How do particles become entangled?

[TheDonk]

alexepascual, I agree with everything you said accept this line.

...I think a causality argument could also be used against the possibility of "instantaneous communication"

Wouldn't instantaneous communication prove that one object isn't causing the other to change at all? Wouldn't it mean that they ARE the same object? It would be like looking at two sides of the same coin. One object (or what we thought was it's own object) is in a different location than the other, but we would now have shown that location has nothing to do with identity.

 

[alexepascual]

TheDonk:
I like the fact that you try to see things from a diffetrent angle. I am the same way. But I have some difficulty with your idea.
If we were only talking about two entangled particles of the same type, then your idea might make some sense and it would deserve being explored further. On the other hand, remember that what is entangled is not the particles themselves but properties of the particles. Also consider the things that have entangled properties do not have to be of the same type. How would you apply your reasoning in that case?
On the other hand, about my comment on a causality argument, someone could reject it on the grounds that there is no reason to exclude the possibility of a violation of causality. But it happens that on a macroscopic scale we have a definite direction in which time "runs" (that direction in which entropy increases). So maybe a violation of causality in a macroscopic scale is not possible.
Think of this: You invent this cute machine that allows you to send instantaneous messages. You send your partner experimentalist to a star a good distance from earth. Then you send a message, which in your frame of reference is received "instantaneously". I another person's frame of reference (someone on a spaceship moving at a great speed in a certain direction) the reception of the message happens before it was sent.
Does this violate causality? Well, thinking again about it maybe not, because if the events are "space related" none of them can be considered to be a cause of the other.
Changing the subject: did you have a chance to look at wikipedia?

 

[Eye_in_the_Sky]

whoops!

There is a problem with some of the things I wrote in an earlier post of this thread.

Back in post #29, I wrote:

By "forcing", it appears to me that you mean some kind of apparatus which will, for example, leave the |+> as it is, but cause the |-> to become a |+>. Can we do this? Sure! ... why not?

The said process, as seen from within the Hilbert space of that particle, is clearly non-unitary. Nevertheless, there must be some physical arrangement which can bring it about ... but I am not precisely sure what it is, nor how to represent it mathematically.

Furthermore, the claim I made that in such a process we would get

(1/√2)(|+>|-> – |->|+>) → (1/√2)|+>(|-> – |+>)

... is definitely wrong! (... because the final state shows a preferred direction which can be identified on the "particle-2 end".)

Moreover, to say that such process is describable by a Hamiltonian such as

H_total = (H₁ + H_mod) x H₂ ,

where H_k is the 'free' Hamiltonian for particle k, and H_mod is the Hamiltonian for the interaction which modifies the status of particle 1.

... is also wrong! (... because such a Hamiltonian implies that the said process, as seen from within the Hilbert space of particle 1, is unitary.)

Nonetheless, the 'spirit' of my answer still stands. It is just as alexepascual puts it at the end of post #30 (note, I have inserted boldface for emphasis):

About changing the state on one side: let me give you a simple classical example: If we have a set of two cards, one has a 1 written on it and the other one has a 0, and we give them to two different peaple that go to different cities (without knowing what their cards are). We know that when they look at their cards, if the first one has a 0 on it, the other one will have a 1 for sure. That is if nobody changed them. If you have someone changing the zero into a one, then the original correlations don't hold, and, what is more important, the other card didn't magically change to keep the correlation. I know this example is not totally appropriate because is classical (superposition is missing) but except for that, I still think it illustrates this issue very well and the argument can still be used for a quantum system.

 

[TheDonk]

If we were only talking about two entangled particles of the same type, then your idea might make some sense and it would deserve being explored further. On the other hand, remember that what is entangled is not the particles themselves but properties of the particles. Also consider the things that have entangled properties do not have to be of the same type. How would you apply your reasoning in that case?

I agree that this reasoning would point towards the entangled objects not being the same object but who's to say that the properties are not objects of some type? I think it could still work either way... maybe not tho, because of the next thing you said:

Think of this: You invent this cute machine that allows you to send instantaneous messages. You send your partner experimentalist to a star a good distance from earth. Then you send a message, which in your frame of reference is received "instantaneously". I another person's frame of reference (someone on a spaceship moving at a great speed in a certain direction) the reception of the message happens before it was sent.
Does this violate causality? Well, thinking again about it maybe not, because if the events are "space related" none of them can be considered to be a cause of the other.

Let's say we "connect" two objects for instantaneous communication. Then one person flies really fast away towards another star. Unless the people come back to the same frame of reference, there is no agreement on who is older than who. I may be wrong but each person would be able to say they are older than the other person. So if one person starts communicating when will the other person get the info? I don't think they could decide on what instantaneous is, could they? I think we need someone who knows special relativity well to answer this. But if I'm right, this will only enforce the point I quoted you on above.

Back to the topic of what causes entanglement. I've seen some explanations but am having big problems learning a few things that I need to. What is the bare minimum concepts I need to know to understand an explanation of the cause of entanglement? Superposition? Wave functions? Do I need to understand the quantum operators? Better understand Hilbert space? All of these I only have a vague idea in. I thought I knew everything to know about Hilbert space, but now I'm not sure.

Thanks for the info so far.

 

[alexepascual]

The Donk:
You definitely need to understand about superposition of states. Also look into the EPR experiment but in the version where they use the spin of particles (not position and momentum).
It may also help to learn a little more about the special theory of relativity. The twin paradox is not related to the topic of our discussion. But you may want to look into instanteneity and get a good understanding of the Lorentz transformations. Different frames of reference refer to observers moving at different velocities.
Again: take a look at Wikipedia
-Alex-

Eye:
Your posts require a lot more thinking. I'll reply in about three weeks. (after classes are over). Thanks again.

 

[Eye_in_the_Sky]

What is the bare minimum concepts I need to know to understand ... entanglement? Superposition? Wave functions? Do I need to understand the quantum operators?

As a bare minimum for a good understanding of "entanglement", I think you will need the following:

1) a "2-dimension vector space" with "inner product";

2) an "orthonormal basis" on that space;

3) a "tensor product" of two such spaces;

4) the "singlet state" in the resulting tensor-product space;

5) "linear operators" on the resulting tensor-product space;

6) the "Projection Postulate" of Quantum Mechanics.

 

[Eye_in_the_Sky]

When a hydrogen molecule is formed did a previous wave function collapse to form the paired electrons?

If we imagine looking at the Schrödinger time-evolution for a pair of hydrogen atoms coming together, we would find that the resulting state shows a superposition of two general scenarios:

(i) the two atoms 'bounced off one another' and no molecule was formed;

(ii) the two atoms 'joined together' and formed an H₂-molecule.

A subsequent "measurement" of some kind is then required to establish which scenario prevails, and (in an established terminology) that measurement then 'collapses' the wavefunction.

This is analogous to what takes place when a particle is incident upon a step potential. In general, an incident wave packet will split up into two wave packets, one corresponding to reflection and another to transmission. A subsequent measurement is then required to 'decide' which scenario prevails.
______________

In a laser beam are the photons entangled?

Here, it sounds like you are alluding to the property of "coherence", which in this context means an extremely large number of photons can (to a very good approximation) be described collectively by the same (single-particle) quantum state.

 

[yanniru]

Well, after reading all these posts, I conclude that no one understands entanglement.

I have read elsewhere of two explanations that make some sense. Particles in superfluids are entangled because their wavefunctions overlap and are in quantum coherence, like photons in a laser beam. Essentially they act like a single particle.

And the second is like unto it: the EPR paradox is explained by overlapping wavefunctions with the conceptual addition of the extra dimensions of string theory.
The entangled particles were made in ten dimensions at a point in space and time. Six of these dimensions compactify and continue to keep the two particles in contact as they separate in space by a thread of the compactified dimension. That is, their wave functions overlap in the compactified dimensions.

Of course I am not sure if any of that is true. What I do understand is that when physicists cannot explain some phenomenon, they give it a name and go on with life. So far entanglement is one of the things that they still cannot explain. Thay can predict, but they cannot explain.

Richard

 

[TheDonk]

I thought wave functions had no cut-off value at a certain distance and spanned the whole universe... If so, when are they combined? Unless there is degrees of entanglement and everything is slightly entangled. I'm quite sure that isn't the case.
Maybe I'm wrong about wave functions... if not, could you explain?

 

[alexepascual]

Richard:
Your version of entanglement using the compactified dimensions of string theory is interesting. Could you give us your source?
I hope this is something someone has worked out to the point of making the idea feaseable.
Thanks,
-Alex-

 

[yanniru]

Sorry. I am the source. Although when I posted it once before on this forum, someone replied that he had also thought of the same thing.

I am not trained to make the idea feasible. Perhaps you are. I take that explanation as evidence that the extra dimensions exist. Not sure why no bono fide string theorist has pursued it.

 

[Louis Cypher]

Qm Entanglement



--------------------------------------------------------------------------------

Posted this on another forum and they just deleted it and wouldn't respond to my questions so I'm assuming it's nonsense, what I wanted too know though is why it's nonsense, to help me learn a bit more about qm; after reading an article on qbits and how they can use entanglement to check parity in a different way so that q information doesn't become lost by observation I had the thought if we entangle a photon and then observe the two photons entangled state a computer could take this information via observing another entangle photon, and do it computationally to check the quantum or just use error bit from then on any information that computer and the other chose to send quantumly would instantaneously be transfereed wireless without broadband information exchange at insatantaneous speed; the applications of the idea are endles, why is this wrong, please answer?!? I'm dying to know.

thanks

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Last edited by Louis Cypher : Today at 03:45 PM.

 

[Louis Cypher]

finding a way to keep the photon hanging around might be a little tricky but then, you could send off for a new photon every once in a wile which could equally be sent by being entangled with the original; obviously a feed back loop from quantum to computer would need to recode information to be transmitted to re entangle the photon after this, but qbit parity check would maintain integrity? Any responses appreciated, even if you do tell me I'm an idiot?

 

[alexepascual]

Sorry. I am the source. Although when I posted it once before on this forum, someone replied that he had also thought of the same thing.
I am not trained to make the idea feasible. Perhaps you are. I take that explanation as evidence that the extra dimensions exist. Not sure why no bono fide string theorist has pursued it.

Richard:
So far I have found the "many worlds" interpretation of quantum mechanics as the most acceptable and when I think about quantum phenomena I think in terms of this interpretation. I have only read popularization articles and books about string theory. Since I read these articles, I have wondered if there is any connection between the extra dimmensions of string theory and the branches of the many worlds interpretation.
I thought you had read some article explaining entanglement in terms of the string theory. I think if you believe your scheme might work, the best for you is to continue learning about QM and string theory until you can prove yourself how these extra dimmensions account for entanglement or that they don't.

instantaneously be transfereed wireless without broadband information exchange at insatantaneous speed; the applications of the idea are endles, why is this wrong, please answer?!? I'm dying to know.

Louis:
Most physicists agree that it is not possible to send information "instantaneously" using entanglement. Of course the applications would be many if you could do this. But it appears you can't. If you think most physicists are wrong, then, I think you should study the subject deep enough so that you can put your ideas in mathematical form.
If you just want to know why your idea is wrong, you can do a Google search for: entanglement + causality or "teleportation".
Other place where you can search about these topics is in Wikipedia. You can actually learn a lot about QM there.
If you wonder why someone can't just give you a detailed explanation. I think it is because the effort it woud take to explain it to you is much, much, greater than the effort you put in formulating your hypothesis in the first place.

Eye_in_the_sky:
I'll be reading your posts in the comming days (done with finals now) . I promise I'll give you a response soon.

TheDonk:
Where are you?

 

[Louis Cypher]

would apreciate a reply before end of week

Any ideas guys?
see previous post

 

[Kea]

entanglement

Elsewhere I posted this:

A categorical semantics of quantum protocols
S. Abramsky and B.Coecke
http://users.ox.ac.uk/~mert1596/QUOXIC/talks/samson.pdf

Y. is no doubt correct in saying that no one understands it, but
this is a pretty good start.

 

[TheDonk]

I looked into superposition and the EPR experiment and now understand what both of them are. (I already had an idea what they were)

But back to my original thread topic question. How do particles become entangled? Let's use spin for the example. When do two electrons that aren't entangled go together and become entangled? What needs to happen for the electrons to enangle? Shouldn't this be easy to answer? It seems like it would be simple to answer this in laymens terms and without math, tho any answer would be appreciated.

Could it be that almost everything entangles particles? Maybe most measurements entangle the particle and the particle measuring it? Can two different particle types (ie electron and photon) be entangled?

Could every single particle be entangled with a pair and the only way to untangle it is to entangle it with something else? But what would happen to the original? Maybe it entangles with the other left over particle. I obviously just made this up and have no evidence at all but it sounds cool. Would this be hard to disprove?

Louis Cypher:
Your idea makes sense, I think, but only if you can send info instantaneously, which we don't know how to do. What you're talking about is a little bit off topic. (tho I'm bad for going off topic too)

 

[Mike2]

I looked into superposition and the EPR experiment and now understand what both of them are. (I already had an idea what they were)

But back to my original thread topic question. How do particles become entangled? Let's use spin for the example. When do two electrons that aren't entangled go together and become entangled? What needs to happen for the electrons to enangle? Shouldn't this be easy to answer? It seems like it would be simple to answer this in laymens terms and without math, tho any answer would be appreciated.

Could it be that almost everything entangles particles? Maybe most measurements entangle the particle and the particle measuring it? Can two different particle types (ie electron and photon) be entangled?

Could every single particle be entangled with a pair and the only way to untangle it is to entangle it with something else? But what would happen to the original? Maybe it entangles with the other left over particle. I obviously just made this up and have no evidence at all but it sounds cool. Would this be hard to disprove?

Obviously if particles were not entangled and then become entangled, then they become entangled when they start to effect one another. This means that at the point that a photon goes from one to the other, then information from one goes to the other so that electron spins must line up accordingly. I suppose that even the most distant particles would eventually transfer a photon between them and become entangled. But distant atoms could also remain hidden from one another for a long time.

 

[TheDonk]

Obviously if particles were not entangled and then become entangled, then they become entangled when they start to effect one another. This means that at the point that a photon goes from one to the other, then information from one goes to the other so that electron spins must line up accordingly. I suppose that even the most distant particles would eventually transfer a photon between them and become entangled. But distant atoms could also remain hidden from one another for a long time.

Thanks for the response Mike. I'm not too familiar with QM yet so I still don't know some of these obvious things. You said they get entangled when they effect each other. In what ways can this happen. Is sending a photon the only way? Does sending a photon always entangle the electron that absorbs it? Can you give me an example of how a different property other than spin can become entangled?

In every atom that has an even amount of electrons, are all the electron pairs entangled? Id that what defines them as a pair in the first place?

 

[Louis Cypher]

reply

Thanks that's the sort of answer I was looking for, the have used photon entanglement to exchange info though haven't they or am I mistaken?

Quantum encription bob and alice thingy, I'm not talking about teleprtation there just firing the light along a wire or at another computer as in the Bob and Alice experiment.

Just starting a couple of A level style maths courses before I go on to the Degree, it's a pain it means I have to do eight years of study to go with the foundation year I've allready done, but then no one ever claimed physics was easy. look at my post as a reflection of quantum encryption not telportation then; ok if entanglement doesn't teleport it at least qould mean we could us light instead of electricity to transmit signals, and in turn a device could interpret this much like a modem and encode back to an electronic signal, dial up users would be chuffed, and we would no longer have need of dial up, everyone could use broadband and light is better than electricity for sending signals plus it would be very safe from hackers compared to the internet now.

Like I say just thinking

ideas are the cornerstone of science, thanks for replying, and for refraining from calling me an idiot

 

[reilly]

Entanglement occurs in boh classical and quantum systems. Entanglement is a fancy word for correlation. The primary source of entangelment/correlations beween FREE particles -- EPR, Stern-Gehrlach (well almost free) is CONSERVATION LAWS, which are at their most striking for two-body systems. Note that without entanglement, the neutrino might well have remained undiscovered.

In two photon or two electron systems, the drill is to work with a s-wave source. Thus total angular momentum is zero. This means, measuring along the quantization axis, that if particle one is in a +1 Jz state, then immediately you know the quantization axis eigenvalue of the other particle -- just go to your already prepared table lookup. You knew, before the experiment, that if one gives spin up, the other necessarily must be spin down. QM allows only two state here; spin up/spin down or vica versa. The rest is Clebsch-Gordon coefficients, and appropriate rotation matricies.
There's no superluminal communication, nor any magic involved; the experimenter knows all the conditionals and probabilities prior to the experiment.

So, what am I missing? What other than conservation laws creates entanglement?

Regards,
Reilly Atkinson

 

[Louis Cypher]

tx

Thanks for that; I do now the postions of entanglement all I was saying is why can't we use a computer to take this Q information and code it into electrons? Or use the computer to take the quantum, change it and parity check it and then use this to transfer information via a modem from electrical to QM and vice versa, what's the problem here? If we can use QM to transfer info why can't we use a computer to decrypt encode and the recrypt and then send; a new internet without band width problems we use light as a medium of internet transfer, not electrons.

Am I being deranged?

I know we couldn't do it now, but if we could set up a network of cables to transfer photons not electrons could we not use it as an internet connection instead of broad band, using q encryption alice and bob stuff to transfer info much more safely than the internet using encryption, I feel I'm missing something here, cos everyones posting facts, which I appreciate alot; why couldn't we in "theory" if we had the technology use the quantum to exchange information via an electrical medium ie a computer?

Am I being dense again someone explain?

 

[alexepascual]

Louis,
Most of internet communications are in fact being transmitted by optical fibers. Up until now these fibers reach some local switching stations which convert the optical signal to electrical. The technology has been evolving so that more and more "conversations" can be sent over the same fiber at the same time. There are discussions about having these optical fibers reach the home, but it appears this is not economically feaseable at the time. If this were done, then you would have a system similar to what you propose except for the entanglement part. There has been a lot of discussion about the possibility of using entanglement to send secure messages over an optical channel. This would not necessarily increase the bandwidth but it would allow for an ultra-secure way to transfer information. Also, of course this system would not allow for transfer of information faster than the speed of light. If you want to learn more about these things, there is plenty of information on the web. Google for: Quantum cryptography, quantum encryption, etc. Every time you come up with an idea, it is better to first check to see if it hasn't been invented yet. Most of the time, whatever idea you (or I) come up with, someone else has thought about it before and even published it. If they haven't, there is a good chance the idea is wrong.

 

[alexepascual]

Reilly,
I am not sure I understand your observation, but it appears to me that you are putting classical correlations and quantum entanglement on the same footing. In quantum entanglement you have a superposition of different possible "realities' all existing at the same time. I think this has implications that are quite troubling (or exciting) depending on how you look at it.
One of the features of entanglement that makes it unique is "non-locality".
I think Einstein was very well versed in classical mechanics, but even so, he found non-locality quite troubling. So I don't think there is anything trivial about this phenomenon. It is true that some of the people posting here, who don't have enough formal training in QM have a wrong idea about what is entanglement, but that doesn't change the fact that there are aspects of entanglement that are quite puzzling.

 

[Wave's_Hand_Particle]

How do particles become entangled? I've heard that it's when two particles bump into each other. How is this "bump" defined? What does it mean for 2 particles to bump? Is it based on distance apart, or something else?

How do particles become entangled? I've heard that it's when two particles bump into each other. How is this "bump" defined? What does it mean for 2 particles to bump? Is it based on distance apart, or something else?

Entanglement is when 'ONE' particle, 'EXISTS' at two different locations, in the same instant.

One can make many speculations about the devision of a quantity?..if one cuts an whole orange in half, there does not appear two whole oranges, in two separate frames of reference, there exists 'ONE' orange at two locations, but their difference is not Time-Dependant.

It is a wonder that nature provides many aspects of observation, a "cause will have an effect".

Entanglement is like an opposite to the Pauli Exclusion Principle, there are other seemingly paradox's, resulting in definate Observation constraints, some may be Conscious derived, for instance:Consciousness is the Effect of the surrounding Spacetime, with Memory the Path Integral of Entangled 'event' States?

Just as you cannot remember all events 100% ( to do so would invoke a present-time event ), because you cannot be at two TIME locations in the same instant, to do this is to nullify the Uncertainty Principle, with Measurment, in fact some have speculatated that Entanglement is really just giving a Product, Two of the same identifyable quantities, or in simplistic terms, a precise measurement of Location/position, and disregarding the Momentum Measurment?

The 'cause' of entanglement (effect), can now be attributted to the QM 'splicing' experimental setups, or forgeting about the value of one property of Measure?

 

[Louis Cypher]

thanks guys, yeah let's go optical, and have a safe internet connection without all the annoying hackers and idiots ruining our fun, people may have concieved of going optical but what I suggested was going entangly optical, a leap I know but just an idea, just wanted sum feedback, if we could use the optical 1bit thing as a bus in a computer it would still travle at the speed of light but we would have a 1 a 0 and a 1 and 0 this would let us calculate faster; or would it as you say; anyway marry existing technology with the quantum is an idea, so how come no ones even trying as such, quantum modems, telephone lines the applications are endless, k its not possible now but why don't people at least try it, if they are theyre doing small scale experiments, thinking small, let's push things forward and try to use the quantum we can use entanglement as a modem, transfer optically, then decode using entanglement as a modem then convert to electricity? I know we can't but it does no harm to think about it, if we could use the quantum then why not use it and marry it to current stuff, instead of trying to ditch the old for the new?

 

[reilly]

Reilly,
I am not sure I understand your observation, but it appears to me that you are putting classical correlations and quantum entanglement on the same footing.

Indeed, I am.


(ap)In quantum entanglement you have a superposition of different possible "realities' all existing at the same time. (Why? In what sense do they exist?))

Arguable. If true in QM, then why not, say, in using Kalman Filtering to deduce aircraft location in airport radar's? Just because it might be does not mean that it is -- still a valid take on the world.

Happy Holidays to all,
Regards,
Reilly Atkinson

 

[alexepascual]

(ap)In quantum entanglement you have a superposition of different possible "realities' all existing at the same time. (Why? In what sense do they exist?))

Reilly,
I think we are getting into interpretation territory here. Heisenberg would have talked about "potentialities" instead of "realities". If a photon has gone through a double slit, I guess you could say that it went through both (in a way). But as you can't say that one quantum went through each slit, I guess you would have to think of these split personalities of the photon I guess as less that full "realities". But again I think this may be a matter of interpretation.
The second part of your post I don't understand and I suspect it may be a little off topic.
But I still think that quantum correlations and classical correlations are fundamentally different. Once again, I think a discussion about this would deserve its own thread and is not essential for our discussion on how two particles become entangled.
-Alex-

 

[Eye_in_the_Sky]

Reilly,
I am not sure I understand your observation, but it appears to me that you are putting classical correlations and quantum entanglement on the same footing.

Indeed, I am.

Classical correlations can be construed in terms of a local "reality", BUT quantum correlations cannot! This is the essential difference between the two.

That quantum correlations imply nonlocality can be seen from the following argument:

1) Suppose that the value obtained in a measurement is 'determined' at the time of the measurement and not before.

Then, clearly, for a pair of spin-entangled particles far apart in space, the joint 'collapse' which ensues on account of a spin-component measurement of only one of the particles implies nonlocality. //

2) On the other hand, suppose that the value obtained in a measurement is 'determined' before the time of the measurement. That is, this value – prior to measurement – is "definite but unknown".

Then, assume locality and derive a "Bell inequality". Such an inequality, however, contradicts the QM formalism. //[/color]

Putting 1) and 2) together leads to the inescapable conclusion that quantum correlations imply nonlocality.


... HOWEVER, there is another separate point which Reilly seems to be making, namely:

A "knowledge-based approach" to the 'collapse' phenomenon is consistent with the QM formalism.[/color]

The above statement is true!

Nevertheless, it should be noted that anyone who subscribes to such a "knowledge-based approach" is necessarily also subscribing to a nonlocal hidden-variable interpretation of QM.

 

[yanniru]

Non-local hidden variable interpretation- why that's Bohmian mechanics. Is that a dirty word that cannot be said?