Quantum Entanglement Question

[INRM]

What exactly is quantum entanglement. If I recall it's when two atoms or particles become connected somehow and information is transferred instantaneously or something.

Can one "dis-entangle" something that's been entangled?

Also, (this is a weird question, but if you got a theory: I'd enjoy hearing it) is quantum entanglement more likely to occur near a quantum singularity like a black hole or something

 

[Schneibster]

"Entanglement" refers to the state of two or more quanta produced from a single event such that parameters of one can be inferred from measurements of the other when the conservation laws are considered.

"Disentanglement" occurs at the next interaction of either quantum, after which their parameters no longer can be inferred exclusively from measurements made on the other.

Entangled quanta occur wherever two or more quanta emerge from a single event; this is no more likely to occur "near a quantum singularity [sic] like a black hole or something" than anywhere else.

 

[RecoveringYuppy]

"Entanglement" refers to the state of two or more quanta produced from a single event such that parameters of one can be inferred from measurements of the other when the conservation laws are considered.

Is that a complete definition?

A photon creates a positron/electron pair. I measure the charge on one of the two particles of the pair and infer the charge on the other. Is that an example of entanglement?

 

[Schneibster]

Is that a complete definition?

A photon creates a positron/electron pair. I measure the charge on one of the two particles of the pair and infer the charge on the other. Is that an example of entanglement?

Considering only the charge? No. Considering the spins, and the momenta, and the positions? Yes. At least until one of them interacts with something else; then the momenta and positions no longer have to correlate, and they've decohered.

 

[Ziggurat]

Is that a complete definition?

No, it's not. It's not even a particularly good definition. Conservation laws are relevant for determining how many particular entanglements occur, but relying upon that for a definition is sloppy.

A complete definition is when the quantum state of a multiparticle system cannot be factorized into the product of single-particle states. The implications of that statement can get quite complicated, but that's the sole requirement for entanglement.

 

[wipeout]

I read a nice little definition of entanglement today:

An entangled state is a quantum superposition of two distinct physical systems.

These systems can be single particles. And entanglement is everywhere.

Also, any "instantaneous" effects between entangled particles are better looked at as just unfortunate apparitions from the old way of looking at quantum theory.

 

[Schneibster]

No, it's not. It's not even a particularly good definition. Conservation laws are relevant for determining how many particular entanglements occur, but relying upon that for a definition is sloppy.

Sure it is, but do you really want to introduce the Hamiltonian to a novice, and show the derivation of the action principle from it?

A complete definition is when the quantum state of a multiparticle system cannot be factorized into the product of single-particle states. The implications of that statement can get quite complicated, but that's the sole requirement for entanglement.

Sigh. OK. Here's the deal:

Entanglement means that you can't represent a single particle's state without referring to the state of the particle(s) it's entangled with. In other words, you can't write that particle's equation without the equation of its "entanglee." Like the new word I invented? To put it another way, if two particles are entangled, their equations of state cannot be expressed without reference to one another's equations of state.

Now, I suppose that's technically correct and relatively complete unless you want to actually see the Dirac notation- but tell me, does anyone actually understand anything more from it? If you did, you don't need me to tell you what "entanglement" means.

 

[Schneibster]

I read a nice little definition of entanglement today:

An entangled state is a quantum superposition of two distinct physical systems.

Sure, that's just another way of saying the same thing that Zig and I did.

These systems can be single particles.

Errr, well, no, not really, I don't think. Just because the eigenstate of a particle is the superposition of all its possible states, doesn't mean it's entangled with itself. What's really necessary I think is that it be impossible to enumerate the particle's eigenstate merely in terms of its own possible states- one must make reference to the eigenstate, and thereby indirectly the possible states, of another particle. It's that old thing about one thing being able to be in one place at one time.

And entanglement is everywhere.

Now that's true, if you're a proponent of decoherence, which just about everybody is these days. Aside from the odd time-reversal or TIQM guy- and even they're pretty uniformly convinced.

Also, any "instantaneous" effects between entangled particles are better looked at as just unfortunate apparitions from the old way of looking at quantum theory.

Well, now, I think that it's too early to say that there's no evidence of non-locality in physics. There is evidence of something weird, for sure, and that something might be non-locality. OTOH, it might be something else weird, at least from the point of view of beings that cannot directly sense quantum phenomena, but only their decoherent and logically consistent macroscopic effects. But there definitely has to be something, because you just can't account for the experiments we can do without a type of logic that just doesn't apply in classical mechanics.

 

[wipeout]

Sure, that's just another way of saying the same thing that Zig and I did.

It's the most straighforward I've seen, that's why I liked it. Now I just need an equally straightforward definition of the superposition it mentions.

Errr, well, no, not really, I don't think. Just because the eigenstate of a particle is the superposition of all its possible states, doesn't mean it's entangled with itself.

I meant simply that each of the systems can be a single particle in the "two systems" part of what I quoted as a definition of entanglement. I probably should have made my statement clearer.

Well, now, I think that it's too early to say that there's no evidence of non-locality in physics. There is evidence of something weird, for sure, and that something might be non-locality. OTOH, it might be something else weird, at least from the point of view of beings that cannot directly sense quantum phenomena, but only their decoherent and logically consistent macroscopic effects. But there definitely has to be something, because you just can't account for the experiments we can do without a type of logic that just doesn't apply in classical mechanics.

Seems like the weirdness is from people thinking in terms of there being classical properties for particles when there's really quantum superpositions of properties.

 

[Schneibster]

It's the most straighforward I've seen, that's why I liked it. Now I just need an equally straightforward definition of the superposition it mentions.

My criticism actually was of the description, not your explanation of it. I think it's too straightforward- it misses some of the essence of what's going on.

I meant simply that each of the systems can be a single particle in the "two systems" part of what I quoted as a definition of entanglement. I probably should have made my statement clearer.

No, your explanation was clear enough. It's not that I had a problem with.

Seems like the weirdness is from people thinking in terms of there being classical properties for particles when there's really quantum superpositions of properties.

That's where I think it's incomplete. I disagree; that's not all there is to it. There's something even stranger than superposition going on.

 

[casebro]

"Quantum Enlargement Question" ?

Imagine my disapointment that this wasn't a post about a Woo system to compete with the Viagra market.... perhaps using a copper pyramid? Or a plastic card with a sticker on it, to be carried in your front pocket, on the appropriate side...

 

[rakovsky]

"Entanglement" refers to the state of two or more quanta produced from a single event such that parameters of one can be inferred from measurements of the other when the conservation laws are considered.

"Disentanglement" occurs at the next interaction of either quantum, after which their parameters no longer can be inferred exclusively from measurements made on the other.

Entangled quanta occur wherever two or more quanta emerge from a single event; this is no more likely to occur "near a quantum singularity [sic] like a black hole or something" than anywhere else.

That seems simple enough. It sounds like the simple concept that any reaction produces an equal opposite reaction.
Why then would Einstein have called this "spooky" physics, been incredibly skeptical about it, and 42% of people in my Skeptics poll replies that they didn't believe in Quantum Entanglement?

I want to please ask you to have a look:

FORUM POLL: Which of these principles do you believe are likely true?
Principle--------------- votes ----- percent
Quantum Entanglement 18 58.06%

http://www.internationalskeptics.com/forums/showthread.php?t=317428&page=2

What I found suggests something different beyond simply their states being predictable until they interacted with something else:

In quantum physics, entangled particles remain connected so that actions performed on one affect the other, even when separated by great distances. The phenomenon so riled Albert Einstein he called it "spooky action at a distance."

The rules of quantum physics state that an unobserved photon exists in all possible states simultaneously but, when observed or measured, exhibits only one state.

http://www.livescience.com/28550-how-quantum-entanglement-works-infographic.html

Click here for a good image:
http://www.livescience.com/images/i...ement-spooky-action-at-a-distance-130408c.jpg

 

[Ziggurat]

That seems simple enough.

It does seem simple, but it's also wrong. Part of his confusion comes from the fact that the easiest way to create entangled states involves conservation laws which constrain the possible states to those which produce entanglement. But there is nothing in principle which says that entangled states have to obey any sort of conservation laws. So for example, the canonical entangled quantum spins are created so that if one is up, the other is down (since because of the way you normally create them, total spin must be zero). But you could also have a quantum state where if one spin was up, the other is also up, and if one spin is down, the other is down. Now the total spin isn't conserved. Conservation laws don't help you in that case.

Schneibster objected to my definition on the basis that if you understand the definition then you already understand entanglement. That's not quite true, but it is true that you need to know a moderate amount of quantum mechanics in order to really get the implications. And that is, admittedly, a shortcoming of my mathematical definition. But that's sort of the sad truth of the matter: you cannot fully grasp entanglement until you grasp the math. Quantum mechanics is a mathematical theory. There's no getting around that.

It sounds like the simple concept that any reaction produces an equal opposite reaction.

Nope, not really. It's much more subtle than that.

Why then would Einstein have called this "spooky" physics, been incredibly skeptical about it, and 42% of people in my Skeptics poll replies that they didn't believe in Quantum Entanglement?

No idea about your poll. It could be that they didn't understand the question, or thought you were trying to get at something else. Or, they just might be wrong.

The issue with Einstein is that quantum entanglement implies nonlocality. This is... unsatisfying. So Einstein and others looked for ways out of that, such as "hidden variables". But Bell pretty much squashed their hopes, and while Einstein might not have been happy with that, I don't think he disbelieved it.

What I found suggests something different beyond simply their states being predictable until they interacted with something else:

Well, yes. Because it is something different from that. If you have two spins and they're both spin up, then that's not an entangled state. But it's perfectly predictable, until one of them interacts with something.

 

[rakovsky]

Well, yes. Because it is something different from that. If you have two spins and they're both spin up, then that's not an entangled state. But it's perfectly predictable, until one of them interacts with something.

It seems that what I cited, Quant. Ent. emphasizes that not only before another interaction they are predictable, but even that interaction itself on one particle will affect the other particle:

In quantum physics, entangled particles remain connected so that actions performed on one affect the other, even when separated by great distances.

I suspect you and I might be on the same page though, just using different wording.

 

[rakovsky]

I like that you are familiar with this stuff.

The issue with Einstein is that quantum entanglement implies nonlocality. This is... unsatisfying. So Einstein and others looked for ways out of that, such as "hidden variables". But Bell pretty much squashed their hopes, and while Einstein might not have been happy with that, I don't think he disbelieved it.

I think he was a genius, but do you think Einstein's own contributions or genius are sometimes overrated?

Who Actually Invented It? Henri Poincaré, mostly. Poincaré was the foremost expert on relativity in the late 19th century and was most likely the first person to formally present the theory of relativity. If you were Einstein and you wanted to write about relativity, you might consider meeting with the foremost expert on relativity, yes? If you answered "yes" to that question, then you're not Einstein at all.

According to Einstein's famous On the Electrodynamics of Moving Bodies, which contains his theories on relativity, Poincaré, despite publishing 30 books and over 500 papers, is not worth mentioning. It's true, pick up Einstein's paper if you don't believe us, (you won't): Poincaré doesn't receive a single reference, unless you consider plagiarism to be some kind of indirect reference. As a matter of fact, Einstein does not reference, footnote or cite a single ........ source in his entire paper.
...
According to Peter Galison's Einstein's Clocks, Poincaré's Maps: Empires of Time, Einstein and a small group of his fellow nerdlings formed a group called The Olympia Academy and would regularly gather to discuss their own works as well as the works of current scientists. The book goes on to specifically mention how Poincaré was one of the scientists that Einstein and his battalion of nerds would discuss.

Shoots that whole "maybe Einstein didn't read any other papers" theory right to ----, doesn't it? It's interesting that Einstein sat studying and discussing the work of Poincaré for years, published a book that featured a theory that was startlingly similar to Poincaré's, and then didn't reference Poincaré once in the entire book.

http://www.cracked.com/article_16072_5-famous-inventors-who-stole-their-big-idea.html

 

[Nowhere Man]

Thread necromancy alert, 10 years.

Fred

 

[Ziggurat]

It seems that what I cited, Quant. Ent. emphasizes that not only before another interaction they are predictable, but even that interaction itself on one particle will affect the other particle:

That... depends.

If I measure the state of one particle, then yes, that measurement will affect the state of the other particle. But there are still typically things I can do to one particle which don't affect the other particle. For example, in the canonical paired spins problem, there are ways to flip a spin which don't depend on which way the spin was pointing. You can thus flip one spin even without knowing what it was, leaving the other spin alone. They will remain entangled, but instead of being opposite spins, they will now have the same spin.

 

[Ziggurat]

I think he was a genius, but do you think Einstein's own contributions or genius are sometimes overrated?

Not really. While it's true that the lay public doesn't really understand how much Einstein built off other people for Relativity, they also don't know much about his other seminal contributions like the photoelectric effect (which is what he actually got his Nobel Prize for) or his Brownian motion theory.

 

[rakovsky]

That... depends.

If I measure the state of one particle, then yes, that measurement will affect the state of the other particle. But there are still typically things I can do to one particle which don't affect the other particle. For example, in the canonical paired spins problem, there are ways to flip a spin which don't depend on which way the spin was pointing. You can thus flip one spin even without knowing what it was, leaving the other spin alone. They will remain entangled, but instead of being opposite spins, they will now have the same spin.

You have a lot of good information.

 

[Roboramma]

The issue with Einstein is that quantum entanglement implies nonlocality. This is... unsatisfying. So Einstein and others looked for ways out of that, such as "hidden variables". But Bell pretty much squashed their hopes, and while Einstein might not have been happy with that, I don't think he disbelieved it.

Shouldn't that be "I don't think he would have disbelieved it"? I'm pretty sure Einstein was dead by the time Bell came along.

Or maybe you are saying something different and I'm misreading.