B Entanglement Distance: Explained for Beginners

[megacal]

Hello,
I had never heard of "entanglement" (in regards to particles) until today when I read
an article on it in Discover Magazine (July-August 2016, p.68).
I searched for and found several threads here, but were too advanced to
understand and/or did not seem to address my questions below.
BTW, have only had under-graduate physics & math for a BS in Microbiology (1972).

1. What distance is one particle theoretically able to be entangled with another?
2. How can they be said to be entangled if trillions of other particles separate them?

Thanks in advance.

 

[phinds]

There is no distance limit. A pair of entangled particles are a single system. It is irrelevant how far apart they are and it is irrelevant that other particles are between them.

 

[megacal]

There is no distance limit. A pair of entangled particles are a single system. It is irrelevant how far apart they are and it is irrelevant that other particles are between them.

Ok, thanks. It's theoretically possible, and no offense, but so are pigs that fly.
Although I do think it's absolutely amazing that all that we see in the observable universe
is made up of incredibly (infinitely?) small particles tugging and repulsing each other, forming
structures, e.g. galaxies, galaxy clusters, super clusters, etc. that are connected by gravity
over such immense distances. So in that regard, I believe we are entangled.

 

[phinds]

Ok, thanks. It's theoretically possible, and no offense, but so are pigs that fly.

Yes, but pigs that fly have never been observed and entanglement has been experimentally shown to be true on numerous occasions so they're not really similar.

 

[megacal]

How did they isolate the particles so they could be studied?
The Discover article described how polarized light, but I don't understand how you isolate a pair
of particles (photons or protons) to show they are entangled. You really are not working with
a specific pair of particles.

Also, you said the distance or number of particles in between the entangled pair is irrelevant...how can that be proven?
It may be mathematically possible, but there's no way to test it.

 

[Comeback City]

Also, you said the distance or number of particles in between the entangled pair is irrelevant...how can that be proven?
It may be mathematically possible, but there's no way to test it.

It could be experimentally proven by simply increasing the distance that separates the entangled particles, and then performing the experiments to see if it yields the same results. I'm pretty sure the distance would be irrelevant, though, as Phinds stated.

 

[Mentz114]

How did they isolate the particles so they could be studied?
The Discover article described how polarized light, but I don't understand how you isolate a pair
of particles (photons or protons) to show they are entangled. You really are not working with
a specific pair of particles.

Also, you said the distance or number of particles in between the entangled pair is irrelevant...how can that be proven?
It may be mathematically possible, but there's no way to test it.

Entangled photon pairs can be created with PSDC, see the Wiki article

https://en.wikipedia.org/wiki/Spontaneous_parametric_down-conversion

Experiments have confirmed this phenomenon many times, starting with Alain Aspect

https://en.wikipedia.org/wiki/Bell_test_experiments

 

[DrChinese]

How did they isolate the particles so they could be studied?
The Discover article described how polarized light, but I don't understand how you isolate a pair
of particles (photons or protons) to show they are entangled. You really are not working with
a specific pair of particles.

Also, you said the distance or number of particles in between the entangled pair is irrelevant...how can that be proven?
It may be mathematically possible, but there's no way to test it.

The first thing is to understand some of the properties that make entangled particles different than un-entangled particles. There is something called "Bell Inequalities". Entangled particle pairs violate the inequality, others do not. So the answer to your question is to run a Bell test on particle pairs and look at the results. So there is a way to test this.

Second, as to distance: progressively longer and longer distances have been measured for entanglement. It went from being a few feet, to a few football fields, to even longer (this is 10 years old):

Abstract: Quantum Entanglement is the essence of quantum physics and inspires fundamental questions about the principles of nature. Moreover it is also the basis for emerging technologies of quantum information processing such as quantum cryptography, quantum teleportation and quantum computation. Bell's discovery, that correlations measured on entangled quantum systems are at variance with a local realistic picture led to a flurry of experiments confirming the quantum predictions. However, it is still experimentally undecided whether quantum entanglement can survive global distances, as predicted by quantum theory. Here we report the violation of the Clauser-Horne-Shimony-Holt (CHSH) inequality measured by two observers separated by 144 km between the Canary Islands of La Palma and Tenerife via an optical free-space link using the Optical Ground Station (OGS) of the European Space Agency (ESA). Furthermore we used the entangled pairs to generate a quantum cryptographic key under experimental conditions and constraints characteristic for a Space-to-ground experiment. The distance in our experiment exceeds all previous free-space experiments by more than one order of magnitude and exploits the limit for ground-based free-space communication; significantly longer distances can only be reached using air- or space-based platforms. The range achieved thereby demonstrates the feasibility of quantum communication in space, involving satellites or the International Space Station (ISS).

https://arxiv.org/abs/quant-ph/0607182

 

[DrChinese]

Ok, thanks. It's theoretically possible, and no offense, but so are pigs that fly.
... So in that regard, I believe we are entangled.

You asked about the theoretical limits, and phinds answered that there were none. The thing that ends entanglement is an observation. I gave you an example of entanglement over a large distance, and even larger are planned. There are even some attempts to perform observations of cosmic light particles - these are still in the very early stages of design. On the other hand, there has entanglement preserved for an hour - quite a technical feat.

As to "theoretical" flying pigs: please note that there is no theory of flying pigs (as in flying like a bird) because pigs do not possesses a mechanism for flight. So as was pointed out: the word "theoretical" has a more specific meaning around here, and it is not a derogatory term.

Lastly: there are many scientists who believe entanglement is pervasive. In some sense, you could say that "we" are entangled. Note that there is nothing that limits entanglement to just 2 particles, and entanglement of billions of particles has been experimentally demonstrated. On the other hand, there seem to be some limits but they are not well understood at this time.

 

[megacal]

Thank you for all the replies. I very much appreciate the time you take to help us who don't
have higher math ability. If I misused the term "theoretical", it was not meant to be derogatory.

If entanglement has been verified by multiple experiments (over 30 yrs?) and accepted as reality by the Physics
Community, then I must accept it as well, even if it seems impossible.

I'm surprised I hadn't heard of it until now, though it appears
to be common knowledge here. I'm way behind the curve!

I can't spend more time on it at the moment, but will in the coming days, and
may have a few more questions later.

 

[phinds]

If entanglement has been verified by multiple experiments (over 30 yrs?) and accepted as reality by the PhysicsCommunity, then I must accept it as well, even if it seems impossible.

This will hardly be the last thing you encounter that you will find impossible if your pursue knowledge of quantum mechanics and/or cosmology.

 

[sanman]

Is it possible for an entangled particle to undergo tunneling, and yet still remain entangled?

 

[Nugatory]

I can't spend more time on it at the moment, but will in the coming days, and
may have a few more questions later.

You should give Louisa Gilder's (fascinating, not technically demanding) book "The age of entanglement" a they.

 

[Nugatory]

Is it possible for an entangled particle to undergo tunneling, and yet still remain entangled?

You are stringing words together in a way that doesn't make sense. There's no such thing as "undergoing tunneling", because tunneling isn't something that happens to or is done to a particle. There's a probability of finding the particle in various locations. These probabilities are different from what classical theory predicts and we use the word "tunneling" to describe the differences.

 

[rockart]

I'd guess that since a photon is two dimensional, not existing in its direction of travel, that once it has been emitted, it will remain entangled with whatever emitted it until it interacts with another three dimensional object.

 

[phinds]

I'd guess that since a photon is two dimensional, not existing in its direction of travel, that once it has been emitted, it will remain entangled with whatever emitted it until it interacts with another three dimensional object.

As far as I am aware, this makes no sense at all. It seems that you misunderstand photons, EM radiation, and entanglement.

Light does not travel as photons, it travels as waves. Photons are the result of an EM wave interacting with a particle. Entanglement is not something that just automatically happens by virtue of a photon having been emitted.

The language on this gets a bit fuzzy. For example, we say that a photon is emitted, but the resulting EM radiation is a wave, not a photon. Actually, to be more precise about it, a photon is a quantum object. It is not a wave and it is not a particle. If you measure it for wave characteristics, it exhibits wave characteristics and if you measure it for particle characteristics, it exhibits particle characteristics, but it is not one or the other. It is a quantum object. Period.

Also, you can, with some rationale, talk about the size (amplitude) of an EM wave, but to say that a photon has 2 dimensions is not meaningful.

 

[Comeback City]

As far as I am aware, this makes no sense at all. It seems that you misunderstand photons, EM radiation, and entanglement.

Light does not travel as photons, it travels as waves. Photons are the result of an EM wave interacting with a particle. Entanglement is not something that just automatically happens by virtue of a photon having been emitted.

The language on this gets a bit fuzzy. For example, we say that a photon is emitted, but the resulting EM radiation is a wave, not a photon. Actually, to be more precise about it, a photon is a quantum object. It is not a wave and it is not a particle. If you measure it for wave characteristics, it exhibits wave characteristics and if you measure it for particle characteristics, it exhibits particle characteristics, but it is not one or the other. It is a quantum object. Period.

Also, you can, with some rationale, talk about the size (amplitude) of an EM wave, but to say that a photon has 2 dimensions is not meaningful.

Photons can be thought of as quantum packets of electromagnetic energy. Can you even measure energy in dimensions?

 

[Nugatory]

Photons can be thought of as quantum packets of electromagnetic energy. Can you even measure energy in dimensions?

No, and this is only one of several reasons why the idea that "a photon is two dimensional, not existing in its direction of motion" is nonsense.

However, all of this is a digression in this thread. Any further discussion should happen in a new thread - but please do look at some of the many threads we already have about what a photon is.

 

[megacal]

This Wiki article, Quantum Entanglement, seems the best overview I've found so far (thanks to earlier links provided by Mentz114),
and just ordered a copy of The Age of Entanglement by Louisa Gilder recommended by Nugatory.

Also appreciate the info by Dr.Chinese, and am studying the pdf of the 144km experiment using
the Optical Ground Station (OGS) of the European Space Agency he linked to.

This is going to take me a while to digest...fascinating stuff!

 

[mikeyork]

1. What distance is one particle theoretically able to be entangled with another?
2. How can they be said to be entangled if trillions of other particles separate them?

First of all, it is a god idea to think of entanglement as a fundamental QM phenomenon that operates at a level in which space-time is irrelevant. So, even though we necessarily see it as happening in our space-time context, it is independent of any space-time co-ordinates or measurements such as distance.
Second, the presence of trillions of other particles is also irrelevant unless the entangled particles have interacted with any of them. If they don't "see" or experience them in any way then they can have no effect. On the other hand, if they did interact then yes, they would interfere with the prior entanglement.

 

[sanman]

You are stringing words together in a way that doesn't make sense. There's no such thing as "undergoing tunneling", because tunneling isn't something that happens to or is done to a particle. There's a probability of finding the particle in various locations. These probabilities are different from what classical theory predicts and we use the word "tunneling" to describe the differences.

Pardon me for articulating myself poorly. So I just wanted to know if an entangled particle which is then observed to have tunneled across some potential barrier, would still be able to maintain its original entanglement.
Is that more clearly stated now?

There are all kinds of interactions which will collapse the wavefunction indeterminacy into some particular state, thus losing the entanglement with other particles. I just want to know if that entanglement would be lost if tunneling subsequently occurs.

 

[DrChinese]

Pardon me for articulating myself poorly. So I just wanted to know if an entangled particle which is then observed to have tunneled across some potential barrier, would still be able to maintain its original entanglement.
Is that more clearly stated now?

There are all kinds of interactions which will collapse the wavefunction indeterminacy into some particular state, thus losing the entanglement with other particles. I just want to know if that entanglement would be lost if tunneling subsequently occurs.

If the entanglement was on a basis not relevant for the tunneling, it would not be affected per se. If it were entangled in energy, then it would serve as a kind of measurement to have tunneled.

 

[megacal]

It thus appears that one particle of an entangled pair "knows" what measurement has been performed on the other, and with what outcome, even though there is no known means for such information to be communicated between the particles, which at the time of measurement may be separated by arbitrarily large distances.

- Quantum Entanglement, Wiki
I accept that it as reality, but how does one particle "know" what the other particle is doing or what is being done to it?
It's as though they are connected by a thread (or string?) that has no elasticity if they react instantly to the measurement of
the other(s) in the system.

 

[mikeyork]

The phrase "It thus appears" is important here. It refers to how things appear from a classical viewpoint -- which we know to be wrong. Confusion continues around this in the popular literature (and in Wikipedia). Understanding QM requires a willingness to abandon classical ideas that no longer serve us.

 

[megacal]

The phrase "It thus appears" is important here. It refers to how things appear from a classical viewpoint -- which we know to be wrong.

Are you saying the Wiki article is inaccurate?

 

[mikeyork]

Are you saying the Wiki article is inaccurate?

I am saying that in trying to explain QM phenomena to a lay audience it faces the same problem that the early pioneers had to tackle and in that piece you quote it would create the same confusion that lasted for decades by trying to describe QM in classical terms if it weren't for the "It thus appears" caveat.

 

[weirdoguy]

Are you saying the Wiki article is inaccurate?

Most of them are inaccurate tbh...

 

[anorlunda]

First of all, it is a god idea to think of entanglement as a fundamental QM phenomenon that operates at a level in which space-time is irrelevant.

Doesn't that overstate it?

I think of two free electrons, initially unentangled, whose trajectories pass very close to each other. They can become entangled into a singlet pair as they pass in proximity, and remain entangled as they separate.

But space and time is highly relevant. If they did not come in close proximity, they would not become entangled, hence spatial relevance. Time is relevant because there are before and after entanglement states. Future events can disentangle them, so time is again relevant.

p.s. "god idea" great pun

 

[megacal]

@mikeyork,
what is not accurate in the Wiki article, Quantum Entanglement?

@Dr.Chinese,
do you agree with Mike about the article?
Was looking at your webpage, and seems like you are an "authority" on the subject.

 

[DrChinese]

@mikeyork,
what is not accurate in the Wiki article, Quantum Entanglement?

@Dr.Chinese,
do you agree with Mike about the article?
Was looking at your webpage, and seems like you are an "authority" on the subject.

I agree with Mike.

As to me being an authority LOL: More like I recognize people who are themselves authorities. After reading enough on the subject, you recognize the ones who can consistently represent facts and theory together - which is what makes them an authority in my eyes. I try to cite my sources, and I encourage anyone to go to those where possible.

 

[mikeyork]

Doesn't that overstate it?

I think of two free electrons, initially unentangled, whose trajectories pass very close to each other. They can become entangled into a singlet pair as they pass in proximity, and remain entangled as they separate.

But space and time is highly relevant. If they did not come in close proximity, they would not become entangled, hence spatial relevance. Time is relevant because there are before and after entanglement states. Future events can disentangle them, so time is again relevant.

p.s. "god idea" great pun

You have described how an external observer sees things when they impose a space-time frame. And, yes, I see your point that we can't ignore that an observer's view of the entangling interaction requires spatial coincidence (or at least proximity). But the intrinsic entanglement that results remains independent of both the observer's space-time frame and the space-time co-ordinates of each electron relative to the other.

 

[megacal]

If the Wiki article is not accurate, would like to know specifically in what way.

 

[mikeyork]

If the Wiki article is not accurate, would like to know specifically in what way.

I have responded to the two questions in your original post. I explained the problem with the piece you quoted from the Wikipedia article. I do not have the time to read the rest of the article and so I do not know how accurate it is. Even if I did have the time, I would not be inclined to do so for someone who just rolled their eyes at me as I would expect it to be a waste of my time. I suggest you spend a little more time thinking about how I have already responded and try reading a modern text-book on QM. Or, better still, since you like DrChinese's webpage, why don't you read that?

 

[DrChinese]

If the Wiki article is not accurate, would like to know specifically in what way.

No one around here really worries too much about what Wiki says in a particular part of an article. I would take what it says with a grain of salt. As Mike says, attempting to translate QM principles to lay language often leads to something that itself cannot be defended in strict terms. Think of it "as if" X is true, even if X is not strictly accurate.

 

[megacal]

Whoa, Mike!
Your answers made no sense to me, though I'm sure they are clear to
others here. You gave a very nebulous answer, imho, and did not quote any part of the article that I can see.
I don't want you to waste your valuable time on it.
Please note that I labeled the thread "B" for Basic.

Dr.Chinese,
could you please show me where the Wiki article is in error? It seemed to correlate with what I've read so
far (Louisa's book is on the way).
Thanks!

 

[megacal]

As Mike says, attempting to translate QM principles to lay language often leads to something that itself cannot be defended in strict terms. Think of it "as if" X is true, even if X is not strictly accurate.

Sorry, didn't realize you had posted...Ok, I won't beat it to death. There is nothing specifically
wrong with the Wiki article, it's just suspect because it's a Wiki article, LOL!

BTW, I don't take the Wiki as gospel for anything, but it seemed like a relatively concise explanation in this case. Always take EVERYTHING with a grain of salt..."Low Salt"...1/2 KCl, 1/2NaCl...cardio friendly.
In the Big Scheme of Things, it ain't that big a deal.
Thanks for your attempt to explain Mike's point, but I still don't get it.
Hopefully Louisa's primer on Entanglement will make it clear (assuming it has the blessing of the QM community).
Peace all.

 

[stoomart]

Whoa, Mike!
Your answers made no sense to me, though I'm sure they are clear to
others here. You gave a very nebulous answer, imho, and did not quote any part of the article that I can see.
I don't want you to waste your valuable time on it.
Please note that I labeled the thread "B" for Basic.

Dr.Chinese,
could you please show me where the Wiki article is in error? It seemed to correlate with what I've read so
far (Louisa's book is on the way).
Thanks!

I'm no doctor, but I know it's common practice for Wiki articles to cite non-peer-reviewed sources, hence why it is not considered a valid source on these forums.

 

[mikeyork]

You gave a very nebulous answer, imho, and did not quote any part of the article that I can see.

You quoted the part I referred to. I merely emphasized the importance of the first three words. Also, my first response was #20. Did you read that?

 

[megacal]

Yes, I read everything you posted, and you still have not shown me any part of the article that
you disagree with. Is your only criticism of the article the phrase, "It thus appears..."?
I'm not trying to be obtuse or argumentative. I just wanted to know what if any of the article was
wrong, according to you or anyone else.
I'm not even disputing that the article is inaccurate.
Please quote the part that is wrong, and explain why it is wrong.
Sorry I rolled my eyes...wasn't meant to be offensive.
Gotta lighten up. Life is too short.

 

[Nugatory]

Is your only criticism of the article the phrase, "It thus appears that..."?

That's not a criticism of the article, it's pointing out the words that save it and the way that you seem to have misunderstood them. If the article said

One particle of an entangled pair "knows" what measurement has been performed on the other, and with what outcome, even though there is no known means for such information to be communicated between the particles, which at the time of measurement may be separated by arbitrarily large distances.

then the article would be wrong. But instead the article carefully says that "It thus appears that...". The article doesn't say that one particle of the entangled pair knows what measurement has been performed, it says that it appears as if it does. The implication is that although it appears to be that way, it isn't really that way and everything following the "It thus appears that..." is actually false.

And stuff like this is the reason why wikipedia is not in general an acceptable source under the Physics Forums rules. There are some things that wikipedia does really well. Competent explanations of quantum mechanics is not one of them.

 

[anorlunda]

then the article would be wrong. But instead the article carefully says that "It thus appears that...". The article doesn't say that one particle of the entangled pair knows what measurement has been performed, it says that it appears as if it does. The implication is that although it appears to be that way, it isn't really that way and everything following the "It thus appears that..." is actually false.

Congratulations @Nugatory , that is an excellent explanation. It also illustrates Wikipedia's challenge and weakness. Even when the article author tries mightily to be careful and accurate, ordinary readers will miss the nuances in the choice of words. Some physics can only be described by mathematics.

 

[megacal]

ordinary readers will miss the nuances in the choice of words

Like I did.
My apologies to MikeYork for not understanding what he was pointing out earlier.
After reviewing his replies together with those of Dr.Chinese & Nugatory more carefully, and I realize I was missing the forest for the trees.
In the future I will not use the Wiki as a resource for QM (or other physics-related disciplines) to avoid the risk of being mislead or
misleading others.

In the mean time, just received my copy of The Age of Entanglement recommended by Nugatory.
According to the author, Louisa Gilder,

"This is a book of conversations, a book about how the give and take between physicists
repeatedly changed the direction in which Quantum Physics developed..."

full of quotes from memoirs & biographies.
Hope to get a better understanding of Entanglement, as well as learn about those who worked to elucidate this "spooky action at a distance"
as Einstein called it.

 

[physika]

- Quantum Entanglement, Wiki
I accept that it as reality, but how does one particle "know" what the other particle is doing or what is being done to it?
It's as though they are connected by a thread (or string?)
that has no elasticity if they react instantly to the measurement of
the other(s) in the system.

or a wormhole

Are entangled particles connected by wormholes? Evidence for the ER=EPR conjecture from entropy inequalities
Phys. Rev. D 89, 066001

http://journals.aps.org/prd/abstract/10.1103/PhysRevD.89.066001

https://arxiv.org/pdf/1308.0289v1.pdf