Some questions regarding quantum entanglement

[Old Guy In Stanton]

I am not a physicist, nor even a scientist, but I would like to put forth an argument for an idea I have that is related to quantum entanglement. I believe that this is a testable/replicable concept.

 

The concept requires, I think, several "leading" assumptions/questions, maybe as many as ten of them, to get everyone thinking on the same page, as it were.

 

I will try to make the questions as plain and unambiguous as possible. I will ask the questions one at a time, evaluating responses and restating the assumption/question until there is consensus of what is meant. I would appreciate plain language responses, if possible.

 

Question #1: I have read that it is now possible to entangle quantum particles. I've read that what is entangled is certain properties, such as spin, or magnetic charge, or electric charge. So: is it possible to entangle two quantum particles in such a way that their magnetic charges or electrical charges are identical, or linked in some way, and what would those ways be?

Edited June 14, 2016 by Old Guy In Stanton

[swansont]

I've not heard of being able to entangle charge, since that's an intrinsic property of a particle, and I can't think of a way that systems would be easily indistinguishable if you tried to entangle charge in some ensemble of atoms.

 

IOW, the charge of a single electron can't be entangled, since it will always have that value. If you tried to entangle the charge of an atom and an ion you'd likely fail, since an atom is easily distinguished from an ion. Any interaction that can tell if one is charged would collapse the wave function.

 

Typically the entangled property is the spin state for an atom (or electron) and the polarization for photons; you are able to make these states the same, or orthogonal to each other — there are options. Other states/properties could be entangled, too.

[imatfaal]

...Other states/properties could be entangled, too.

 

Is there a connexion between states which admit to the possibility of entanglement and those which form part of non-commutable pairs of observables; ie you can know the charge both positive and negative - these are not entangeable, you cannot know both the spin on the x and the z to arbitrary accuracy - particles are spin entangleable.

[Old Guy In Stanton]

I've not heard of being able to entangle charge, since that's an intrinsic property of a particle, and I can't think of a way that systems would be easily indistinguishable if you tried to entangle charge in some ensemble of atoms.

 

IOW, the charge of a single electron can't be entangled, since it will always have that value. If you tried to entangle the charge of an atom and an ion you'd likely fail, since an atom is easily distinguished from an ion. Any interaction that can tell if one is charged would collapse the wave function.

 

Typically the entangled property is the spin state for an atom (or electron) and the polarization for photons; you are able to make these states the same, or orthogonal to each other — there are options. Other states/properties could be entangled, too.

 

OK, let me restate my question: Make the quantum particles electrons, and entangle their spins. What, then, IS spin, how will each electron be affected, does spin affect charge in any way, and what happens to the two electrons when the wave function is collapsed by "looking at" one of the electrons?

[Strange]

What, then, IS spin

 

 

It is a measurement of the angular momentum of the parcel. Like many other properties it is quantised.

 

 

 

how will each electron be affected

 

By being entangled? Not at all except that their spins will be correlated when measured.

 

 

 

does spin affect charge in any way

 

No.

 

 

 

, and what happens to the two electrons when the wave function is collapsed by "looking at" one of the electrons?

 

Their spins become known. (Before then, they are undetermined.

[imatfaal]

I think it is also necessary to state that quantum spin differs from classical angular momentum due to the fact that it is intrinsic to the the particle - you cannot change the quantity of the quantum spin of a particle whereas the classical angular momentum is changeable with an external torque

[Velocity_Boy]

Don't feel bad, a lot of scientists didn't understand or even believe in quantum entanglement. I think that maybe some still don't. Hell, Even Albert Einstein was a little baffled about it – which caused him to speak the oft-quoted words “spooky action at a distance”.

Basically, at least how I see it, quantum entanglement posits that acting on a particle here, can instantly influence a particle far away. This is also described as theoretical teleportation. And obviously it has huge implications for quantum mechanics, quantum communication and could offer some really awesome progress for quantum computing.

[Strange]

I think it is also necessary to state that quantum spin differs from classical angular momentum due to the fact that it is intrinsic to the the particle - you cannot change the quantity of the quantum spin of a particle whereas the classical angular momentum is changeable with an external torque

 

 

And, despite being called "spin" nothing actually rotates ...

[Old Guy In Stanton]

>>>what, then, IS spin

 

It is a measurement of the angular momentum of the parcel. Like many other properties it is quantised.

 

>>>how will each electron be affected?

 

By being entangled? Not at all except that their spins will be correlated when measured.

 

>>>does spin affect charge in any way?

 

No.

 

>>>how will each electron be affected

 

Their spins become known. (Before then, they are undetermined.

 

 

OK, then Question #2: is it possible to capture these two electrons in two boxes of some sort? Perhaps inside a magnetic or electric field that keeps each electron stable (without triggering wave collapse)? IOW, two electrons with entangled spins, 10 feet apart in two little boxes of some nature?

Edited June 14, 2016 by Old Guy In Stanton

[Strange]

 

 

OK, then Question #2: is it possible to capture these two electrons in two boxes of some sort? Perhaps inside a magnetic or electric field that keeps each electron stable (without triggering wave collapse)? IOW, two electrons with entangled spins, 10 feet apart in two little boxes of some nature?

 

 

I believe it is possible (but very difficult in practice).

 

https://www.technologyreview.com/s/420664/physicists-build-a-memory-that-stores-entanglement/

[elfmotat]

Is there a connexion between states which admit to the possibility of entanglement and those which form part of non-commutable pairs of observables; ie you can know the charge both positive and negative - these are not entangeable, you cannot know both the spin on the x and the z to arbitrary accuracy - particles are spin entangleable.

All observables have corresponding operators, and the non-commutativity of two operators is what defines the uncertainty relation between these observables. Since there needs to be uncertainty for there to be entanglement, the answer to your question would be: yes.

[swansont]

 

Is there a connexion between states which admit to the possibility of entanglement and those which form part of non-commutable pairs of observables; ie you can know the charge both positive and negative - these are not entangeable, you cannot know both the spin on the x and the z to arbitrary accuracy - particles are spin entangleable.

 

 

You measure the spin in the same coordinate system, since the spin projection along any one axis commutes.

Basically, at least how I see it, quantum entanglement posits that acting on a particle here, can instantly influence a particle far away.

 

 

Which is not how it's described by physicists. There is no interaction or influence. It's also not teleportation, although entanglement is involved in teleportation.

All observables have corresponding operators, and the non-commutativity of two operators is what defines the uncertainty relation between these observables. Since there needs to be uncertainty for there to be entanglement, the answer to your question would be: yes.

 

Commutation is not the only source of uncertainty. The Heisenberg uncertainty, which is what you get from commutation relations, would kill entanglement. One of the basic descriptions is that Alice and Bob must use the same basis — that's only only way to get correlated spins. If one uses a different basis than the other, then you won't have a unique measurement.

 

IOW, if I measure my particle to be spin up, the only way I know the other spin is if it's measured along the same axis (and those measurements commute). If it's measured along another axis, I can get up or down.

[disarray]

"All fundamental particles have a property called spin, which doesn’t really mean they’re spinning around, but it does mean they have an orientation in space and an angular momentum."

I, as a lay person, was under the impression that "measuring" the spin of one particle then tells you what the various opposite "spins" of the other particle are, and that, moreover, though the entangled particles always have opposing spins in theory, the neither particle really has a definite spin (according to Bell's calculations about what logically would happen if they did) until one of them is "measured":

"effectively, the two entangled particles don’t actually have a well-defined spin until you measure them. They're basically spin-less until the first one is measured - and that value then determines the spin of the second particle."

The same article concludes that "new research also shows that the phenomenon won’t allow us to communicate faster than the speed of light, which means it doesn't violate the special theory of relativity" though no further explanation is given.

 

I have read that every particle, or perhaps every electron, has its (symmetrical) twin, and that this is related to the idea that "because the total angular moment of the Universe must stay constant, you can then predict that if you measure one of these entangled particles and they have an up spin, then the other one in the pair must have a down spin - otherwise the laws of the Universe would be breached."

 

Thus, when we apparently force a particle to "reveal its true colors" (for lack of a better metaphor), every particle is somehow affected in order to maintain the balance of various properties, (as if one is merely dropping one more coin into the carnival arcade coin pusher and every coin/particle shifts automatically), including the other particle in the pair of twins.

[swansont]

 

Thus, when we apparently force a particle to "reveal its true colors" (for lack of a better metaphor), every particle is somehow affected in order to maintain the balance of various properties, (as if one is merely dropping one more coin into the carnival arcade coin pusher and every coin/particle shifts automatically), including the other particle in the pair of twins.

 

 

You have to be careful here. "somehow affected" means that entangled particles are now in a known state, rather than that they have changed from one state to another.

[Markus Hanke]

Since there needs to be uncertainty for there to be entanglement

 

 

That is interesting, I have not come across this statement before. Could you elaborate on this a bit ?

Let's say I have a composite system of two fermions, and I create an entanglement relationship between them. I am interested only in their spin state, so either particle can be either spin-up or spin-down. Where does uncertainty come into this ?

[disarray]

swansont: You write that “You have to be careful here. "somehow affected" means that entangled particles are now in a known state, rather than that they have changed from one state to another.”

 

But don’t we say that they are not in any particular state until measured, whether talking about the position of an electron or the spin of entangled electrons or an electron passing through a board with double slits.

 

How can a physicist say that half the electrons in the universe have up spin (of some type of “spin”) and the other half have down spin if they really don’t have any particular spin until measured?

 

How would even a hypothetical God of Einstein know if there was an equal number of electrons with up and downs, so that the total spin balanced out, if they don’t really have any determinate spin…as if they had superposition of spin?

 

I have read that even if we could each particle in a pair of entangled spins to have the same direction, we would still find, if we got them together at a party to compare notes later, that they somehow had opposite spins, as the universe won’t let them have the same spin.

 

(By the way, thanks to those who have the patience to explain things to beginners).

[swansont]

 

 

That is interesting, I have not come across this statement before. Could you elaborate on this a bit ?

Let's say I have a composite system of two fermions, and I create an entanglement relationship between them. I am interested only in their spin state, so either particle can be either spin-up or spin-down. Where does uncertainty come into this ?

 

 

You can't be able to distinguish between the particles in a way which would let you correlate some other variable with the one you wanted to entangle.

 

If you have photons of two different polarizations from two different sources you can entangle them by combining them on a beamsplitter, so you get two photons out, one of each polarization. But if they are a different wavelength they won't be entangled, because you can associate a polarization with the color. Or if they don't come out at the same time, because then there is a time-ordering. In parametric downconversion (one photon in, two photons out) the emission directions have a correlation to the polarization. It's only the direction(s) that coincide that allow for entanglement.

 

So for spin states, you can't have some other variable (direction of motion, time ordering of arrival, etc.) have a correlation with the spin state.

swansont: You write that “You have to be careful here. "somehow affected" means that entangled particles are now in a known state, rather than that they have changed from one state to another.”

 

But don’t we say that they are not in any particular state until measured, whether talking about the position of an electron or the spin of entangled electrons or an electron passing through a board with double slits.

The error I'm trying to steer you away from is the common one that declares that changing one spin state causes the other one to change. If you read through more than two pop-sci articles on the subject, you are likely to see that "explanation" pop up.

 

 

How can a physicist say that half the electrons in the universe have up spin (of some type of “spin”) and the other half have down spin if they really don’t have any particular spin until measured?

 

How would even a hypothetical God of Einstein know if there was an equal number of electrons with up and downs, so that the total spin balanced out, if they don’t really have any determinate spin…as if they had superposition of spin?

 

I have read that even if we could each particle in a pair of entangled spins to have the same direction, we would still find, if we got them together at a party to compare notes later, that they somehow had opposite spins, as the universe won’t let them have the same spin.

 

(By the way, thanks to those who have the patience to explain things to beginners).

 

 

Because angular momentum is conserved in the absence of an external torque. So if we start out with zero total spin, and all of the sudden we have two particles which we know to have spin 1/2, we know one of them is spin up and the other must be spin down.

[Strange]

Because angular momentum is conserved in the absence of an external torque. So if we start out with zero total spin, and all of the sudden we have two particles which we know to have spin 1/2, we know one of them is spin up and the other must be spin down.

 

I am slightly doubtful about this claim that half the electrons must be spin up for a couple of reasons. Firstly, as you say, it can only apply to electrons that are in a determinate state and secondly, even if their total angular momentum were conserved it isn't obvious that it must be zero.

[swansont]

 

I am slightly doubtful about this claim that half the electrons must be spin up for a couple of reasons. Firstly, as you say, it can only apply to electrons that are in a determinate state and secondly, even if their total angular momentum were conserved it isn't obvious that it must be zero.

 

 

I said IF we start with spin zero. That's a given condition of the problem. One particle will then be measured spin up, and the other as spin down.

[Strange]

 

 

I said IF we start with spin zero. That's a given condition of the problem. One particle will then be measured spin up, and the other as spin down.

 

Ah, I missed that you had qualified it like that. I was re-reading the original post about it.

[disarray]

I confess that I am an amateur here and I gathered that finding out the spin of the first one tells you that the other electron has opposite spin (but does not make the other one have an opposite spin), though I was under them impression that the other that they both had indeterminate spin until someone looked (measured one of them).

 

However, when I read about experiments, I often read that a physicist somehow changed the spin of the first electron and then the other one changed its spin (to the opposite) accordingly.

 

But ultimately, I was wondering about explanations wherein a physicist claims that there is some balance between spin up and spin down if one somehow knew what all the electrons in the universe were doing as a prelude to explaining how entanglement works, as I mentioned earlier with reference to coin arcade.

 

If electron spins are indeterminate (as if there is some sort of superposition of spins) until measured, how can one even say that half are one way and half the other?

[imatfaal]

I confess that I am an amateur here and I gathered that finding out the spin of the first one tells you that the other electron has opposite spin (but does not make the other one have an opposite spin), though I was under them impression that the other that they both had indeterminate spin until someone looked (measured one of them).

 

However, when I read about experiments, I often read that a physicist somehow changed the spin of the first electron and then the other one changed its spin (to the opposite) accordingly.

 

But ultimately, I was wondering about explanations wherein a physicist claims that there is some balance between spin up and spin down if one somehow knew what all the electrons in the universe were doing as a prelude to explaining how entanglement works, as I mentioned earlier with reference to coin arcade.

 

If electron spins are indeterminate (as if there is some sort of superposition of spins) until measured, how can one even say that half are one way and half the other?

 

When entangled both particles share a state of quantum superposition (this is a joint state which cannot be simplified into two normal states) - measurement perforce means that the measured particle is now in a normal state as it breaks the entanglement; the non-measured particle is also in a normal state

[Strange]

However, when I read about experiments, I often read that a physicist somehow changed the spin of the first electron and then the other one changed its spin (to the opposite) accordingly.

I think that is an example of sloppy journalism (quite possibly, the journalist doesn't understand either).

 

If electron spins are indeterminate (as if there is some sort of superposition of spins) until measured, how can one even say that half are one way and half the other?

 

I don't think you can.

[swansont]

I confess that I am an amateur here and I gathered that finding out the spin of the first one tells you that the other electron has opposite spin (but does not make the other one have an opposite spin), though I was under them impression that the other that they both had indeterminate spin until someone looked (measured one of them).

 

However, when I read about experiments, I often read that a physicist somehow changed the spin of the first electron and then the other one changed its spin (to the opposite) accordingly.

 

 

That's the bad reporting I was referring to.

 

If electron spins are indeterminate (as if there is some sort of superposition of spins) until measured, how can one even say that half are one way and half the other?

 

Half will be measured to be one state and the other half in the opposite state, if you started with zero spin, because of conservation laws.