[Dimensional (OP)]

How do we know that two electrons, for example, are entangled?  When we observe one electron, how do we know that the other has taken on the opposite spin at that moment?

I hope this isn't the most obvious question ever asked here, but I don't think I have ever understood the answer.

I think I have a pretty decent understanding of QM and physics in general, but if someone can explain it simply that would be very much appreciated.

[Zer0]

I don't understand QM.
But I'll still give it a go...

I suppose they search for an atom home.
& Push out the hubby & wifey electrons.
Then separate them thru vast distances.
Whenever they ask the hubby a question for which the answer is a resounding Yes!
By commonsense they know the wifey shall say No!
(ha ha)

Perhaps the vast distances matter not.
Perhaps it's about fundamental differences.
But still, a wave of emotions keeps them connected.
Maybe they simply cordially agree to disagree.

[Zer0]

When we observe one electron, how do we know that the other has taken on the opposite spin at that moment?

You don't obviously. If you could, a message could be sent faster than light using such detection.

Can entangled electrons have an identical spin Sometimes?
Or do they have to or must have opposite spins Always?
(Total Angular Momentum)


ps - could QE be used as a Coin Toss?

Say I'm on planet A & U on planet B.
Distance between A & B is 10lys.
A BORG Armada approaches Us.
We can either fight or flight.

We predecided if ever such a grave time comes, one of Us shall stay back n fight, giving the other an opportunity to flee n colonize elsewhere.

Rather than send encrypted SOS signals to each other which would take 10yrs to arrive & 10yrs for a meaningful response & which also would be detected by & decrypted by the enemy at the gates...

We both open up the little entangled black boxes We exchanged while departing..

Instantaneously, We both shall know which one of Us, has an opportunity to Survive & which one gets the opportunity to make the ultimate Sacrifice.

[Dimensional (OP)]

When we observe one electron, how do we know that the other has taken on the opposite spin at that moment?

You don't obviously. If you could, a message could be sent faster than light using such detection.

Hmmm, that definitely goes against everything that I have ever read about quantum entanglement.  Are you adopting some kind of QM interpretation outside of the more mainstream interpretations?

The reason why I ask is that the information that gets sent faster than light is said to be unuseful information.  In at least two mainstream interpretations of QM that I can think of off the top of my head, the information that gets sent has no causal effect on anything. 

I am interested to know how you came to your answer. 

[Halc]

Hmmm, that definitely goes against everything that I have ever read about quantum entanglement.

You don't reference anything that makes the claim that there is some way of detecting when the other side has made its measurement, so if you find a site that claims such a possibility, you need to upgrade the quality of pop website from which you're getting your information.

Are you adopting some kind of QM interpretation outside of the more mainstream interpretations?

You're asking about an empirical test, and no interpretation makes different empirical predictions, hence I'm speaking from quantum theory, not any particular interpretation.

The reason why I ask is that the information that gets sent faster than light is said to be unuseful information.  In at least two mainstream interpretations of QM that I can think of off the top of my head, the information that gets sent has no causal effect on anything.

Counterfactual (or as some say: realist) interpretations posit faster than light causality, yes, but that's nothing that can be verified, else all the local interpretations would be falsified.

Can entangled electrons have an identical spin Sometimes?

Your speaking of a counterfactual here: the unmeasured spin of an electron. There's no evidence that there is such a thing. We only have measurements. It's similar to saying that photons exist and have a location 'en-route' before they are measured.
If you measure (along the same axis) the spin of entangled particles, the spins will always be correlated. They therefore won't be found to have identical spin when compared.

(Total Angular Momentum)

Spin of an electron has nothing to do with angular momentum. They don't have a moment, so the classical property of angular momentum cannot apply.

ps - could QE be used as a Coin Toss?

Any quantum measurement (spin say) can serve as a random coin toss. There's no requirement for using anything entangled.

Instantaneously, We both shall know which one of Us, has an opportunity to Survive & which one gets the opportunity to make the ultimate Sacrifice.

You're describing sending of a faster than light message via entangled particles. This cannot be done. Each side can choose to measure the particle at any time, and unless the result of the measurement of the other side has already been communicated (by say flashing lights), it will be totally unpredictable. There's no message in that.

[Dimensional (OP)]

Hmmm, that definitely goes against everything that I have ever read about quantum entanglement.

You don't reference anything that makes the claim that there is some way of detecting when the other side has made its measurement, so if you find a site that claims such a possibility, you need to upgrade the quality of pop website from which you're getting your information.

It's not pop science.  Read,

"The postulates of quantum theory seem to indicate that the state collapse happens instantaneously, regardless of the distance separating the particles.  Imagine that we prepare the two-particle state in a laboratory on Earth. Particle  A
  is then transported to the laboratory of Alice, in the Alpha Centauri star system, and particle  B
  is transported to the laboratory of Bob, in the Betelgeuse system, separated by  ∼640
  light years. ... Once ready, Alice measures  S^z
  on particle  A
 , which induces an instantaneous collapse of the two-particle state. Immediately afterwards, Bob measures  S^z
  on particle  B
 , and obtains?with 100% certainty?the opposite spin."

This was from a free online textbook from, https://phys.libretexts.org/Bookshelves/Quantum_Mechanics/Quantum_Mechanics_III_(Chong)/03%3A_Quantum_Entanglement/3.03%3A_The_Einstein-Podolsky-Rosen_Paradox

[Halc]

It's not pop science.  Read

It doesn't look accurate at all to me.

"The postulates of quantum theory seem to indicate that the state collapse happens instantaneously[/quote]Perhaps it only 'seems' this way if other postulates are assumed, such as typical postulates of classical physics. This is simply wrong since there are valid interpretations which don't suggest collapse at all, and many (most of them) which do not posit "instantaneous" (in scare quotes because it's totally undefined) action at a distance.

Immediately afterwards, Bob measures  S^_z on particle  B, and obtains --with 100% certainty-- the opposite spin."

Quantum theory says no such thing. For one thing, the statement is a counterfactual. Alice has measured Bob, so what Bob is doing is a meaningless assumption in absence of assuming counterfactuals (realism). The there's the "immediately" which is totally ambiguous given relativity of simultaneity. Depending on the frame of choice, any time over the course of over 1200 years could be considered simultaneous with Alice's measurement. So the statement seems to presume absolute time, which is unforgivable for physics textbook. Bob has not measured Alice and so obtains a random outcome every time. That's what empirical tests say, and is what the theory says.

The only thing the entanglement brings into the picture is that when the two measurements are compared by some common entity, the two will be found to be correlated. No mention of that I see. The whole page doesn't mention the word 'correlate' at any point. Ouch...

Find a better book than the Chong one.

[Dimensional (OP)]

It's not pop science.  Read

It doesn't look accurate at all to me.

"The postulates of quantum theory seem to indicate that the state collapse happens instantaneouslyPerhaps it only 'seems' this way if other postulates are assumed, such as typical postulates of classical physics. This is simply wrong since there are valid interpretations which don't suggest collapse at all, and many (most of them) which do not posit "instantaneous" (in scare quotes because it's totally undefined) action at a distance.

Immediately afterwards, Bob measures  S^_z on particle  B, and obtains --with 100% certainty-- the opposite spin."

Quantum theory says no such thing. For one thing, the statement is a counterfactual. Alice has measured Bob, so what Bob is doing is a meaningless assumption in absence of assuming counterfactuals (realism). The there's the "immediately" which is totally ambiguous given relativity of simultaneity. Depending on the frame of choice, any time over the course of over 1200 years could be considered simultaneous with Alice's measurement. So the statement seems to presume absolute time, which is unforgivable for physics textbook. Bob has not measured Alice and so obtains a random outcome every time. That's what empirical tests say, and is what the theory says.

The only thing the entanglement brings into the picture is that when the two measurements are compared by some common entity, the two will be found to be correlated. No mention of that I see. The whole page doesn't mention the word 'correlate' at any point. Ouch...

Find a better book than the Chong one.

No Halc, I will not find a better reference.  This has got to stop.  You are way out of control.  You can't possibly expect me to believe you over a MIT graduate working for a very reputable university at NTU (not to mention the hundreds articles of "non-popsci" I have read over the years that say the exact same thing). 

And your understanding of simultaneity is wrong.  Two locations in space can have the same time as long as they are at rest relative to one another and be in locations with the same gravitational potential.

[evan_au]

How do we know that particles are entangled?

Requirements include:
- they came from the same entanglement "factory", ie
- they have a shared history,
- and a shared quantum state.
- so their state is correlated

Some quantum "factories" are very inefficient; for example, the parametric down-converter method has just 1 photon in millions entangled. Much better methods of entanglement are needed for a workable quantum computer - typically these involve electromagnetic fields operating on electrons.
https://en.wikipedia.org/wiki/Spontaneous_parametric_down-conversion#Example

Can entangled electrons have an identical spin Sometimes?
Or do they have to or must have opposite spins Always?

In the Spontaneous parametric down-conversion example provided above, Wikipedia lists two crystals with different chemistries - one generates entangled photons with parallel spins; another crystal generates entangled photons with opposite spins.
- But the dominant non-entangled photons are not correlated.

[Dimensional (OP)]

How do we know that particles are entangled?

Requirements include:
- they came from the same entanglement "factory", ie
- they have a shared history,
- and a shared quantum state.
- so their state is correlated

Some quantum "factories" are very inefficient; for example, the parametric down-converter method has just 1 photon in millions entangled. Much better methods of entanglement are needed for a workable quantum computer - typically these involve electromagnetic fields operating on electrons.
https://en.wikipedia.org/wiki/Spontaneous_parametric_down-conversion#Example

Can entangled electrons have an identical spin Sometimes?
Or do they have to or must have opposite spins Always?

In the Spontaneous parametric down-conversion example provided above, Wikipedia lists two crystals with different chemistries - one generates entangled photons with parallel spins; another crystal generates entangled photons with opposite spins.
- But the dominant non-entangled photons are not correlated.

I will give a basic example of what I mean.

I know that they will entangle 2 electrons for example.  Then they separate them.  When they observe the spin of one electron, they instantly know the spin of the other.  How do they know that the spins of each electron were not predetermined when they were entangled. 

This is a very common question from people like me that do not know a lot about the details of entanglement.  They do have an answer to this question, but I can never understand it.

[Origin]

How do they know that the spins of each electron were not predetermined when they were entangled. 

Bells inequality.

[Zer0]

Can entangled electrons have an identical spin Sometimes?

Your speaking of a counterfactual here: the unmeasured spin of an electron. There's no evidence that there is such a thing. We only have measurements. It's similar to saying that photons exist and have a location 'en-route' before they are measured.

I'm not quite sure what you mean by, or how you define
 " Counterfactual " .
The inquiry is about Spin of Entangled Electrons.
Only when We are sure about Them being in the ' Singlet State ' & then Measure the Spins & always find a Correlation, then can We not conclude that Their spins would Always be Correlated?
(along same axis)

If you measure (along the same axis) the spin of entangled particles, the spins will always be correlated. They therefore won't be found to have identical spin when compared.
This is Exactly what i was asking, Thanks!

[Zer0]

(Total Angular Momentum)

Spin of an electron has nothing to do with angular momentum. They don't have a moment, so the classical property of angular momentum cannot apply.

Would it have made sense if i would have brought up ' Total Angular Momentum ' w/r/t
' Entangled Particles ' ?

I had read if 2 particles form from a single source of Energy.
They should be considered as not Independent, but part of a same system.

Somewhat like an extended wave function.
& the total angular momentum of the Universe must stay Constant, hence They ought to have Opposite spins, which can then cancel out.

Spin = 0.
(makes sense?)

[Zer0]

ps - could QE be used as a Coin Toss?

Any quantum measurement (spin say) can serve as a random coin toss. There's no requirement for using anything entangled.
Yes!
Any quantum spin measurement could indeed serve as a Coin Toss.
But my limited imagination is Not capable of fantasizing how that would work out between two points A & B separated by 10blys.

All i can think of is A making the measurement & sending it to B, or vice versa.

Anyhow, that would take 10blyrs of Time.
Kinda futile, don't U think?

[Zer0]

Instantaneously, We both shall know which one of Us, has an opportunity to Survive & which one gets the opportunity to make the ultimate Sacrifice.

You're describing sending of a faster than light message via entangled particles. This cannot be done. Each side can choose to measure the particle at any time, and unless the result of the measurement of the other side has already been communicated (by say flashing lights), it will be totally unpredictable. There's no message in that.

What i am trying to describe or propose is to Create a Quantum Coin.

Then Slice it thru the middle in such a manner that both the Sides of the Coin(heads & tails) can be Separated spatially.

Take them apart at a distance of 10 billion light years by carefully orienting them along the same Axis.

Provided the Singlet state does Not break down due to Decoherence.
Concluding both sides shall be Correlated to preserve total angular momentum of the Universe.
Spin = 0.

Until any side(A or B) makes their measurements, it would be Unknown who has the Heads & who the Tails.

Once measured by (A), there will Not be any need to wait for the other side(B) to take their measurements or even convey the message to each other at a snail pace of " c " .

If (A) gets Heads.
They know (B) has or will get Tails.

Nothing will be conveyed between A & B at FTL.

Commonsense shall Prevail!
(Workable?)

[Eternal Student]

Hi.

@Zer0   --->   Yes you could make a quantum coin toss in some way and use it for the purposes you described much earlier   (having one person sacrifice themselves to the Borg Armada to buy time for the other,  while the other tries to get away).    However, you could do this without any quantum entagled particles ever being needed.
     Just use bits of paper and a hat.   Cut two equal sized squares of paper.   Write on one   "You will sacrifice yourself",   write on the other   "You will try to get away".    Put them in a hat and shake the hat.   Have one person pull a piece of paper out (as usual without looking).   The other person takes the last remaining piece of paper.   Just to keep it in the style you wanted, let's say no-one unfolds their square of paper and reads it,  they just put it away in their top pocket.   Then they separate and go off to do whatever,  with the understanding that if and when a Borg Armada does arrive they must read their bits of paper and do what is written.
     Now, provided no-one looks at their paper until (or unless) a Borg Armada arrives, neither person did know what they were supposed to do and when the Aramda does come into sight they do both know what to do without needing to relay messages between themselves.   Although it's a big Armada if the two people really were 10 billion light years away from each other and they did both see it - but that's a different issue.

Best Wishes.

[Zer0]

@Eternal

Well, at the Rate at which We are cuttin & burnin em...
Even before We figure out the WarpDrive..
I don't suppose there's gonna be any Paper left!
(Thanks for your response thou)


@HaIc

Even if the Quantum Coin doesn't work, that should be No reason to Worry about the BORG.
Perhaps We aren't as Strong & as Superior as we'd like to think...
Maybe We are simply Not worth it!
(We're Safe)