since who opens the envelope first depends on the (relativistic) frame of reference, you can't tell who should be the communicator. You can only establish that there is a correlation. Am I making sense?

 

[bhobba]

Am I making sense?

Sure. Its just a correlation.

Thanks
Bill

 

[Daniel K]

Sure. Its just a correlation.

Yes, it's correlation. Through the correlation one can infer that there is some kind of communication being transferred between the two entangled states. You stated yourself that within the confines of quantum mechanics two entangled states are in a superposition of being both state A and B when not measured. So my question is simple: if it is merely correlation and there is no communication, then how does an entangled particle know to be opposite to the other particle?

Let me try to be my clear and give an example in what I am asking.Let's say we have an entangled pair of particles that are separated by an arbitrarily long distance. We'll call the particles system 1 and system 2.
Both systems are in a superposition of being in state A and B.
If then system 1 is measured to contain state A, then we can infer that system 2 contains state B. However how does system 2 itself know that it has to be in state B? Prior to measurement, it should be in a superposition of both A and B. And so then how can a measurement that is occurring in some far off galaxy affect the of state of it?

 

[zonde]

Since who opens the envelope first depends on the (relativistic) frame of reference, you can't tell who should be the communicator. You can only establish that there is a correlation. Am I making sense?

Not quite. It does not make sense to incorporate into single model symmetries of relativity and FTL communication. Of course you get contradiction because symmetries of relativity are simply incompatible with any FTL communication or causality.

 

[entropy1]

However how does system 2 itself know that it has to be in state B?

In my view, it isn't in state B. The correlation measured leaves room for the interpretation it may have been in state B, but the measurements and their correlations is all we have!

 

[bhobba]

Yes, it's correlation. Through the correlation one can infer that there is some kind of communication being transferred between the two entangled states.

That's incorrect. It knows it the same way the colored papers knows it ie you have arranged it so by the entanlement. The only difference is in QM properties do not exist until measured.

To be specific because |a>|b> and |b>|a> are in superposition you have arranged things so that the only outcomes of an observation are |a>|b> or |b>|a>. That is the exact analogue of the coloured papers where the only outcomes are red and green or green and red.

If you still don't see it you need to explain exactly why there is no communication between the slips of paper but there must be communication between entangled particles.

Thanks
Bill

 

[entropy1]

Not quite. It does not make sense to incorporate into single model symmetries of relativity and FTL communication. Of course you get contradiction because symmetries of relativity are simply incompatible with any FTL communication or causality.

But they both apply to the observable world, don't they? I am just combining the two in this context!

By the way, I am on the contrary not talking about FTL communication. I think that decoherence is an important contributor in this case. Hoewever, I have little knowledge of it right now.

 

[zonde]

By the way, I am rather thinking about non-local hidden variables or the deBroglie-Bohm model

For deBroglie-Bohm model you have to use preferred frame and incorporate relativity using so called Lorentz relativity interpretation.

 

[Daniel K]

If you still don't see it need to explain exactly why there is no communication between the slips of paper but there must be communication between entangled particles?

Within the confines of the paper analogy, there are definite values.
However for the quantum mechanical one, there are not. Once measurement occurs in one entangled particle, you are destroying the superposition of the other entangled particle that could be light years apart instantly.

 

[bhobba]

It seems to me that your explanation is putting inherit values within the particles.

I am not.

All that has happened is the entanglement has ensured the only outcomes are |a>|b> or |b>|a>. It says nothing about what properties it has prior to observation.

I know with the pop-sci half truths that surrounds this its difficult to shake the confusion they engender. But really its quite simple.

But don't take my word for it - you can read Bell's original paper:
https://cds.cern.ch/record/142461/files/198009299.pdf

The only difference is he uses Bertlmann's socks.

Thanks
Bill

 

[Daniel K]

But don't take my word for it - you can read Bell's original paper:
https://cds.cern.ch/record/142461/files/198009299.pdf

Thanks I'll take a look at it.
Also, if you are correct and there is no communication between the particles, is non-locality not necessary then?

 

[bhobba]

Within the confines of the paper analogy, there are definite values.
However for the quantum mechanical one, there are not. Once measurement occurs in one entangled particle, you are destroying the superposition of the other entangled particle that could be light years apart instantly.

So? Entanglement is not some kind of mystical connection between objects - its simply a superposition.

Thanks
Bill

 

[bhobba]

Thanks I'll take a look at it.
Also, if you are correct and there is no communication between the particles, is non-locality not necessary then?

It's exactly as I explained. You only need locality broken if you want it to be like the slips of paper ie have the properties irrespective of observation.

Thanks
Bill

 

[entropy1]

Thanks I'll take a look at it.
Also, if you are correct and there is no communication between the particles, is non-locality not necessary then?

It is non-local in the sense that there is a correlation between two measurement values that could not have communicated.

 

[jerromyjon]

But they both apply to the observable world, don't they?

Tell me how to observe FTL communication... I doubt you can because it still hasn't. And I don't mean group velocities or phase velocities (or whatever it is I mean and can't think of a better term for) Information can never be transferred from A to B instantly. I choose to think of entanglement as existing in complex tension between two states so that an action affects that tension which is separate from the "observable" reality of spacetime.

 

[zonde]

It is non-local in the sense that there is a correlation between two measurement values that could not have communicated.

You must be using very uncommon definition of non-locality.

 

[entropy1]

You must be using very uncommon definition of non-locality.

Tell me yours! Well, perhaps it goes a little limp formulated this way...

 

[zonde]

Tell me yours!

I try to avoid it. Basically there are two possible meanings:
first, FTL causality or communication;
second, distance is meaningless.

For the first meaning it is less ambiguous to call it FTL whatever.
About the second meaning, it is novel philosophical concept that does not really have much to do with science.

 

[Daniel K]

So? Entanglement is not some kind of mystical connection between objects - its simply a superposition.

Could the crash of a superposition between two objects be instantaneous?

 

[DrChinese]

Entanglement is sometimes referred to as "Quantum Nonlocal" in order to distinguish it amongst the many definitions. The underlying physical process whereby entangled particles become entangled and/or cease to be such are not understood, even though the formalism itself is relatively well understood. You can entangle particles that do not even exist at the same time, for example. That too is "quantum nonlocal". So if you want to describe entanglement in a manner outside the theoretical formalism, there really is nothing that works better as a definition. Because there is no further physical description possible except by adopting an interpretation of QM.

Please note that even individual particles have nonlocal attributes. A particle's position can be [1/2 here] and [1/2 there], and then collapse to a state of [all here]. That collapse is also quantum nonlocal.

 

[Daniel K]

You only need locality broken if you want it to be like the slips of paper ie have the properties irrespective of observation.

DrChinese, do you then believe that particles have properties irrespective of observation?

 

[bhobba]

Could the crash of a superposition between two objects be instantaneous?

There is no could about it - it is.

But it doesn't mean anything.

Thanks
Bill

 

[Daniel K]

There is no could about it - it is.

Alright, so just to be clear of your interpretation on quantum entanglement-

You believe what occurs is that two particles become entangled, and upon measurement of one particle, the superposition of both systems instantly crashes and they become defined states.

However the instant superposition crash of both particles does not imply faster than light communication/non-locality; rather just correlation.

Is this correct?