Why does quantum entanglement not allow ftl communication

[Ken G]

That's interesting, it would be somewhat ironic if gravity waves are first detected via their interaction with sublimely constructed entangled quantum states, rather than the more brutely classical application of watching them make masses jiggle!

 

[foxy1968]

By using a minimum of 2 sets of qubits in isolation and by freezing the spin of the entangled particles and specifying that set 1 is used to indicate the start of a message and set 2 is used to send the message. By influencing the spin of the particles at one site and monitoring the spin changes at the other site why is this not possible. In this manner would it not be possible to send data over an infinite distance with no delay and therefore ftl.

 

[K^2]

Because you can't check whether the spin was "influenced" by the sender or by your own attempt to check whether it was influenced. Both give you exactly the same result, and so no information is carried.

 

[Demystifier]

Sec. 3 of
http://xxx.lanl.gov/abs/1006.0338
gives a simple explanation why entanglement cannot be used for ftl signalization.
It also proposes how this inability (to use it for ftl signalization) could, in principle, be overcame.

 

[Brokinarrow]

So basically the reason FTL communication is not possible using quantum entanglement: Currently we cannot control the state of the entangled particles, we can only observe the changes that nature is making to the state of the particles. If we could figure out a way to control the state of these particles, FTL communications would be possible.

 

[Albert V]

I've read some papers that were aiming to use linearly and circularly polarized light as a protocol for communication - however, it seems difficult/impossible to distinguish these two when you have to rely on incident photons (eg. Physics Letters A
Volume 251, Issue 5, 1 February 1999, Pages 294-296). Anyone with an idea?

I recently saw another ideá from Arxiv.org. I am not able to discover the flaw in his argument, but I suspect that there will be no interference?

http://arxiv.org/abs/1106.2257

 

[Albert V]

This is a discussion of the Cornwall paper on superluminal communication from

http://arxiv.org/abs/1106.2257

quantum theory says that whatever you do on one side does not change what you observe on the other:

Total state |Phi> = (|H>|V> + |V>|H>)/sqrt2.

Not using the polarizing filter (no modulation)

rho = |Phi><Phi|
= (1/2) ( |H>|V><H|<V| + |H>|V><V|<H| + |V>|H><H|<V| + |V>|H><V|<H| ).

In order to see what we observe on the left side we have to "trace out" the right side

rho_right = Tr_left(rho) = (1/2) (|H><H|+|V><V|),

which is eihter a photon in the mode H or a photon in the mode V, which will give no interference.

Am I mistaken?

 

[N468989]

If I understand correctly, then quantum entanglement is explained by the simple fact that two particles behave the same way after being separated.

Take Machine A and B, each compute numbers from 1 to 10 and are synchonized. Separate the machines and get the output at a given moment in time. We know what the other machine reads, is this correct?

Another thing is to assume that something is propagating through space... ()

 

[DrChinese]

If I understand correctly, then quantum entanglement is explained by the simple fact that two particles behave the same way after being separated.

Take Machine A and B, each compute numbers from 1 to 10 and are synchonized. Separate the machines and get the output at a given moment in time. We know what the other machine reads, is this correct?

This is true in a sense. And the description you give works fine for identical measurements on the individual particles. But it does not yield a suitable explanation for Bell tests. I.e. it predicts the wrong results. This fact was not noticed for many years after the EPR paper appeared, until Bell discovered it around 1964.

Best way to think of it is to imagine polarization of a pair of Type II entangled photons Alice and Bob at angles 0, 120 and 240 degrees. I.e. 1/3 of the way around a circle. After a while, you will realize that using your example, there is an average of at least a 1/3 chance that 2 adjoining measurements (one on Alice, the other on Bob) yielding the same value. However, experiments yield a value of 25% which is in agreement with the quantum expectation value.

 

[unified]

This is true in a sense. And the description you give works fine for identical measurements on the individual particles. But it does not yield a suitable explanation for Bell tests. I.e. it predicts the wrong results. This fact was not noticed for many years after the EPR paper appeared, until Bell discovered it around 1964.

Best way to think of it is to imagine polarization of a pair of Type II entangled photons Alice and Bob at angles 0, 120 and 240 degrees. I.e. 1/3 of the way around a circle. After a while, you will realize that using your example, there is an average of at least a 1/3 chance that 2 adjoining measurements (one on Alice, the other on Bob) yielding the same value. However, experiments yield a value of 25% which is in agreement with the quantum expectation value.

Agreed. But this leaves the question of how the entangled particles "know" what to do. If the correlation can't be explained in terms of a past interaction, I don't see how you can ever escape from "what I do over hear influences what happens over there". I think that's the whole point of Bell's theorem. It's not that hidden variables must be non-local, but any theory explaining this must be non-local.

 

[DrChinese]

Agreed. But this leaves the question of how the entangled particles "know" what to do. If the correlation can't be explained in terms of a past interaction, I don't see how you can ever escape from "what I do over hear influences what happens over there". I think that's the whole point of Bell's theorem. It's not that hidden variables must be non-local, but any theory explaining this must be non-local.

Welcome to PhysicsForums, unified!

You've probably seen some of the different interpretations that are currently in play. Of course the Bohmian view should be right up your alley. There are several others, including the time symmetric group. In those, the mantra is: "what I do now affects the past" and locality is preserved in the sense that influences do not propagate faster than c.

 

[questionpost]

quite simply the above question.
Why does quantum entanglement not allow for faster than light communication?
Thanks

They are the same particle, there's nothing to send information between. It would be like bouncing a ball and asking "why aren't other balls magically bouncing now?"

 

[unified]

Welcome to PhysicsForums, unified!

You've probably seen some of the different interpretations that are currently in play. Of course the Bohmian view should be right up your alley. There are several others, including the time symmetric group. In those, the mantra is: "what I do now affects the past" and locality is preserved in the sense that influences do not propagate faster than c.

Thanks for the welcome!

I've actually been reading your posts for quite some time now. I thought we could have some good talks. I'm not so interested in alternative theories to quantum mechanics, with the single exception of Bohmian Mechanics. I sometimes wonder why there are so few Bohmians. Bell thought it was almost scandalous. More interesting than Bohmian mechanics to me is the question of non-locality in quantum mechanics. Is it local? The most interesting thing I've come across lately is William Unruh, who is no fringe scientist. He makes the argument that quantum mechanics is completely local, and that there can be a simple answer to my question, "how do the electrons" know what to do. He explains everything in terms of past interaction, which confuses me because I thought the point of Bell's theorem is that this explanation is wrong. Unruh, though, certainly understands this theorem better than I do. Link included below.

Also, I would like to mention that I have never, ever come across someone who said quantum mechanics was non-local who was NOT also pursuing realistic theories, eg. Bohmian mechanics, GRW, etc. I suppose it's possible that this biases their opinions on quantum mechanics. Bell, surely could fall into this category.

 

[J Edwin Gray]

I find this subject interesting, If two particles were really entangled and mirrored each others patterns, in order to prove they are not just reacting on a past interaction. you would have to separate them by elevation for instance for several weeks for enough time has gone by that time dilation can be observed. After this time if the two particles are brought together and they are still in-sync they have been in active communication and not repeating a pattern. Is this an adequate experiment?

 

[San K]

I am pointing out the distinct difference between "random information" and
"no information" according to Shannon's information theory.

Hans, if something is truly random then how can it have/carry information?

Can you send any references/link/papers that distinguish between the two categories mention above?

 

[audioloop]

I find this subject interesting, If two particles were really entangled and mirrored each others patterns, in order to prove they are not just reacting on a past interaction. you would have to separate them by elevation for instance for several weeks for enough time has gone by that time dilation can be observed. After this time if the two particles are brought together and they are still in-sync they have been in active and not repeating a pattern. Is this an adequate experiment?

can delve it more ?

 

[J Edwin Gray]

I am relatively new to the forum but as a researcher and amateur scientist please forgive me for not quoting information for this is my first attempt in joining a conversation with what would seem more educated individuals than myself.

(By pattern I mean anything observable that would indicate the particles or structures are in sync.)


From some of the earlier post in this string of discussion it would seem as though there is a question of if the entangled particles actually are in constant communication mirroring each other in a form observable or if it is a pattern like that of two in-sync machines behaving off of a past interaction or setting.

I do not know if Shannon's theory of information can be used for this sort of information structure but in the before mentioned experiment (current abilities of science may not suffice) if the two particles or atomic structures were in communication be it random or ordered information it will be in-sync proving information transference , even random information is transferred. If it is not in sync and the patterns are identical it is not random information at all but a complex pattern we can not understand because if it is truly random the information would be different from both particles.

Both outcomes would advance a study but I would much rather see them in sync for the probability of structured information transmission in an instantaneous manner.

With time dilation in mind the pattern would occur faster for the particle in a reference frame with more time dilation, the observer would see it happen faster than the observer that is in a more rested frame (lower elevation) or it could be opposite. The possibility of a parent particle influencing another may be true. One particle may follow a specific one of the pair or would they produce a mean frequency of the both?

This is theoretical and I do not know if any such experiment has been performed as of yet but it would shed some light on the subject.

As to the idea that the constant velocity of light is broken it is not broken unless it travels in a quanta of space faster than the speed of c when this signal or transference of information is instantaneous. Take Plank's constant for instance of the smallest measurable distance, is is similar to a node in space time, distance is a number but nothing says one node cannot communicate with anther node some distance away. This is another theoretical view but if it skips a quanta of space time then it did not travel it appeared to travel instead. This keeps the constant max velocity of c without breaking any accepted laws. I could say more on this idea of node like theory of the space time fabric or structure but maybe for another discussion and not this thread.

 

[DrChinese]

I am relatively new to the forum but as a researcher and amateur scientist please forgive me for not quoting information for this is my first attempt in joining a conversation with what would seem more educated individuals than myself.

(By pattern I mean anything observable that would indicate the particles or structures are in sync.)


From some of the earlier post in this string of discussion it would seem as though there is a question of if the entangled particles actually are in constant communication mirroring each other in a form observable or if it is a pattern like that of two in-sync machines behaving off of a past interaction or setting.

I do not know if Shannon's theory of information can be used for this sort of information structure but in the before mentioned experiment (current abilities of science may not suffice) if the two particles or atomic structures were in communication be it random or ordered information it will be in-sync proving information transference , even random information is transferred. If it is not in sync and the patterns are identical it is not random information at all but a complex pattern we can not understand because if it is truly random the information would be different from both particles.

Both outcomes would advance a study but I would much rather see them in sync for the probability of structured information transmission in an instantaneous manner.

With time dilation in mind the pattern would occur faster for the particle in a reference frame with more time dilation, the observer would see it happen faster than the observer that is in a more rested frame (lower elevation) or it could be opposite. The possibility of a parent particle influencing another may be true. One particle may follow a specific one of the pair or would they produce a mean frequency of the both?

This is theoretical and I do not know if any such experiment has been performed as of yet but it would shed some light on the subject.

As to the idea that the constant velocity of light is broken it is not broken unless it travels in a quanta of space faster than the speed of c when this signal or transference of information is instantaneous. Take Plank's constant for instance of the smallest measurable distance, is is similar to a node in space time, distance is a number but nothing says one node cannot communicate with anther node some distance away. This is another theoretical view but if it skips a quanta of space time then it did not travel it appeared to travel instead. This keeps the constant max velocity of c without breaking any accepted laws. I could say more on this idea of node like theory of the space time fabric or structure but maybe for another discussion and not this thread.

I am not sure there is a specific question in here, but I can comment.

Experiments have been performed to attempt to determine the time sequencing of entanglement. The current lower bound on the speed of entanglement is 10^4 c (10,000 times the speed of light). Quantum theory does not give time or distance parameters as part of an entangled state description. So the "communication" is presumed to be instantaneous. Actually, that is not even a good term when you get down to it. The "communication" can depend on future events and in fact does not even need to be between particles that existed at the same time. So normal concepts of temporal sequence do not apply.

http://arxiv.org/abs/0808.3316
Lower limit of spooky action at a distance

http://arxiv.org/abs/quant-ph/0201134
Particles can be entangled after they are detected

 

[audioloop]

(By pattern I anything observable that would indicate the particles or structures are in sync.) From some of the earlier post in this string of discussion it would seem as though there is a of if the entangled particles actually are in mirroring each other in a form observable or if it is a pattern like that of two in-sync behaving off of a past interaction or setting.

mirrored because of collapse ?
after the collapse the englament is broken.

With time dilation in mind the pattern would occur faster for the particle in a reference frame with dilation, the observer would see it happen faster than the observer that is in a more rested frame (lower elevation) or it could be opposite.
This is theoretical and I do not know if any such experiment has been performed as of yet but it would shed some on the subject.

in a relativistic scheme.
done.

Experimental test of nonlocal quantum correlation in relativistic configurations
in/and
http://arxiv.org/abs/quant-ph/0007009-----
and if you ask for the speed of collapse itself (per se, not entanglement time), that is another question.

 

[audioloop]

I do not know if 's theory of information can be used for this sort of information structure but in the before mentioned experiment (current abilities of science may not suffice) if the two particles or atomic structures were in communication be it random or ordered information it will be in-sync proving information transference , even random information is transferred. If it is not in sync and the patterns are identical it is not random information at all but a complex pattern we can not understand because if it is truly random the information would be different from both particles.

Both outcomes would advance a study but I would much rather see them in sync for the of structured information transmission in an instantaneous manner.

you wish to explain or justify the correlations ?

 

[GwtBc]

I'm not a professional physicist, neither am I very familiar with quantum physics, so the following question might seem silly to you. Suppose there were two side, a and b, that wanted to relay information using quantum entanglement. They have two pairs of entangled particles, one to send information from a to b and one to send data from b to a. They agree that if they wanted to communicate information they would increase or decrease the spin rate of their transmission particle, the other side would constantly monitor their receiver particle for changes in it's spin rate. Could information, in binary form, be communicated using this method?

 

[Vanadium 50]

Did you read this thread? This very thing (it's spin direction, not rate, BTW) is discussed. If there is something you didn't understand, please point it out.

 

[GwtBc]

Did you read this thread? This very thing (it's spin direction, not rate, BTW) is discussed. If there is something you didn't understand, please point it out.

Sorry the spin rate issue was caused by a blunder.

 

[DrZoidberg]

Here is an interesting talk that explains in detail what would happen if you tried to use entangled particles to send information.

 

[Hamzaa]

What if there was a measurable speed for the transfer of information about the state of an entangled particle that we simply haven't calculated yet?

 

[DrChinese]

What if there was a measurable speed for the transfer of information about the state of an entangled particle that we simply haven't calculated yet?

Welcome to PhysicsForums, Hamzaa!

Assuming you are referring to the speed of collapse of entanglement, otherwise believed to be instantaneous: this has been measured to occur no less than 10,000 c.

http://arxiv.org/abs/0808.3316

 

[StrangeCoin]

Correlations can still be preserved even by measurements like that.

Correlations. How is some probability percentage between two binary streams of data supposed to contain such information to lead us into all these metaphysical conclusions we somehow indirectly infer from it? Shouldn't we label it as "inconclusive", rather than jump to such extreme conclusions standing in contradiction with the rest of the physics?

 

[DrChinese]

Correlations. How is some probability percentage between two binary streams of data supposed to contain such information to lead us into all these metaphysical conclusions we somehow indirectly infer from it? Shouldn't we label it as "inconclusive", rather than jump to such extreme conclusions standing in contradiction with the rest of the physics?

What is inconclusive? Metaphysics is only involved when trying to interpret the known (and predicted) results.

 

[StrangeCoin]

What is inconclusive?

Correlations, the experiments. Two polarizers are set to 30 degrees relative angle and we measure some probability percentage of 25%. Then we set them to 60 degrees apart and we measure 75%. Then we go like, wow 75%, that number means photons are communicating faster than light! To me that sounds like a pretty big leap to unnecessary conclusion. Based on what logic we ever expected to get any other number but 75% in the first place?

 

[DrChinese]

Correlations, the experiments. Two polarizers are set to 30 degrees relative angle and we measure some probability percentage of 25%. Then we set them to 60 degrees apart and we measure 75%. Then we go like, wow 75%, that number means photons are communicating faster than light! To me that sounds like a pretty big leap to unnecessary conclusion. Based on what logic we ever expected to get any other number but 75% in the first place?

And that correlation alone does not prove FTL influences.

But add Bell's Theorem, and you now know that either there are FTL (nonlocal) influences OR that particles do not have well determined properties outside of the context of a measurement (contextuality or nonrealistic). Could be either (or both). So it depends on which interpretation of QM you choose to adhere to. Many Worlds or Bohmian Mechanics are examples interpreting differently. Both follow Bell. You merely have to decide *which* classically intuitive idea you wish to abandon. There is nothing inconclusive about needing to drop one or the other.