Quantum Entanglement spin measurement

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Hi,

When a quantum entangled photon is measured to determine spin does it's spin stay in that orientation as long as it's measured it or does it immediately go back to a superpositioned state? In other words if you determined the spin of a quantum entangled particle at say 12:00 pm and constantly measured it, would it's spin stay in the same direction at 12:02 pm. or is the measurement only good for the exact moment it was initially measured?

Thanks in advance for the answer.
 

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  • #2
Nugatory
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(If it's a photon, we aren't talking about the spin, we're talking about the polarization).

But with that said, the first measurement on either of a pair of entangled particles breaks the entanglement so there is no return to the entangled state.

Once the entanglement is broken, both particle evolve independently. Suppose we start with a pair of photons with entangled polarization (the state of the quantum system consisting of both photons is a superposition of "photon A is horizontally polarized, photon B is vertically polarized" and "photon A is vertically polarized , photon B is horizontally polarized"). When we measure either photon the wave function collapses to either "photon A is horizontally polarized, photon B is vertically polarized" or "photon A is vertically polarized , photon B is horizontally polarized", whichever is consistent with our measurement result.

And now we just have two photons, each with a definite polarization. They'll stay that way unless and until we do something to one of them that changes its polarization; if we do the other one will be unaffected.
 
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Hi Nugatory,

Thank you for your reply, explanation and clarification on spin vs polarization in reference to photons. I thought measuring the photons might brake the entanglement.

So if I understand this correctly, we can't know the polarization of either of the entangled photons without measuring them. Is it possible when the entangled photons are created what appears to be a superpositioned state is simply, photon "a" is in a fixed polarized state and photon "b" is in a fixed but opposite polarized state?
Wouldn't this be a simpler explanation of why when we simultaneously measure the polarization of entangled photons separated by great distances they are opposite?

Thanks Again,
 
  • #4
Nugatory
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Wouldn't this be a simpler explanation of why when we simultaneously measure the polarization of entangled photons separated by great distances they are opposite?
It would be.... but it turns out that there are differences between any such theory and the quantum mechanical prediction; these differences are experimentally testable; the experiments have been done; and the results agree with quantum mechanics and are inconsistent with the two photons having been created with fixed but opposite polarizations.

Google for “Bell’s Theorem” and look at the web page managed by our own @DrChinese https://www.drchinese.com/Bells_Theorem.htm
 
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  • #5
hmmm27
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So, if I read that right then the simple(ish) explanation is that
a) particles retain attributes once set, and
b) entanglement isn't forever, therefore
c) since observation of entangled particle pairs show complementary attributes, and
d) observation of particle pairs which have lost entanglement show random attributes, therefore
e) attributes of an entangled pair aren't set until an observation.

Does the above make any sense ?

(the logic looks a bit circular though, ie: how can you tell if a particle pair is entangled?)
 
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Did the above make any sense ? Was it correct ?
For #a, it depends on what it is we are measuring. If we measure the position of a moving particle, we get a result but because the particle is moving that result is only good for a single moment. On the other hand, if we measure the electric charge of an electron we will get ##-e##, every single time. Schrödinger’s equation will tell us which observables are fixed and which may change as the quantum system continues to evolve.

in #c and #d the results are always random. The interesting question is how the random results at one detector are correlated with the random results at the other.

The ”therefore” in #e isn’t right - #e is not a consequence of the other four. All five follow from the mathematical formalism of quantum mechanics.
(the logic looks a bit circular though, ie: how can you tell if a particle pair is entangled?)
We can’t. We have to know up front that it was created by a process that creates entangled pairs.

We can, however, test whether whatever process we’re using is in fact creating entangled pairs. A particle pair comes out of our particle source and we measure the vertical spin of both particles; one is up and one is down. That will happen half the time with any two particles that just happen to be wandering through our lab (it’s the same situation as flipping two coins - they’ll match half the time). So we do it again with the next pair, and again, and again..... after we get mismatches every single time in a few hundred trials we can be certain that the pairs coming out of our particle source really are entangled
 
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Does the above make any sense ?
Not really, no. "particles retain attributes once set" - what does that mean? What is an "attribute"? What does it mean to "set" it? Can it never be set to another value? (Similar points could be raised with the other statements) I'm not saying what you wrote is wrong - I am saying it is so unclear as to preclude communication.

Many people have gone down this path - to try and get the exact collection of words needed to understand a physics theory. This seldom works.
 
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  • #8
DrChinese
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So, if I read that right then the simple(ish) explanation is that
a) particles retain attributes once set, and
b) entanglement isn't forever, therefore
c) since observation of entangled particle pairs show complementary attributes, and
d) observation of particle pairs which have lost entanglement show random attributes, therefore
e) attributes of an entangled pair aren't set until an observation.

Does the above make any sense ?

(the logic looks a bit circular though, ie: how can you tell if a particle pair is entangled?)
Sometimes explanations such as yours are convenient for casual conversation. Physicists use such shortcuts all the time. But the "full context" of a particular setup may have many details that change one or more items. And often you get into situations where any explanation will fail.

Generally, it is not possible to determine (by measurement) if a single particle pair is entangled. Normally, it takes statistical analysis of a sufficiently large group. On the other hand, there are sources of entangled photon pairs with high fidelity (almost all pairs are actually entangled).
 
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vanhees71
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State determination is not a simple issue, but of course it can be done (at least in principle) with a large ensemble of equally prepared systems. For a thorough discussion, see

Ballentine, Quantum Mechanics - A Modern Development, World Scientific
 
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hmmm27
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Thankyou ; I think I'll take a closer look at the basic experiments.
 
  • #11
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If you measure the spin along some direction d, on one of an entangled pair of electrons at 12:00 pm,
its spin in the direction d, will thereafter remain what it was measured to be.

If it's + along d at 12:00pm it will still be + along d at 12:02 pm, and 12:03 pm etc...

But it will thenceforward no longer be entangled with its formerly entangled twin.
However ....
If you THEN measure its spin in some other direction, x, you will find that it is either + along direction x or - along direction x, and this orientation in the x direction will likewise be stable over time.

BUT... if you now REmeasure the spin in direction d, you will sometimes find it to be the OPPOSITE along direction d to what it was before you made the x-direction mesurement.

If direction x is perpendicular to direction d, then the probability of the spin being the same as it was before the x-direction measurement, will be 50%.

In general the probability is given as "the square of the cosine of half the angle" between directions d and x.

[Unacceptable reference deleted by Mentors]
 
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