# How do you quantumly entangle particles?

1. Mar 16, 2013

### 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

2. Mar 16, 2013

### Staff: Mentor

3. Mar 16, 2013

### 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

4. Mar 16, 2013

### Staff: Mentor

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

5. Mar 16, 2013

### San K

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

Last edited: Mar 17, 2013
6. Mar 18, 2013

### 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

7. Mar 18, 2013

### 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

8. Mar 18, 2013

### f95toli

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.

9. Mar 18, 2013

### DrChinese

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.

10. Mar 18, 2013

### 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!

11. Mar 18, 2013

### 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?

12. Mar 18, 2013

### DrChinese

That is correct.

13. Mar 18, 2013

### DrChinese

14. Mar 18, 2013

### friend

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?

Last edited: Mar 18, 2013
15. Mar 18, 2013

### DrChinese

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.

16. Mar 18, 2013

### friend

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?

17. Mar 23, 2013

### ftr

you can check out this paper

Gravity as Quantum Entanglement Force

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

18. Mar 26, 2013

### DrChinese

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.

19. Mar 29, 2013

### 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.

20. Mar 29, 2013

### Staff: Mentor

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

21. Mar 29, 2013

### DrChinese

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

22. Mar 29, 2013

### ChiralWaltz

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

23. Mar 29, 2013

### DrChinese

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.

24. Mar 29, 2013

### ChiralWaltz

Thank you!

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

Last edited: Mar 29, 2013
25. Mar 29, 2013

### DrChinese

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

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