What Happens When We Measure Entangled Particles?

[itoero]

Hi,
I have some questions concerning entanglement.

1. If it's possible (theoretically) to simultaneously measure both entangled particles.
Then what will the measurements give?

2. The wave function is supposed to hold the info about both entangled particles...it's a superposition.
When you apply a measurement on one of the entangled particles, the superposition/wave function/entanglement collapses. So which info do you measure? Does the collapsing of the superposition make the particles go back to there quantum state from before they got entangled (+the state that caused the entanglement to collapse)?

3. Measurement breaks the entanglement, so how can you study entanglement? How can you know particles behave as twins without measuring them?

4. I've heard from another physicist that information transfer demands a classical channel between the particles. And the wave function contains the info about both particles.
So then that classical channel is the superposition?

 

[ddd123]

Hi, I think direct answers to your questions would take more time for you to gain an idea than studying the matter from scratch (even on a non-specialized level, there are introductory materials all over the place), because you're asking about the very basics of the matter and after you've learned a bit on the topic you'll discover some of your questions are badly phrased or meaningless.

 

[Nugatory]

For definiteness, let's work with spin-entangled particles with opposite spin (there are many other possibilities, but this is one of the easier ones to work with). Then...

1. If it's possible (theoretically) to simultaneously measure both entangled particles.
Then what will the measurements give?

It is possible and easy to measure both particles simultaneously: set up two detectors near one another and put the source of entangled particles in the middle, equidistant from both detectors. When the source emits an entangled pair, one particle will go one way, the other will go the other way, and they'll both reach a detector at the same time. If the two detectors are aligned the same way (vertically, horizontally, some angle in between) they will always produce opposite results - one particle is spin-up and the other is spin-down along that axis.

2. The wave function is supposed to hold the info about both entangled particles...it's a superposition.
When you apply a measurement on one of the entangled particles, the superposition/wave function/entanglement collapses. So which info do you measure? Does the collapsing of the superposition make the particles go back to their quantum state from before they got entangled (+the state that caused the entanglement to collapse)?

The system wave function before the measurement was a superposition of "left-moving particle is spin-up and right-moving particle is spin-down" and "left-moving particle is spin-down and right-moving particle is spin-up". When the particles reach the detectors the wave function collapses into one of these two states, and the results at the detectors tell us which.

3. Measurement breaks the entanglement, so how can you study entanglement? How can you know particles behave as twins without measuring them?

We get one measurement on each, as above. That's enough to study the behavior.

4. I've heard from another physicist that information transfer demands a classical channel between the particles. And the wave function contains the info about both particles.
So then that classical channel is the superposition?

The "classical channel" is just fancy words for how we compare the results at both detectors after the measurement. The simplest classical channel is just to shout across the lab to your lab partner: "Hey, my detector just recorded a spin-up particle! What did yours see?".

 

[itoero]

Thanks, that clears things up.

I've read several times that spacetime is built by or emerges from entanglement.
How do they relate?
Is there a simple way of explaining this?.

 

[Heinera]

Thanks, that clears things up.

I've read several times that spacetime is built by or emerges from entanglement.
How do they relate?
Is there a simple way of explaining this?.

It would be easier to comment on this if you could give us some references to where you've read this, but absent that, there is nothng in current and mainstream theory of physics that implies that spacetime emerges from entanglement.

 

[itoero]

It would be easier to comment on this if you could give us some references to where you've read this, but absent that, there is nothng in current and mainstream theory of physics that implies that spacetime emerges from entanglement.

Hirosi Ooguri should have written a paper about it.
Juan Maldacena and Leonard Susskind did research which suggested that gravity emerged from entanglement.

 

[Strilanc]

It would be easier to comment on this if you could give us some references to where you've read this, but absent that, there is nothng in current and mainstream theory of physics that implies that spacetime emerges from entanglement.

Nothing mainstream maybe, but out is something being looked into. I don't know anything about it but here's a post by Sean Carol:

http://www.preposterousuniverse.com/blog/2015/05/05/does-spacetime-emerge-from-quantum-information/

 

[stevendaryl]

Nothing mainstream maybe, but out is something being looked into. I don't know anything about it but here's a post by Sean Carol:

http://www.preposterousuniverse.com/blog/2015/05/05/does-spacetime-emerge-from-quantum-information/

Interesting stuff. But if gravity arises from pure quantum mechanics, then there should be a way to relate Newton's constant G to some property of the quantum system. I skimmed the papers, and it seems that the relationship is through the formula for entropy, but I don't understand that.

 

[Strilanc]

Interesting stuff. But if gravity arises from pure quantum mechanics, then there should be a way to relate Newton's constant G to some property of the quantum system. I skimmed the papers, and it seems that the relationship is through the formula for entropy, but I don't understand that.

That sounds reasonable, though I don't know if they're even near that far along. I'd defend the idea if I knew anything about it beyond how to point you in the general direction of competent people working on it.

John Preskill has given a talk about it. That's probably more accessible than a paper... also I could listen to him talking all day.

 

[itoero]

Here is an article about the paper of Hirosi Ooguri:
http://www.sciencedaily.com/releases/2015/05/150527112953.htm

 

[Nugatory]

Here is an article about the paper of Hirosi Ooguri:
http://www.sciencedaily.com/releases/2015/05/150527112953.htm

Paper itself is behind a paywall unfortunately: http://journals.aps.org/prl/accepted/5b078Y2cM051f24665e658c4cb56b6c2e9f335027

Maybe there's a preprint available on arXiv?

 

[Heinera]

The arXiv preprint is here: http://arxiv.org/abs/1412.1879

(They changed the title in the published version.)

 

[itoero]

How does entanglement enables teleportation?
When you teleport a photon, do you bring it in the wave and does the collapsing of the wave make it look like it's been teleported?
If so, then why does the teleportation work in one direction?

 

[Nugatory]

How does entanglement enables teleportation?
When you teleport a photon, do you bring it in the wave and does the collapsing of the wave make it look like it's been teleported?
If so, then why does the teleportation work in one direction?

Although it is called "teleportation", it's not teleportation as we usually understand the word. We don't make a photon move from one location to another, all we do is recreate the state of the original at some other location - there are some pretty decent summaries of how it works out on the web.

 

[itoero]

ok
And how can I 'visualize' this in terms of collapsing of the wave?