Entanglement - lack of "symmetry" in no-hidden variables explanation

In summary: What do you mean by "measure the spin of the system and obtain the singlet state"? If you measure the spins of two random electrons that are spatially separated,...
  • #1
mgkii
138
42
TL;DR Summary
Despite clear proof and demonstrations that there are no local-hidden-variables at play, entanglement itself has to be local to start, even though un-entanglement happens at a distance. This seem non-symmetric and I want to learn more!
Hi. Over the years I've read LOT of "popular science" (i.e. non-textbook) books on entanglement, and on the explanations / objections / arguments Einsten, Bohr, Bohm and others had that still remain today. There's one aspect which never seems to get covered in these books and I wondered if anyone could point me to a source?

There seems to be a lack of symmetry in entanglement, in that for two particles to be entangled they have to have been at the same place at the same time to get entangled, even though the un-entanglement occurs at a distance. As far as I understand, there is no mechanism for two particles to become entangled at a distance.

I've read enough accounts of thought experiments and real experience designed to test Bell's inequality to understand (well - more accurately I should say accept !) there are no local hidden variables at play, and read accounts of Bohm's theory that if there are hidden-variables, then they have to be non-local. But again, there seems to be a lack of symmetry in that there can be no local-hidden variables, but there has to be something that goes on locally to cause the entanglement.

Any suggestions of further reading would be appreciated. I've tried and failed a couple of times to go the textbook route, but I really don't have the maths skills to keep up with the texts; I'm afraid I'm firmly stuck in the laymen / popular-science explanation field. I've also read enough Edward Frenkel to know I've probably committed a few sins with my use of the word symmetry - so sorry for that as well!

Thanks
Matt
 
Last edited:
Physics news on Phys.org
  • #2
mgkii said:
There seems to be a lack of symmetry in entanglement, in that for two particles to be entangled they have to have been at the same place at the same time to get entangled, even though the un-entanglement occurs at a distance. As far as I understand, there is no mechanism for two particles to become entangled at a distance.

I'm not sure there is any maximum distance over which an entagled state can be created. Take the example of two electrons, say. If we measure the spin of the system and obtain the singlet state, then that is an entangled spin state. That's doesn't imply that the particles are in the "same place at the same time", which isn't a very quantum mechanical thing to say in any case.

It certainly must be practically easier to obtain an entangled state if the electrons are isolated or if an electron and positron are created together. But, in principle, there is nothing to say that an entangled state must be initially localised. The entangled state persists even when the spatial wavefunction of the state is extended over a large region.
 
  • #3
Entanglement is pretty hardcore standart quantum mechanics. It was predicted to exist way before the first experimental test came around. Within the framefork there is nothing questionable or controversial. It's how the world is at the smallest scales - indeterminate prior to measurement. Entaglements are ubiquitous.
 
Last edited:
  • #4
PeroK said:
I'm not sure there is any maximum distance over which an entagled state can be created. ... But, in principle, there is nothing to say that an entangled state must be initially localised. The entangled state persists even when the spatial wavefunction of the state is extended over a large region.

Thanks PeroK - definitely my very clumsy wording to use the phrase "same place"! It's probably one of the many issue with limiting reading to non-textbooks; in all the books and articles I've ever read on entanglement, the starting point is always something that happens "at very short distance" be it at two particles' creation, through interaction, a beam splitter, etc. Your reply is the first time that someone has pointed out that "local" entanglement isn't the only way of being entangled. Much appreciated
 
  • #5
EPR said:
Entanglement is pretty hardcore standart quantum mechanics. It was predicted to exist way before the first experimental test came around. Within the framefork there is nothing questionable or controversial. It's how the world is at the smallest scales - indeterminate prior to measurement. Entaglements are ubiquitous.
Thanks for taking the time to reply EPR. I certainly wasn't suggesting there was anything questionable, just that in the explanations I've read (that have been limited to non-textbook articles and books), there has always been local interaction to kick things off. PeroK has kindly pointed out that this does not have to be the case. Thanks again
 
  • #6
PeroK said:
Take the example of two electrons, say. If we measure the spin of the system and obtain the singlet state, then that is an entangled spin state

What do you mean by "measure the spin of the system and obtain the singlet state"? If you measure the spins of two random electrons that are spatially separated, and obtain opposite results (z-spin up and z-spin down, say), that does not mean the electrons are in the singlet state. To get a pair of electrons in the singlet state, you have to prepare them in that state using a process that entangles them. As far as I know, all such processes are local--the electrons have to be co-located.
 
Last edited:
  • Like
Likes vanhees71 and bhobba
  • #7
mgkii said:
There seems to be a lack of symmetry in entanglement, in that for two particles to be entangled they have to have been at the same place at the same time to get entangled, even though the un-entanglement occurs at a distance. As far as I understand, there is no mechanism for two particles to become entangled at a distance.
... I'm afraid I'm firmly stuck in the laymen / popular-science explanation field.
I you want a laymen / popular-science explanation, consider the analogy between entanglement and love relationship.
(For a similar analogy see also https://www.physicsforums.com/threa...anglement-and-decoherence.871801/post-5481090)
To get into a love relationship with a woman, you need to get close to her. But to break with her, you don't need to get close to her again. It may be enough that you get close to another woman somewhere else.

More fundamentally, this asymmetry (for both entanglement and love relationships) is related to the 2nd law of thermodynamics, but that would not be a layman explanation.

mgkii said:
But again, there seems to be a lack of symmetry in that there can be no local-hidden variables, but there has to be something that goes on locally to cause the entanglement.
That's a different kind of asymmetry, I don't know how to explain it in lay terms.
 
  • #8
mgkii said:
Summary:: Despite clear proof and demonstrations that there are no local-hidden-variables at play, entanglement itself has to be local to start

Just to add to what others have already said: You can entangled objects that have never been in contact. One way to do it is called "entanglement swapping".

https://arxiv.org/abs/quant-ph/0201134
 
  • Like
Likes Lord Jestocost and bhobba
  • #9
PeterDonis said:
Saying that the entangled state persists when the state is extended over a large region, is not the same as saying the entangled state can be created over a large region. The latter is a much stronger statement, and per my previous post just now, I don't think it's true.

I couldn't see a reason why the spatial wavefunction of the two-particle state, theoretically, had to be initially localised. Practically, of course, it may be difficult to achieve beyond a microscopic extent.
 
  • #10
PeterDonis said:
To get a pair of electrons in the singlet state, you have to prepare them in that state using a process that entangles them. As far as I know, all such processes are local--the electrons have to be co-located.

Perhaps for electrons, this requirement may be correct. But there are other things (photons for example) that can be entangled remotely via entanglement swapping. They need not share a common light cone to the past.
 
  • Like
Likes PeroK
  • #11
PeterDonis said:
What do you mean by "measure the spin of the system and obtain the singlet state"? If you measure the spins of two random electrons that are spatially separated, and obtain opposite results (z-spin up and z-spin down, say), that does not mean the electrons are in the singlet state. To get a pair of electrons in the singlet state, you have to prepare them in that state using a process that entangles them. As far as I know, all such processes are local--the electrons have to be co-located.

If you prepare a system of two electrons in some entangled state
$$\psi(x_1, x_2) \chi$$
Where ##\chi## is the singlet state. What constraints are there on the spatial wavefunction ##\psi##? I can't see why there should, theoretically, be some constraints at time ##t=0## than don't apply at other times.
 
  • #12
Demystifier said:
That's a different kind of asymmetry, I don't know how to explain it in lay terms.
I wonder if the OP means time symmetrical.

Can you (in principle) reverse the process, and have a pair of unentangled particles at a distance become entangled?
 
  • #13
DaveC426913 said:
I wonder if the OP means time symmetrical.

It was part of the question, but location is the other part. The lack of symmetry that never seems to be addressed in the non-academic texts that I have been (self) limited to, is that the "birth" of entangled particles always seems to have to involve very local processes (even during entanglement swapping as DrChinese has kindly noted), yet the un-entanglement can happen at any distance. The fact that local mechanisms are mandated, yet local-variables are demonstrably/provably not part of the picture is another aspect that feels "asymmetric"
 
  • #14
You should stop thinking in term of particles. At least in the quantum realm.

Two particles that have interacted have joint uncertainties and share joint physical properties.

There are no particles in the quantum world and this is what's crucial in this setup. No particles at all.

The 2 interacting 'particles' are 1 system. Space appears to be dependent on the state of the quantum system, not vice versa.
 
  • #15
mgkii said:
DaveC426913 said:
I wonder if the OP means time symmetrical. Can you (in principle) reverse the process, and have a pair of unentangled particles at a distance become entangled?
It was part of the question, but location is the other part.
This is what I meant. I thought perhaps this is the symmetry you are looking for, and can it happen?
entanglement.png


But I realize I am only muddying the waters. Others are doing a better job.
 
  • #16
They can be re-entangled only if they are made to interact locally(e.g. in a collision). Entanglement is simply the inability to separate joint uncertainties. As far as i can see, interactions remove quantumness and well... entanglements. Certain interactions which leave no which-way info turn into entanglements(these should be everywhere, all the time). Broken entanglements are a special case, sort of like 'classical reality' is a certain special case of the quantum world. We certanly live in quite a special case of quantum reality(esp. if judged by qm alone)
 
  • #17
PeterDonis said:
To put it in short form, entanglement can be swapped remotely, but it can't be created remotely; remote entanglement swapping relies on a prior local creation of entanglement somewhere.

I think we've had some discussion around this previously. The "standard" view is that entanglement can be created between 2 particles that have never shared a common light cone to the past. The decision to entangle them is made at a location which is likewise not local to either of the entangled particles.

Myself, I wouldn't call it local - it is quantum nonlocal in the same way that entangled particle correlations are. But as you say: the entangled pair originated as a result of prior local entanglement operations (though with a different entangled partner).

Usually, a proper description of a quantum system requires consideration of the entire system, and individual components don't really have the usual separate description. In this case, the entire system lacks a place that could be reasonably called a single "source location".
 
  • Like
Likes Lord Jestocost and PeroK
  • #18
PeterDonis said:
Because there is a preparation process that creates entanglement happening at ##t = 0## and not at other times. As far as I know, any process that creates entanglement (as opposed to just swapping it--see my response to @DrChinese just now) must be local: the particles involved have to be co-located.

I must admit the theoretical process of creating an entangled system without some sort of initial local interaction is subtler than I first thought.

How local is "local". Theoretically, if the total spin AM of a hydrogen atom is zero, then the proton and electron must be in the singlet state and entangled. In what sense were these two particles in the same place at the same time?

Similarly, if you have two identical particles in a box, measure the total energy and get the ground state energy, then you know you have an entangled combination of the two lowest single-particle energy states. What limitations are there on the size of the box?

Is this really a question for QFT? It feels to me like there are limitations of non-relativistic QM in thinking about this.
 
  • #19
PeroK said:
if the total spin AM of a hydrogen atom is zero, then the proton and electron must be in the singlet state and entangled. In what sense were these two particles in the same place at the same time?

"In the same place at the same time" within the limitations of the uncertainty principle.

PeroK said:
if you have two identical particles in a box, measure the total energy and get the ground state energy, then you know you have an entangled combination of the two lowest single-particle energy states. What limitations are there on the size of the box?

The uncertainty principle limitations on that particular ground state.
 
  • #20
PeterDonis said:
I assume you mean never shared a common light cone prior to the entanglement swapping interaction? Once the entanglement swapping interaction takes place, the two particles must have overlapping past light cones; see below.
Yes, and this means that each particle of the entanglement pair must have those local entanglement operations in its past light cone when it undergoes the entanglement swapping interaction. Which means that both particles must have overlapping past light cones once the entanglement swapping interaction takes place.

Doesn't the diagram of the experiment in the paper in post #9 show that photons 0 and 3 don't?
 
  • #21
PeterDonis said:
I assume you mean never shared a common light cone prior to the entanglement swapping interaction? Once the entanglement swapping interaction takes place, the two particles must have overlapping past light cones; see below.

Yes, and this means that each particle of the entanglement pair must have those local entanglement operations in its past light cone when it undergoes the entanglement swapping interaction. Which means that both particles must have overlapping past light cones once the entanglement swapping interaction takes place.

No, the entangled particles never existed in the other's light cone. Here's the reference, and I included several additional ones as some are clearer than others. In a typical swap setup, particles 0 & 3 end up entangled even if they never come close to each other.

a) Particles 0 & 1 are created and entangled.
b) Particles 2 & 3 are created and entangled, elsewhere - and far enough away not to be local.
c) Particles 1 & 2 are sent to Charlie's Bell State Analyzer (BSA or BSM) in such a way that they are indistinguishable. If that happens, and the Analyzer so indicates, 0 & 3 are now entangled with each other rather than their previous partners. (There is NO specific time at which that change can be said to occur.)
d) Meanwhile, 0 & 3 continue on their paths to the usual A/Alice and B/Bob, who perform a normal Bell test to prove they are entangled (i.e. have the quantum correlations).

[Note that in each paper, the labels are different than above, and Alice/Bob and Charlie/Victor may be doing different things. Each setup is slightly different but the essentials are the same. All of these are by top teams.]

https://arxiv.org/abs/quant-ph/0201134
Diagram page 8.

https://arxiv.org/pdf/0911.1314.pdf
"It is natural to expect that correlations between distant particles are the result of causal influences originating in their common past — this is the idea behind Bell’s concept of local causality [1]. Yet, quantum theory predicts that measurements on entangled particles will produce outcome correlations that cannot be reproduced by any theory where each separate outcome is locally determined by variables correlated at the source. This nonlocal nature of entangled states can be revealed by the violation of Bell inequalities.

However remarkable it is that quantum interactions can establish such nonlocal correlations, it is even more remarkable that particles that never directly interacted can also become nonlocally correlated. This is possible through a process called entanglement swapping [2]. Starting from two independent pairs of entangled particles, one can measure jointly one particle from each pair, so that the two other particles become entangled, even though they have no common past history. The resulting pair is a genuine entangled pair in every aspect, and can in particular violate Bell inequalities. "
https://arxiv.org/abs/1508.05949
This is a setup that entangles electrons using photonic entanglement swapping. The electrons themselves do not travel. So strictly speaking, they are in each other's light past cones although they are isolated from each other during the period in question.
 
  • Like
Likes Lord Jestocost and PeroK
  • #22
PeterDonis said:
The term "entanglement swapping" has two possible meanings, and I should have made that distinction clearer before. In the experiment in the paper in post #9, "entanglement swapping" means only "by post-processing the data to filter out a certain subset of events, we can show correlations between photons 0 and 3 that indicate entanglement".

...

"Entanglement swapping" might not be the correct term for the latter kind of experiment, but I believe that, if you want to have a pair of spatially separated entangled particles that are usable for future experiments, you have to do it by that kind of process; the kind of process described in the paper linked in post #9 won't work because it has to consume the particles.

Post selection is not what is occurring although it is a component. It is a nonlocal swap operation, and if it is not done, there is no entanglement between 0 & 3. True, only some swaps succeed and that certainly requires post selection to determine which did. You will NOT find a quote in a swapping paper that in any way implies that the final entangled particles are anything other than non-interacting to each other, or that the event ready ones only represent a group that has been post selected (implying that the swap is not an actual quantum operation. (Please note that post selection is performed with ALL parametric down conversion sources as well, but it is still a quantum operation. No one dismisses the results just because there is selection.)

The 3rd reference in my post #28 leads to exactly the situation you say shouldn't occur. You have 2 electrons sitting in an entangled state, ready for you to do what you want to them, and they never interacted.

The only people I have EVER heard make statements you are making are you and vanhees71 - certainly nothing like you say appears in the literature. As I have provided references from 3 top teams all saying the exact same thing: you can non-locally entangle particles that have never interacted via what is commonly called "entanglement swapping" (or sometimes "quantum teleportation"). I would kindly request you to provide suitable reference for your statement contradicting me (or saying that my statements do not apply as I say they do). I am not trying to be combative in any way. However, this step has been declined by you in all of our previous discussions. I believe if you will read these papers again, you will see that every objection you have is handled in one paper or the other. And please note these additional references, although my first should have been enough:

https://arxiv.org/abs/quant-ph/0409093
Long distance entanglement swapping with photons from separated sources (2004)
"Quantum teleportation is a process that enables the quantum state of an object to be transferred from one place to a distant one without ever existing anywhere in between. The quantum teleportation channel is nothing like an ordinary channel: it follows no path in space, but consists of entangled particles. "

https://arxiv.org/abs/quant-ph/0609135
Non-local generation of entanglement of photons which do not meet each other (2006)
Note that in this one, there are just 2 photons that are entangled without meeting each other - there is no swap operation at all.

https://arxiv.org/abs/1209.4191
Entanglement Between Photons that have Never Coexisted (2012)
Photons 1 & 4 are entangled even though they never coexisted - proving they could not have interacted.

And I can cite as many more as it takes.
 
  • Like
Likes Lord Jestocost
  • #23
PeterDonis said:
1. And if I'm reading things right, the post selection requires making a destructive measurement on photons 0 and 3, so they no longer exist. I don't see where there is any way of using this to produce an entangled pair of photons 0 and 3 that exist after the whole experiment is done and can be used in further experiments.

2. None of your cites appear to contradict what I said above about a destructive measurement being required. If I am mistaken, please explain why. Please note that I am not disputing any of the experimental results. I am only trying to understand whether my statement about destructive measurements being required is correct. If it isn't, I would like specific cites that explain why not.

1. You are reading incorrectly. 1 & 2 are destructively measured by the BSA, leaving 0 & 3 entangled. You are then free to perform any measurement you like on 0 & 3 to show they are in fact entangled. And I also provided a citation for electrons ending up entangled that have never interacted in the past.

2. PeterDonis: you should provide a citation to support what you say. It is not on me to disprove it when I have provided direct quotes from reference after reference that exactly match what I say. Specifically tell me what you disagree with below:

"Quantum teleportation is a process that enables the quantum state of an object to be transferred from one place to a distant one without ever existing anywhere in between. "

"However remarkable it is that quantum interactions can establish such nonlocal correlations, it is even more remarkable that particles that never directly interacted can also become nonlocally correlated. This is possible through a process called entanglement swapping [2]. Starting from two independent pairs of entangled particles, one can measure jointly one particle from each pair, so that the two other particles become entangled, even though they have no common past history. The resulting pair is a genuine entangled pair in every aspect, and can in particular violate Bell inequalities. "

In fact just read the titles of my references, they say it all.

I know it is poor form to say this, but seriously: where is your citation saying anything remotely like what you say? You really should dig deeper in this, as you will discover that you are far out of sync with the community on these points. I recognize implicitly your vast knowledge (far exceeding mine) and that you are a mentor. But I am shocked that my repeated requests for references from you are dismissed in thread after thread. My references are from a few of the best minds in the field, and none of them are close to your position.

To the OP's question: there is no requirement that particles must interact, collide or otherwise have causal communication to become entangled, and that is true for electrons as well as photons.
 
  • Like
Likes Lord Jestocost
  • #24
PeterDonis said:
None of your cites appear to contradict what I said above about a destructive measurement being required. If I am mistaken, please explain why. Please note that I am not disputing any of the experimental results. I am only trying to understand whether my statement about destructive measurements being required is correct. If it isn't, I would like specific cites that explain why not.

Your requirement about the destructive measurements does not in any way invalidate (or even apply) to my position. But regardless, here is a version where entangled photons are created without interacting and no destructive measurement occurs.

https://arxiv.org/abs/quant-ph/0609135
Non-local generation of entanglement of photons which do not meet each other (2006)
Note that in this one, there are just 2 photons that are entangled without meeting each other - there is no swap operation at all.
 
  • #25
DrChinese said:
You are reading incorrectly. 1 & 2 are destructively measured by the BSA, leaving 0 & 3 entangled.

So in other words, in Fig. 1 of the paper linked to in post #9, we could leave out Bob's polarization analyzers altogether, and use the apparatus as a source of entangled photon pairs (each 0 & 3 pair), with the specific state of each pair being determined by the recorded result of the Alice measurement for the corresponding photons 1 & 2?

DrChinese said:
Specifically tell me what you disagree with below:

I already said I'm not disputing any of the experimental results. I'm talking about a scenario which has not been realized in any of the experiments (a scenario where, for example, we don't destructively measure photons 0 & 3 in the setup in Fig. 1 of the paper linked to in post #9), and trying to understand what we can say about that scenario based on what we know from the experiments.

Also, you keep giving quotes in ordinary language, but we are talking about QM here, which is very, very difficult to describe properly in ordinary language, and ordinary language descriptions often use words in ambiguous ways which are over-hyped as meaning things they don't actually mean when you look at the math. For example, here's the very last sentence at the end of the paper linked to in post #9:

"Alice’s measurement projects photon 0 and 3 in an entangled state, at a time after they have already been registered."

If I had a dollar for every thread here on PF that has been based on confusion among lay people caused by such language, I'd be retired on my island in the Caribbean by now.

DrChinese said:
You really should dig deeper in this, as you will discover that you are far out of sync with the community on these points.

It's quite possible that I am. If I can get a yes or no answer to the question I asked at the top of this post, that will help.
 
  • #26
PeterDonis said:
So in other words, in Fig. 1 of the paper linked to in post #9, we could leave out Bob's polarization analyzers altogether, and use the apparatus as a source of entangled photon pairs (each 0 & 3 pair), with the specific state of each pair being determined by the recorded result of the Alice measurement for the corresponding photons 1 & 2?

To refine the question somewhat (while still being a yes/no question), can the following type of apparatus be built?

A box with a button and four pairs of output ports, corresponding to the four different Bell states that photons 0 & 3 can be projected into. Every time the button is pressed, a pair of photons comes out one of the four pairs of output ports. It can't be predicted in advance which pair of ports for each button press, but the photons that come out of a given pair of ports will satisfy all of the properties of the corresponding Bell state. In other words, we can't predict in advance which state a pair of photons we get on a button press will be in, but once we see which pair of ports the photons come out of, we know which state they're in.
 
  • #27
PeterDonis said:
So in other words, in Fig. 1 of the paper linked to in post #9, we could leave out Bob's polarization analyzers altogether, and use the apparatus as a source of entangled photon pairs (each 0 & 3 pair), with the specific state of each pair being determined by the recorded result of the Alice measurement for the corresponding photons 1 & 2?

I already said I'm not disputing any of the experimental results. I'm talking about a scenario which has not been realized in any of the experiments (a scenario where, for example, we don't destructively measure photons 0 & 3 in the setup in Fig. 1 of the paper linked to in post #9), and trying to understand what we can say about that scenario based on what we know from the experiments.

...

It's quite possible that I am. If I can get a yes or no answer to the question I asked at the top of this post, that will help.

If 0 & 3 are not measured, they are simply entangled with each other. They could be sent elsewhere (still nonlocal to each other). They could have any entangled basis measured. Or they could even have their entangled state stored for a while. Nevertheless, they were never in causal contact with each other.

If you are asking about experiments that have NOT been performed: honestly, that is nothing I can do anything about. I am not an experimental scientist. If you have a criticism of Zeilinger's work, Gisin's work, Pan's work, Weihs' work, etc you should take it up with them. Please don't pick at me when I am fairly presenting the essence of their work as should be clear from my repeated references and direct quotes. Again, and with respect to you: it is totally unfair to use your "authority" to criticize their work here using me as a surrogate. It has been peer reviewed and meets all forum standards.

And yes, it is true that English does not always do justice to descriptions of QM which are better presented in the language of the math. Of course, all of these references provide that - along with the authors' English summary. So once again, I will state as follows in plain English (not my words, but identical in my meaning):

"Quantum teleportation [entanglement swapping] is a process that enables the quantum state of an object to be transferred from one place to a distant one without ever existing anywhere in between. particles that never directly interacted can [...] become nonlocally correlated. This is possible through a process called entanglement swapping. Starting from two independent pairs of entangled particles, one can measure jointly one particle from each pair, so that the two other particles become entangled, even though they have no common past history. The resulting pair is a genuine entangled pair in every aspect, and can in particular violate Bell inequalities."

And further, multiple experiments demonstrate "non-local generation of entanglement of photons which do not meet each other" exactly refuting the question of the OP. There does not need to be a local starting point for entanglement. Instead, you must look at the entire setup and the quantum system it creates. Should be no surprise there. And making comments about "post selection" as some kind of theoretical deficiency is far off the mark, and should be taken up with the authors of the references if you really feel it is an issue.
 
  • #28
PeterDonis said:
To refine the question somewhat (while still being a yes/no question), can the following type of apparatus be built?

A box with a button and four pairs of output ports, corresponding to the four different Bell states that photons 0 & 3 can be projected into. Every time the button is pressed, a pair of photons comes out one of the four pairs of output ports. It can't be predicted in advance which pair of ports for each button press, but the photons that come out of a given pair of ports will satisfy all of the properties of the corresponding Bell state. In other words, we can't predict in advance which state a pair of photons we get on a button press will be in, but once we see which pair of ports the photons come out of, we know which state they're in.

Yes, of course (and we are using the "ports" just as a loose idea, since 0 & 3 are gone when this happens). And there is a fifth output port that is the most common encountered: no Bell state achieved at all.
 
  • #29
DrChinese said:
If 0 & 3 are not measured, they are simply entangled with each other. They could be sent elsewhere (still nonlocal to each other).

Ok, so this would be a "yes" answer to my question. In that case, yes, you're right, I need to re-evaluate my understanding. I'll take more time to read the references you provided when I can (and probably follow up with further papers referenced in those).

DrChinese said:
there is a fifth output port that is the most common encountered: no Bell state achieved at all.

Would this mean the photons aren't entangled? Or just that nothing comes out at all?
 
  • #30
DrChinese said:
If you are asking about experiments that have NOT been performed: honestly, that is nothing I can do anything about. I am not an experimental scientist.

Of course not. But you answered my question about a hypothetical device that nobody has actually built just fine. That's all I was looking for.

DrChinese said:
it is totally unfair to use your "authority" to criticize their work here using me as a surrogate.

I'm going to use some emphasis here: I am not doing that.

I am simply not doing the things you think I am doing. I am just trying to get a better understanding of what these experiments are actually telling me.

DrChinese said:
I will state as follows in plain English

Here's the problem with this "plain English": I can't be sure that the words mean what they would mean in ordinary, non-scientific conversation in English. Indeed, some of the words wouldn't mean anything in that context, since they're technical terms that reference a highly complex body of theoretical and experimental understanding.

That's why I'm trying to phrase questions in terms of things like what would happen when you pushed the button on the device I described.
 
  • #31
Moderator's note: I have deleted some posts (of mine) that are incorrect or misleading in the light of further discussion.
 
  • Like
Likes Dale
  • #32
PeterDonis said:
Would this mean the photons aren't entangled? Or just that nothing comes out at all?

Sometimes there is only 1 photon (in the 0 & 3 channels) rather than two within the required time window. So there is one in the 0 channel but none in the 3 channel, or vice versa. That is because you are actually looking at the 1 & 2 channels, and one of those is missing a photon within the time window. There must be both a 1 & 2 that are indistinguishable for the 0 & 3 to be primed and ready. Keep in mind that the 0 & 1 is one source, 2 & 3 is a separate source, and there is no requirement that they create pairs at exactly the same time. Only occasionally does that happen, and in fact most of the time there is nothing present.
 
  • #33
DrChinese said:
Just to add to what others have already said: You can entangled objects that have never been in contact. One way to do it is called "entanglement swapping".

https://arxiv.org/abs/quant-ph/0201134
But their partners interacted locally widening the entanglements to the other 2 co-entangled partners nonlocally. How is this unexpected? Everything is entangled all the time.
 
  • #34
Isn't ubiquitous entanglement breaking the only explanation why we observe a seemingly 'classical' reality arise from an infinitely entangled quantum world upon observation/interaction?
 
  • #35
DaveC426913 said:
I wonder if the OP means time symmetrical.

Can you (in principle) reverse the process, and have a pair of unentangled particles at a distance become entangled?
The Schrodinger equation contains such solutions, so in principle yes.
 
<h2>1. What is entanglement and why is it important in quantum mechanics?</h2><p>Entanglement is a phenomenon in quantum mechanics where two or more particles become connected in such a way that the state of one particle cannot be fully described without considering the state of the other particles. This is important because it challenges our classical understanding of how particles behave and has implications for technologies such as quantum computing.</p><h2>2. How does entanglement relate to the concept of symmetry in quantum mechanics?</h2><p>In the no-hidden variables explanation of entanglement, it is believed that the entangled particles do not have individual states but rather share a single state. This lack of individual states violates the principle of symmetry in quantum mechanics, where all particles are assumed to have their own unique states and properties.</p><h2>3. What are "hidden variables" and why do they not fully explain entanglement?</h2><p>Hidden variables refer to the idea that there may be unknown or unobserved properties of particles that can fully explain their behavior. However, in the case of entanglement, it has been shown that no hidden variables can fully explain the observed correlations between entangled particles. This suggests that there are inherent limitations to our understanding of quantum mechanics.</p><h2>4. Can entanglement be used for communication or teleportation?</h2><p>Entanglement cannot be used for communication as the state of one particle cannot be directly determined by observing the state of the other particle. However, it can be used for teleportation in the sense that the state of one particle can be transferred to another particle instantaneously, without physically moving the particle itself.</p><h2>5. How is entanglement being studied and applied in the field of quantum computing?</h2><p>Entanglement is a crucial component in quantum computing, as it allows for the creation of qubits (quantum bits) that can hold multiple states simultaneously. This enables quantum computers to perform certain calculations much faster and more efficiently than classical computers. Researchers are currently studying ways to create and control entangled qubits for practical applications in quantum computing.</p>

1. What is entanglement and why is it important in quantum mechanics?

Entanglement is a phenomenon in quantum mechanics where two or more particles become connected in such a way that the state of one particle cannot be fully described without considering the state of the other particles. This is important because it challenges our classical understanding of how particles behave and has implications for technologies such as quantum computing.

2. How does entanglement relate to the concept of symmetry in quantum mechanics?

In the no-hidden variables explanation of entanglement, it is believed that the entangled particles do not have individual states but rather share a single state. This lack of individual states violates the principle of symmetry in quantum mechanics, where all particles are assumed to have their own unique states and properties.

3. What are "hidden variables" and why do they not fully explain entanglement?

Hidden variables refer to the idea that there may be unknown or unobserved properties of particles that can fully explain their behavior. However, in the case of entanglement, it has been shown that no hidden variables can fully explain the observed correlations between entangled particles. This suggests that there are inherent limitations to our understanding of quantum mechanics.

4. Can entanglement be used for communication or teleportation?

Entanglement cannot be used for communication as the state of one particle cannot be directly determined by observing the state of the other particle. However, it can be used for teleportation in the sense that the state of one particle can be transferred to another particle instantaneously, without physically moving the particle itself.

5. How is entanglement being studied and applied in the field of quantum computing?

Entanglement is a crucial component in quantum computing, as it allows for the creation of qubits (quantum bits) that can hold multiple states simultaneously. This enables quantum computers to perform certain calculations much faster and more efficiently than classical computers. Researchers are currently studying ways to create and control entangled qubits for practical applications in quantum computing.

Similar threads

  • Quantum Physics
Replies
7
Views
962
Replies
7
Views
986
Replies
25
Views
2K
Replies
80
Views
3K
Replies
41
Views
2K
  • Quantum Physics
3
Replies
78
Views
7K
Replies
19
Views
2K
  • Quantum Interpretations and Foundations
2
Replies
45
Views
3K
Replies
18
Views
2K
  • Quantum Physics
Replies
12
Views
2K
Back
Top