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Another question about entangled pairs of particles


geordief
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Suppose we have such a pair(A and B) and place detectors (not sure if more than one detector is possible or of use) along the path of particle A  but do not do so along the path of particle B

 

Is there any way to determine at which point (or between which points) particle B has any interaction?

 

Alternatively, is there any way at all it is possible to state  that there has not been any interaction (and so the particles are still entangled)? 

 

As a related  (perhaps almost identical)  question , is there anything at all we can say(or infer?)  about the state of a particle in between observations? (I assume the answer must be "No")

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5 minutes ago, geordief said:

Suppose we have such a pair(A and B) and place detectors (not sure if more than one detector is possible or of use) along the path of particle A  but do not do so along the path of particle B

 

Is there any way to determine at which point (or between which points) particle B has any interaction?

 

Alternatively, is there any way at all it is possible to state  that there has not been any interaction (and so the particles are still entangled)? 

 

As a related  (perhaps almost identical)  question , is there anything at all we can say(or infer?)  about the state of a particle in between observations? (I assume the answer must be "No")

 

The answers you get from QM are often already rather vague.

As a consequence you need to be a lot more specific with any questions you ask.

 

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1 minute ago, studiot said:

 

The answers you get from QM are often already rather vague.

As a consequence you need to be a lot more specific with any questions you ask.

 

How more specific?I don't see immediately how to be more specific.

 

I assume you don't mean I should specify the type of particle and the particular state do you?(I had photon ×spin in mind  even if I can"t claim to understand  that phenomenon)

 

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Any one particle does not set a flag, or any other indicator, to let an observer know it is entangled with some other distant partner.
You have no way of knowing if any particle you choose to observe/interact with is part of an entangled group, or not.
You can only be sure if you produce the entangled pair.

I'm not sure if that answers your question; like Studiot, I'm unsure what exactly you are asking.

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4 minutes ago, MigL said:

Any one particle does not set a flag, or any other indicator, to let an observer know it is entangled with some other distant partner.
You have no way of knowing if any particle you choose to observe/interact with is part of an entangled group, or not.
You can only be sure if you produce the entangled pair.

I'm not sure if that answers your question; like Studiot, I'm unsure what exactly you are asking.

It is not possible to produce pairs of entangled particles "to order"?

The same time and location  of source does not guarantee that?

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1 hour ago, geordief said:

Suppose we have such a pair(A and B) and place detectors (not sure if more than one detector is possible or of use) along the path of particle A  but do not do so along the path of particle B

 

Is there any way to determine at which point (or between which points) particle B has any interaction?

 

Alternatively, is there any way at all it is possible to state  that there has not been any interaction (and so the particles are still entangled)? 

 

As a related  (perhaps almost identical)  question , is there anything at all we can say(or infer?)  about the state of a particle in between observations? (I assume the answer must be "No")

What you're suggesting is measuring without interaction or measuring counterfactuals. It's better to deal with it for one particle in the double-slit experiment.

You place a detector intercepting particles only in one of the branching paths.

You fire your particles one by one and observe where they land in a faraway screen.

You gather only the results that didn't do "click" at the detector.

You do the statistics of all of those that didn't make the detector click.

What's observed is that decoherence is broken. Counterfactual measurements, or interaction-free measurements --as they're also called-- break decoherence.

This effect is so real that there is a bomb tester to know if a bomb would go off without actually making it go off:

https://en.wikipedia.org/wiki/Elitzur–Vaidman_bomb_tester

Sorry for re-directing your question, but I think the essential aspect that you want to understand is contained here more simply than considering entanglement.

I hope that has to do with your question at least.

Look up for "interaction-free measurement," or perhaps, "counterfactual measurement."

Also:

https://en.wikipedia.org/wiki/Renninger_negative-result_experiment

Edited by joigus
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1 hour ago, geordief said:

As a related  (perhaps almost identical)  question , is there anything at all we can say(or infer?)  about the state of a particle in between observations? (I assume the answer must be "No")

The answer is “yes”

If you measure the particle states and they do not have the expected correlation, you know there had been a prior interaction. This is the idea behind quantum key encryption.

If there was no prior interaction, you can infer that the state is undetermined.

20 minutes ago, geordief said:

It is not possible to produce pairs of entangled particles "to order"?

The same time and location  of source does not guarantee that?

You can choose the correlation. e.g. photons with the same polarization state or orthogonal polarization states.

You can’t choose the state of one particle, since the individual states are undetermined.

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39 minutes ago, swansont said:

The answer is “yes”

If you measure the particle states and they do not have the expected correlation, you know there had been a prior interaction. This is the idea behind quantum key encryption.

If there was no prior interaction, you can infer that the state is undetermined.

Thanks.I also deduce from your answer that one can  indeed  produce  identifiable pairs** of entangled particles "to order"

** ie we know they are an entangled pair 

Edited by geordief
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4 minutes ago, geordief said:

Thanks.I also deduce from your answer that one can  indeed  produce individually identifiable pairs of entangled particles "to order"

Using “individual” in this way is ambiguous at best. Individual particle? No. I said they are in an undetermined state. You can only choose the correlation.

Individual pair? Why not just say pair?

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48 minutes ago, joigus said:

What you're suggesting is measuring without interaction or measuring counterfactuals. It's better to deal with it for one particle in the double-slit experiment.

You place a detector intercepting particles only in one of the branching paths.

You fire your particles one by one and observe where they land in a faraway screen.

You gather only the results that didn't do "click" at the detector.

You do the statistics of all of those that didn't make the detector click.

What's observed is that decoherence is broken. Counterfactual measurements, or interaction-free measurements --as they're also called-- break decoherence.

This effect is so real that there is a bomb tester to know if a bomb would go off without actually making it go off:

https://en.wikipedia.org/wiki/Elitzur–Vaidman_bomb_tester

Sorry for re-directing your question, but I think the essential aspect that you want to understand is contained here more simply than considering entanglement.

I hope that has to do with your question at least.

Look up for "interaction-free measurement," or perhaps, "counterfactual measurement."

Also:

https://en.wikipedia.org/wiki/Renninger_negative-result_experiment*

Thanks.I will take a look.I think Swansont's  brief allusion to quantum key encryption  probably  contains the answer to my question 

 

18 minutes ago, swansont said:

Using “individual” in this way is ambiguous at best. Individual particle? No. I said they are in an undetermined state. You can only choose the correlation.

Individual pair? Why not just say pair?

18 minutes ago, swansont said:

Using “individual” in this way is ambiguous at best. Individual particle? No. I said they are in an undetermined state. You can only choose the correlation.

Individual pair? Why not just say pair?

Sorry,that is what I meant.....is "identifiable  pair" (identifiable as a pair)

 

I will edit  that post

Edited by geordief
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2 hours ago, joigus said:

What you're suggesting is measuring without interaction or measuring counterfactuals. It's better to deal with it for one particle in the double-slit experiment.

You place a detector intercepting particles only in one of the branching paths.

You fire your particles one by one and observe where they land in a faraway screen.

You gather only the results that didn't do "click" at the detector.

You do the statistics of all of those that didn't make the detector click.

What's observed is that decoherence is broken. Counterfactual measurements, or interaction-free measurements --as they're also called-- break decoherence.

This effect is so real that there is a bomb tester to know if a bomb would go off without actually making it go off:

https://en.wikipedia.org/wiki/Elitzur–Vaidman_bomb_tester

Sorry for re-directing your question, but I think the essential aspect that you want to understand is contained here more simply than considering entanglement.

I hope that has to do with your question at least.

Look up for "interaction-free measurement," or perhaps, "counterfactual measurement."

Also:

https://en.wikipedia.org/wiki/Renninger_negative-result_experiment

Very interesting +1

 

4 hours ago, geordief said:

How more specific?I don't see immediately how to be more specific.

 

I assume you don't mean I should specify the type of particle and the particular state do you?(I had photon ×spin in mind  even if I can"t claim to understand  that phenomenon)

 

Firstly the question of scale.

How long are these paths relative to the size of the particles concerned and thier immediated entangled environment ?

Note real world 'detectors' tend to be many many orders of magnitude larger than the particles themselves.

Secondly what do the detectors detect ?

Just the presence of a particle ?

How do you know it is entangled and what it is entangled with ?

What about random interfering particles ?

and yes all this would become much clearer if you told us what particles you are referring to.

For instance If you are talking about photons (or electrons) and spin if you know that it is spin up, how did you fnd this out without interacting with it ?

(Entanglement for photons is usually about polarisation)

 

These are samples of what I mean by being more specific.

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4 hours ago, joigus said:

break decoherence.

Sorry, "break coherence." I mix up these opposites from time to time. 

Yes, I prefer Swansont's answer too. He's as brief as surgically precise. There are many instances in which you learn something about a quantum particle long before you do anything else with it. That's my understanding of filtering, for example. Measurement is not necessarily interaction. But some interaction is necessary at some point, of course, as Studiot just pointed out. Otherwise, as he said, how are you to tell anything about it?

Sorry for not having addressed your specific concerns about entanglement. I've been talking about entanglement for like a month.

I also have a feeling that some people see in entanglement something that's not involved in the principles of QM themselves, some new law, some extra magic. There isn't. All the mystery is already in the double-slit experiment already, or the "paradox" of partial reflection, etc.

I also totally concur with MigL's comments:

5 hours ago, MigL said:

Any one particle does not set a flag, or any other indicator, to let an observer know it is entangled with some other distant partner.
You have no way of knowing if any particle you choose to observe/interact with is part of an entangled group, or not.
You can only be sure if you produce the entangled pair.

 

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8 hours ago, MigL said:

Any one particle does not set a flag, or any other indicator, to let an observer know it is entangled with some other distant partner.
You have no way of knowing if any particle you choose to observe/interact with is part of an entangled group, or not.
You can only be sure if you produce the entangled pair.

I'm not sure if that answers your question; like Studiot, I'm unsure what exactly you are asking.

Trying to cross my t's, don't we know they are an entangled pair if they are produced simultaneously at the same location?

And ,once we know that can we track their progress?

(I am not familiar with the actual process whereby a pair of entangled  particles are created but I assumed it was because they arose out of a common interaction.Is it not inevitable that such pairs are entangled or are some  entangled and some not?)

Edited by geordief
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7 minutes ago, geordief said:

Trying to cross my t's, don't we know they are an entangled pair if they are produced simultaneously at the same location?

And ,once we know that can we track their progress?

(I am not familiar with the actual process whereby a pair of entangled  particles are created but I assumed it was because they arose out of a common interaction.Is it not inevitable that such pairs are entangled or are some  entangled and some not?)

To entangle two particles they have to have some (common) property that can only take two different values, but possess the same energy.

In a bonding orbital you have two electrons that have this property since they must have opposite spins, due to the Pauli exclusion principle.

Conservations laws, eg momentum may provide a pair of entangled particles in the right circumstances such as pair creation.

But you can't just take any old pair of particles that happen to be close and say they are or will become entangled, for example adjacent electrons in the cathode ray stream impinging on your cathode ray tube are not entangled.

 

Does this help ?

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5 hours ago, studiot said:

Well this is what was in my mind when I asked the question

 

"Firstly the question of scale.

How long are these paths relative to the size of the particles concerned and thier immediated entangled environment ?"

Just as long as practically possible

 

"Note real world 'detectors' tend to be many many orders of magnitude larger than the particles themselves.

Secondly what do the detectors detect ?

Just the presence of a particle ?"

Its properties(I don't know the mechanics of this)

 

"How do you know it is entangled and what it is entangled with ?"

Think I have discussed this in my last reply to MigL

5 hours ago, studiot said:

"What about random interfering particles ?"

Hadn't occured to me

"and yes all this would become much clearer if you told us what particles you are referring to."

Well I suppose I had photons in mind

"For instance If you are talking about photons (or electrons) and spin if you know that it is spin up, how did you fnd this out without interacting with it ?

(Entanglement for photons is usually about polarisation)"

Yes I see that.You can't know anything except (I think) that it is one half of an entangled pair

I see you have replied  but will finish this post  first  before looking at it

5 hours ago, studiot said:

 

These are samples of what I mean by being more specific.

 

9 minutes ago, studiot said:

To entangle two particles they have to have some (common) property that can only take two different values, but possess the same energy.

In a bonding orbital you have two electrons that have this property since they must have opposite spins, due to the Pauli exclusion principle.

Conservations laws, eg momentum may provide a pair of entangled particles in the right circumstances such as pair creation.

But you can't just take any old pair of particles that happen to be close and say they are or will become entangled, for example adjacent electrons in the cathode ray stream impinging on your cathode ray tube are not entangled.

 

Does this help ?

Yes thanks.I am completely ignorant of the actual mechanism of entanglement  generation.

I will have to bone up kn that to some extent at least  in the  next while.

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1 hour ago, studiot said:

To entangle two particles they have to have some (common) property that can only take two different values, but possess the same energy.

In a bonding orbital you have two electrons that have this property since they must have opposite spins, due to the Pauli exclusion principle.

Same energy is not a requirement. Entangled photons can be different colors as long as the entangled parameter is not tied to one energy e.g. if vertical polarization has the higher energy, it won’t be entangled with a photon having horizontal polarization, since you can tell the photons apart.

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8 hours ago, swansont said:

Same energy is not a requirement. Entangled photons can be different colors as long as the entangled parameter is not tied to one energy e.g. if vertical polarization has the higher energy, it won’t be entangled with a photon having horizontal polarization, since you can tell the photons apart.

Thank you for that correction, can you elaborate please ? +1

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2 hours ago, studiot said:

Thank you for that correction, can you elaborate please ? +1

Sorry; I just recall reading about entangling different color photons. If you can arrange it so that energy doesn’t identify the entangled property, or that the energy otherwise doesn’t matter, such as energy-time entanglement, which depends on the photons being created at the same time.

“Each of the photons is directed into its own unbalanced Mach-Zehnder interferometer (see figure 3), giving it a long path (L)and a short path (S)to the detectors. Because the path length difference is much longer than the coherence length of the photons, no interference is observed in the single rates at either of the detectors when the phase in, say, one of the long paths is changed. However, there is interference in the coincidence rate between detectors. The reason is that there are two processes that could lead to such a coincidence count-both photons could have taken their respective long paths or both could have taken their respective short paths”

http://research.physics.illinois.edu/QI/Photonics/papers/My Collection.Data/PDF/Hyper-Entangled States.pdf

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If Studiot and I each pull a coin out of our pockets and palm them, there is a possibility that we pull out heads and tails.
That doesn't mean there is an'entanglement' or anti correlation; but if you repeat the process many many times, with the same result, there could be an anti-correlation.

Similarly with any two random quantum particles;  the two particles may show opposing states, but that doesn't mean there is an entanglement correlation.
Further, you have no idea whether the entanglement was lost along the way to your observations, due to some interaction.
And particles are indistinguishable, so there is no way to tell if a single particle has an entangled partner, or not.

The only way to be sure is to produce the entangled pairs yourself.

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20 hours ago, swansont said:

The reason is that there are two processes that could lead to such a coincidence count-both photons could have taken their respective long paths or both could have taken their respective short paths”

Aha, two processes.  Thank you, +1 I will read further.

 

9 hours ago, MigL said:

If Studiot and I each pull a coin out of our pockets and palm them, there is a possibility that we pull out heads and tails.
That doesn't mean there is an'entanglement' or anti correlation; but if you repeat the process many many times, with the same result, there could be an anti-correlation.

Similarly with any two random quantum particles;  the two particles may show opposing states, but that doesn't mean there is an entanglement correlation.
Further, you have no idea whether the entanglement was lost along the way to your observations, due to some interaction.
And particles are indistinguishable, so there is no way to tell if a single particle has an entangled partner, or not.

The only way to be sure is to produce the entangled pairs yourself.

Agreed and this is why I said that prior information is also required by the observer.

In your example this would mean there must also be some law akin to Pauli  acting,  stating that although either of us could pull out a coin at random, the other must then be opposite. This would correspond to the events not being statistically truly independent.

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