Measuring Entangled Particles

[CWingfield]

I have a question about how we measure entangled particles. As I understand it, once we measure entangled particles - the connection is severed. I was wondering: Is it the act of severing the connection that allows us to know that the particles were indeed entangled, or is it more the byproduct of the measurement?

 

To put it another way: could it be like trying to operate with a machete when you need a scalpel. Can our current "tools" be refined, or will it always be that we break the connection no matter how fine-tuned the measurement device is?

 

I guess the real question is: What is it about how we currently measure entanglement that severs the connection, and could that possibly change in the future?

[swansont]

I have a question about how we measure entangled particles. As I understand it, once we measure entangled particles - the connection is severed. I was wondering: Is it the act of severing the connection that allows us to know that the particles were indeed entangled, or is it more the byproduct of the measurement? To put it another way: could it be like trying to operate with a machete when you need a scalpel. Can our current "tools" be refined, or will it always be that we break the connection no matter how fine-tuned the measurement device is? I guess the real question is: What is it about how we currently measure entanglement that severs the connection, and could that possibly change in the future?

 

Once you measure the entangled property, it is no longer in a superposition. That breaks the entanglement.

[MigL]

In much more layman's terms...

Entangled particles share a 'probability distribution'.

Once you interact with one to determine its actual state, you also know the state of the other.

 

From that point forward the particles will have separate 'probability distributions', so that subsequent measurements will no longer correlate.

[CWingfield]

I am sorry for being dense. What is it about the measurement that breaks the superposition?

I see, thank you MigL. I guess a followup question would be: Is there any way to just tell that two particles are entangled without having to measure one end or the other? More or less, would there be anyway to just tell if the two are entangled just by the connection itself, or is the connection more or less invisible?

 

As a secondary question, is it possible for a particle to be entangled with more than one particle?

[swansont]

I am sorry for being dense. What is it about the measurement that breaks the superposition?

I see, thank you MigL. I guess a followup question would be: Is there any way to just tell that two particles are entangled without having to measure one end or the other? More or less, would there be anyway to just tell if the two are entangled just by the connection itself, or is the connection more or less invisible?

 

As a secondary question, is it possible for a particle to be entangled with more than one particle?

Entangled particles are not in a definite state. Once you measure, they are. You can't have entanglement if you can tell the difference between the particles.

 

There is no "connection" to measure. That's one of the weird aspects of it.

 

Yes, you can entangle more than two particles.

[CWingfield]

Very informative, Swansont. Thank you for the answer.

 

Do we currently know what the overall purpose of entanglement is? Or is it more of a "we can see it and measure it, but just don't yet understand why it is happening" sort of thing?

[swansont]

Very informative, Swansont. Thank you for the answer.

 

Do we currently know what the overall purpose of entanglement is? Or is it more of a "we can see it and measure it, but just don't yet understand why it is happening" sort of thing?

 

Purpose isn't the right word. Physics effects don't really have a purpose. We something about know why it happens, since it falls out of quantum mechanics, and we know we can do it, because it's been done experimentally.

[CWingfield]

Yes, I agree that purpose is the wrong word. I guess I was wondering more about the "Why" of entanglement. As, I doubt entanglement happens for no good reason, I would assume that it serves some important "functional role" in life/universe. Of course, I may be assuming way too much (it wouldn't be a first). Do we having any current hypothesis on the "Why" of entanglement?

 

I did just last night read a really great article in space.com that was a quick question and answer session with a experimental quantum physicist and a theoretical quantum physicist. Had I been able to read this article before I made the original post, it would have explained nearly everything - leaving no need for the original post. I think it has done the best job of describing in layman's terms what we currently know about quantum science. They were able to cram a lot of easily digestible information in a rather short article.

 

Two things from the article I found fascinating that I hadn't known was that, not just particles, but atoms can become entangled, and how quantum science affects our perception of free will.

[swansont]

Yes, I agree that purpose is the wrong word. I guess I was wondering more about the "Why" of entanglement. As, I doubt entanglement happens for no good reason, I would assume that it serves some important "functional role" in life/universe. Of course, I may be assuming way too much (it wouldn't be a first). Do we having any current hypothesis on the "Why" of entanglement?

 

Looking for reasons/purposes is outside the realm of science. As for why, it's inherent in QM. You can have superpositions of particle states that involve more than one particle.

[CWingfield]

Let me try putting it a different way. If tomorrow all of a sudden entanglement stopped occurring, do we have any idea what would change, if anything? For instance, If i ask the same question about gravity, then the response is that things start flying apart. It may be that we don't currently know, may never know, or it doesn't have any real effect whatsoever. I was just curious if there was any speculation about it.

[ajb]

If tomorrow all of a sudden entanglement stopped occurring, do we have any idea what would change, if anything?

The problem is that entanglement, which is to do with the superposition of states, is written into the foundations of quantum mechanics. It is hard to say how we can keep 'quantum-ness' by completely removing entanglement.

 

Attempts at theories involving hidden variables, so theories that essentially do not have entanglement have other problems.

[CWingfield]

Thank you both for your responses. As a final question about this, where does the current scientific majority stand on whether or not we will ever be able to reconcile the classical and quantum realms? Do most think eventually we will see how they work together, or do most think we will never be able to reconcile the differences, and they more or less stand apart?

[ajb]

As a final question about this, where does the current scientific majority stand on whether or not we will ever be able to reconcile the classical and quantum realms?

You will need to be more specific here. We have some reasonable ideas about the classical limit of quantum theories, though a quantum system need not have a unique classical limit. Also most of our knowledge of quantum systems comes from quantisation of classical systems, which of course has many different meanings.

 

Do most think eventually we will see how they work together, or do most think we will never be able to reconcile the differences, and they more or less stand apart?

This depends on what you mean.

[CWingfield]

I have been thinking about the best way to describe my question about this, and may have a somewhat clearer question.

 

Let's say we have two entangled particles, both showing "heads". What happens when I take one of the particles and flip it to "tails"? Does it's entangled pair also flip to "tails"? If it also flips, at what speed does this occur - the speed of light or nearly simultaneously?

[Strange]

Once you know what the orientation is, they are no longer entangled; measurement destroys the entangled state. So if you flip one of them after that, it will have no effect on the other.

[CWingfield]

Thanks, Strange.

 

So, would it be correct to think of entanglement as an ever spinning coin that never shows heads or tails unless measured?

[Strange]

Thanks, Strange.

 

So, would it be correct to think of entanglement as an ever spinning coin that never shows heads or tails unless measured?

 

I guess that is quite a good analogy for the "non realistic" or "no hidden variables" aspect; in other words the value is not just unknown but undefined until it is measured.