How do you quantumly entangle particles?

[tinypositrons]

Hi physicists,

I understand that the particles that were there at the big bang are all quantumly entangled, but if I create a new particle (let's call it 'x') it obviously won't be quantumly entangled, right? So if I create another particle ('y'), how can I make 'x' and 'y' quantumly entangled? Secondly, do 'x' and 'y' have to be the same type of particle (both electrons for example), or can you quantumly entangle an electron and an up-quark?

Thanks,
Joe

 

[jedishrfu]

one way is through pair production where a particle interaction results in two particles splitting off but remain entangled.

http://en.wikipedia.org/wiki/Quantum_entanglement

 

[tinypositrons]

So they become quantumly entangled through specific particle interaction, but what kind of reaction? What specifically happens that makes theses two particles quantumly entangled? Secondly, with regard to the last question in my original post, do two particles have to be the same type of particle to be able to become quantumly entangled?

Thanks,
Joe

 

[jedishrfu]

from every example I've seen the particles are identical but with different spin.

 

[San K]

So they become quantumly entangled through specific particle interaction, but what kind of reaction?

some resource that deals with the fundamentalness of the Law of conservation of momentum.

 

[tinypositrons]

Thank you. But with regards to jedishrfu's comment on the fact that the particles have to be identical except for spin. Is this true? I only question this because I spoke with one of my acquantances today and he seemed to believe that they had to be completely the same, including the spin. Clarification on this would be brilliant.

Thanks,
Joe

 

[cabrera]

Hi tinypositron,

First of all, don't take me for granted. Your questions are valid and difficult to answer. I am an 'aficionado' but I would tell you what my understanding is.

Two particles became entangled when a single wave function describe a physical state of both of them ,i.e spin. But It can also be momentum. The important thing is that such quantity is part of the two particles at the same time before the measurement. It could happens ,for instance, in a decay of an atom where the magnetic moment is zero before the reaction and after a reaction/decay only two electrons are emitted. Electrons have spin, but in order to keep the total magnetic momentum equal to zero, both are opposite.

When you measure the spin of one of them (up) you'd know inmediatly that the other is down. But remember the spin before the measurement was not up or down. Your measurement created that state.

The wave function of the two particles is ψ=1/√2(|+1>|-2>+|+2>|-1>). After the measurement it will collapse to one of the sates

 

[f95toli]

Thank you. But with regards to jedishrfu's comment on the fact that the particles have to be identical except for spin. Is this true?

You can entangle completterly different types of systems. "Obvious" examles would be entanglement of electronic and nuclear degrees of freedom. However, if you want an extreme example you can always look up experiments where an "artifical atom" (a qubit, basically an electronic circuit element) is entangled with a microscopic system such as a molecule.

 

[DrChinese]

Thank you. But with regards to jedishrfu's comment on the fact that the particles have to be identical except for spin. Is this true? I only question this because I spoke with one of my acquantances today and he seemed to believe that they had to be completely the same, including the spin. Clarification on this would be brilliant.

Thanks,
Joe

There is no requirement that they be identical nor that they have opposite spin. There are always conservation rules at work, in which the system has some total observable or quantity in play and two or more possible configurations in which that observable can be expressed. There is no theoretical upper limit to the number of particles which can be entangled. However, when more than 2 are involved, the entangled nature can become quite complicated to follow.

The most common experimental method of entangling particles is through what is called SPDC or just PDC : Spontaneous Parametric Down Conversion. A laser photon source goes through a non-linear crystal, and an entangled pair of photons is produced. I can provide a reference on PDC if that helps.

 

[vanhees71]

A source on PDC would be great, particularly one that explains also the theory behind, how the entangled photon pairs are produced. I've never been able to find a clear explanation for this at all. Of course, one can live with the fact that the entanglement is experimentally proven to an overwhelming degree of accuracy, but I think it would be great to understand this more from the theoretical side too!

 

[friend]

Is it true that a measurement on one particle of a two particle entangled system collapses the wave function of the two particle system so that you know what the other particle would measure?

 

[DrChinese]

Is it true that a measurement on one particle of a two particle entangled system collapses the wave function of the two particle system so that you know what the other particle would measure?

That is correct.

 

[DrChinese]

A couple of sources on PDC, keep in mind that all of these assume you know certain things and present from that perspective:

Type II PDC (1 crystal):
http://www.ino.it/~azavatta/References/JMO48p1997.pdf

Type I PDC (2 crystals required):
http://arxiv.org/abs/quant-ph/0205171

I would also look at Wiki and anything else you can find, because it is a little confusing sometimes. After you read a few, you get a better idea of what is going on.

 

[friend]

That is correct.

and since we cannot force one of those entangled particles to collapse in a desired way, we cannot force the other particle to measure in a particular way, so we cannot send messages FTL, right?

 

[DrChinese]

and since we cannot force one of those entangled particles to collapse in a desired way, we cannot force the other particle to measure in a particular way, so we cannot send messages FTL, right?

That too is correct! It is axiomatic that unless the value was already known for a particular observable, a specific value from a measurement cannot be made to occur. Specific values will each have a probability of being seen.

 

[friend]

That too is correct! It is axiomatic that unless the value was already known for a particular observable, a specific value from a measurement cannot be made to occur. Specific values will each have a probability of being seen.

So as soon as you make a measurement on one of the two entangled particles, you collapse the wavefunction and lose the entanglement associated with that wavefunction, right?

They can no longer be entangled until they interact with each other, right?

Does all interaction necessarily entangle?

 

[ftr]

Hi physicists,

I understand that the particles that were there at the big bang are all quantumly entangled, but if I create a new particle (let's call it 'x') it obviously won't be quantumly entangled, right? So if I create another particle ('y'), how can I make 'x' and 'y' quantumly entangled? Secondly, do 'x' and 'y' have to be the same type of particle (both electrons for example), or can you quantumly entangle an electron and an up-quark?

Thanks,
Joe

you can check out this paper


Gravity as Quantum Entanglement Force

http://arxiv.org/pdf/1002.4568v1.pdf

 

[DrChinese]

you can check out this paper


Gravity as Quantum Entanglement Force

...

Sorry, I don't consider this a suitable reference for this thread. This is an extremely speculative paper. And it doesn't really have anything to do with the OP.

 

[ChiralWaltz]

Sorry guys,
I laughed really hard over the title. I really liked it, no offense but I never had heard of it. General Physics I was the last class I took. Trying to understand what I read about Spontaneous parametric down-conversion wiki article.

1) A beam is projected through a BBO crystal
2) A Type II SPDC dual cone projection is observed.
3) Math that occurs within the BBO crystal and the projected image is what determines that quantum entanglement occurs?

I'm trying to understand how the experiment works. Something like:
a) Shine beam through crystal
b) Observes/Measure shapes

Please don't write me a long, complicated response. I'm trying to understand the wiki article on an elementary level. Just say no if it isn't possible. I won't be offended.

 

[jedishrfu]

Sorry guys,
I laughed really hard over the title. I really liked it, no offense but I never had heard of it. General Physics I was the last class I took. Trying to understand what I read about Spontaneous parametric down-conversion wiki article.

1) A beam is projected through a BBO crystal
2) A Type II SPDC dual cone projection is observed.
3) Math that occurs within the BBO crystal and the projected image is what determines that quantum entanglement occurs?

I'm trying to understand how the experiment works. Something like:
a) Shine beam through crystal
b) Observes/Measure shapes

Please don't write me a long, complicated response. I'm trying to understand the wiki article on an elementary level. Just say no if it isn't possible. I won't be offended.

I think you menat to place this in its own thread, right?

 

[DrChinese]

Sorry guys,
I laughed really hard over the title. I really liked it, no offense but I never had heard of it. General Physics I was the last class I took. Trying to understand what I read about Spontaneous parametric down-conversion wiki article.

1) A beam is projected through a BBO crystal
2) A Type II SPDC dual cone projection is observed.
3) Math that occurs within the BBO crystal and the projected image is what determines that quantum entanglement occurs?

I'm trying to understand how the experiment works. Something like:
a) Shine beam through crystal
b) Observes/Measure shapes

Please don't write me a long, complicated response. I'm trying to understand the wiki article on an elementary level. Just say no if it isn't possible. I won't be offended.

I don't see the question. What is it that you don't understand in the process?

 

[ChiralWaltz]

I don't see the question. What is it that you don't understand in the process?

The question is, the way I'm repeating SPDC, is it correct?

 

[DrChinese]

The question is, the way I'm repeating SPDC, is it correct?

Sure, seems fine. Areas in which there is overlap means you cannot distinguish the signal from the idler, so they are entangled on the polarization basis.

 

[ChiralWaltz]

Sure, seems fine. Areas in which there is overlap means you cannot distinguish the signal from the idler, so they are entangled on the polarization basis.

Thank you!

What type of polarization does the entanglement express? It appears circular. I know of linear and circular polarization types.

 

[DrChinese]

Thank you!

What type of polarization does the entanglement express? It appears circular. I know of linear and circular polarization types.

Both, they will either be symmetric or anti-symmetric on either basis. They will also be momentum entangled.

 

[Thecla]

Is linear momentum of each atom in the dissociation of a hydrogen molecule entangled? If you measure the linear momentum of one atom you instantly know the linear momentum of the other.

 

[ftr]

Sorry, I don't consider this a suitable reference for this thread. This is an extremely speculative paper. And it doesn't really have anything to do with the OP.

I don't have the time to elaborate on my reason, but what do you think of this reference

Quantum entanglement

http://arxiv.org/abs/quant-ph/0702225

 

[DrChinese]

I don't have the time to elaborate on my reason, but what do you think of this reference

Quantum entanglement

http://arxiv.org/abs/quant-ph/0702225

Yes, this does have the words "quantum entanglement" in the title. There are lots of articles in arxiv about this subject. However, the norm around here is to explain WHY a reference relates to the topic.