# Quantum entanglement

1. Nov 23, 2014

### imsmooth

I have read that one particle's state responds to the partner's measured state, and this can occur billions of miles away. Since we can not conduct the experiment "billions of miles away", how have physicists come to the conclusion about the apparent limitless distance for this action? What experiment was conducted to show the distance is that vast?

2. Nov 23, 2014

### phinds

You don't really need "billions of miles" you just need to demonstrate that the connection happens faster than light, and that has been done. I'm sure the internet has lots of data on entanglement experiments.

3. Nov 23, 2014

### Staff: Mentor

First there is no action - just a strange correlation. Its the general property of entanglement that follows from the basic principles of QM. Either they are wrong or the correlation remains valid regardless of distance.

Its sort of like if you had a red slip of paper and a blue slip. You put each in an envelope and randomly keep one and the other is sent on a journey to the other side of the universe. You open your envelope and see red. There is no doubt the other will be blue regardless of if its on the other side of the universe or not. No one has ever done such an experiment but the outcome is not in doubt.

Thanks
Bill

Last edited: Nov 23, 2014
4. Nov 23, 2014

### Staff: Mentor

Indeed it will, but be aware that there are entanglement scenarios that are appreciably weirder than this. Googling for "Bell's theorem" will find some.

However, Bhobba's point about it being a matter of correlation still stands. I look at my slip of paper, I see it's red, I know that if you look at your slip of paper you will see blue. That's not the same thing as saying that my looking at my slip of paper turned your slip blue, nor saying that your slip was blue "all along"

5. Feb 28, 2015

### MisterAmrani

I had the same question. Because I thought that there is merely a correlation (if one particle is measured to have a particular spin, then another 'entangled' particle should have the opposite spin). But then I came across some tekst where they state that communication is possible through entanglement. So now I am confused again.

6. Feb 28, 2015

### jfizzix

It's more than simply a correlation, because if your measurements violate a Bell inequality, then a complete knowledge of the history of each particle won't be enough information to explain the correlations you see. Violating Bell's inequality rules out the possibility that the particles simply were going to give those outcomes all along (like slips of paper). Whatever complete explanation there is for this is still under some debate.

7. Feb 28, 2015

### Staff: Mentor

Yes it's a 'special' type of correlation as implied by QM and different from the slip of paper example which is also a correlation (the difference is in the paper example it has that colour irrespective of if its observed or not - that may or may not be the case with Bell type correlations). But a correlation it most certainly is - if you measure one spin you know the other automatically - that's a correlation by the very definition of correlation. Its the meaning of this 'funny' kind of correlation that's debated.

And it cant be used to send information since the sender can't determine what their observation will give.

Thanks
Bill

Last edited: Feb 28, 2015
8. Feb 28, 2015

### Staff: Mentor

It's not. Either what you read is wrong or you misunderstood it.

9. May 13, 2015

### jk22

The prediction is based on a conservation law like s1+s2=0 but this has a prerequisite that your system does not interact with a third part.

10. Jun 16, 2015

### ShawnDBaum

If I understand correctly, the affected particles appear to obey classical mechanics by adopting opposing states (conservation of momentum) and then retaining those states until acted upon by dissimilar outside forces (inertia).

It seems like quantum entanglement is just a mathematical way of saying, "We can't set up this reaction with sufficient precision to control for every state of the products and haven't gotten around to measuring each product of that reaction yet but when we finally do measure one of the products, we'll have enough info to figure out what the other one looks like."

If that was the case then there wouldn't be anything "spooky" about knowing the physical state of one product by measuring the other (e.g., x*y=1), but it is obviously much more complicated than that if scientists have spent so much effort trying to figure it out all these years. Can anyone provide a high-level summary or point me to a layman's reference of what makes quantum entanglement different?

11. Jun 16, 2015

### Staff: Mentor

12. Jun 16, 2015

### zonde

The problem with this type of reasoning is that it respects Bell inequality (under idealized conditions).

Drchinese site is good but I myself like this informal proof of Bell inequality (btw Drchinese site contains similar proof, but I still like this one better):

13. Jun 16, 2015

### ShawnDBaum

Thank you bhobba and zonde for those links. I will keep working to understand.

14. Jun 16, 2015

### Ravi Mohan

It is possible to send quantum information using entangled qubits and a classical bit. This is called "Quantum Teleportation" https://en.wikipedia.org/wiki/Quantum_teleportation. You can also read "Quantum Computation and Quantum Information" by Nielsen and Chuang.

15. Jun 16, 2015

### jfizzix

There's also a second way to send information via entanglement.
If two parties A and B share a pair of particles prepared in one of the four Bell states, party A can perform actions on their particle to change the joint state describing the pair to be any other one of the four Bell states. When party A sends their particle to B, party B can do measurements to see what bell state the pair is in, and therefore, what actions party A performed.

If A and B share a series of pairs of particles, then A can communicate a message to B, by performing a series of manipulations on the halves they have, and sending them to B for measurement.
Encoding information in this way is called quantum dense coding.
Using entangled pairs of systems as a resource, you can send up to twice as many bits at a time as you would if you didn't share that quantum entanglement.

16. Jun 16, 2015

### atyy

Physicists do not conclude that. Rather, it is a prediction of quantum mechanics which remains to be tested.