# Entangled Particle Detection

My question about quantum entaglement is: is a Quantum particle's spin [altered] into another spin position at the moment of detection or is it just a 'snapshot picture' of the spin at the moment of detection (without alteration)? It seems this is an important differentiation. If there is no true alteration, we are seeing something obvious.

Since the entangled 'twin' particle always has an opposite spin relative to the other, would it not be obvious that a detector for Entangled particle number 1 show a spin which then is always opposite to the spin for particle 2? In other words, what really is actually 'spooky' about a state which [originated] at the time the particles were one and the same, before they were split and after they became entangled, always remain constant relative to the other? They would always be spinning opposite to each other and detector # 2 would show this. Would this not be a natural outcome of what is expected?
Does the detector truly alter the spin of Quantum particle#1 ? Has this been proven?

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Nugatory
Mentor
In other words, what really is actually 'spooky' about a state which [originated] at the time the particles were one and the same, before they were split and after they became entangled, always remain constant relative to the other?
The problem appears when we choose to measure the spins of the two particles on different axes.

Say we measure the spin of one particle along one axis, and then measure the spin of the other particle along another one at an angle ##\theta## relative to the first. The probability that both particles will be aligned along their axis is predicted by quantum mechanics and experimentally confirmed to be ##\sin^2\frac{\theta}{2}## (try this for ##\theta=0## and ##\theta=\pi## to verify that it says that if one particle is spin-up in a given direction the other one will always be spin-down in that direction).

In 1965 John Bell proved that no relationship between the spins when the pair is created can produce this ##\sin^2## probability for all values of ##\theta##. That's Bell's theorem, and you will find much detailed explanation at https://www.drchinese.com/Bells_Theorem.htm (maintained by our own @DrChinese).

• ScienceMike33
PeroK
Homework Helper
Gold Member
My question about quantum entaglement is: is a Quantum particle's spin [altered] into another spin position at the moment of detection or is it just a 'snapshot picture' of the spin at the moment of detection (without alteration)? It seems this is an important differentiation. If there is no true alteration, we are seeing something obvious.

Since the entangled 'twin' particle always has an opposite spin relative to the other, would it not be obvious that a detector for Entangled particle number 1 show a spin which then is always opposite to the spin for particle 2? In other words, what really is actually 'spooky' about a state which [originated] at the time the particles were one and the same, before they were split and after they became entangled, always remain constant relative to the other? They would always be spinning opposite to each other and detector # 2 would show this. Would this not be a natural outcome of what is expected?
Does the detector truly alter the spin of Quantum particle#1 ? Has this been proven?
1) Entanglement only lasts until the system is measured. Any measurement of the system breaks the entanglement.

2) Quantum particles do not have a definite value of spin until they are measured. A measurement (or detector) does not so much change the spin of an electron, as the spin is the result of the measurement.

3) Prior to a measurement, the question of which way a particle is spinning, in general , makes no sense. It's not just something that is unknown or unknowable.

4) Enganglement arises through interaction.

5) Modern QM has been around for almost 100 years, and there has been a lot of rigorous experimental testing in that time. There is a wealth of experimental evidence for entanglement.

Thank you to both for your help in this. See my thought was that entangled particles, having a defined relationship to each other from the beginning, would maintain that relationship even at the superposition level after separation. So that any time the spin of Particle A for example, was detected, then the spin of Particle B, when it was detected, would have to be in a defined relationship to Particle A (no matter what the distance between them was)..

What I am understanding here is that the superpositons of both particles are independent (and random) relative to each other (so no mathematical relationship in terms of superposition relative to each other exists) at all times until detection, and hence, when Particle A is detected, it necessarily at that point of detection, [and only then], influences Particle B. That influence, over a very long distance, is thus the 'spooky influence at a distance that Einstein talked about.

PeroK
Homework Helper
Gold Member
Thank you to both for your help in this. See my thought was that entangled particles, having a defined relationship to each other from the beginning, would maintain that relationship even at the superposition level after separation. So that any time the spin of Particle A for example, was detected, then the spin of Particle B, when it was detected, would have to be in a defined relationship to Particle A (no matter what the distance between them was)..

What I am understanding here is that the superpositons of both particles are independent (and random) relative to each other (so no mathematical relationship in terms of superposition relative to each other exists) at all times until detection, and hence, when Particle A is detected, it necessarily at that point of detection, [and only then], influences Particle B. That influence, over a very long distance, is thus the 'spooky influence at a distance that Einstein talked about.
That's a very difficult post to answer because generally there is a confusion of ideas and terminology. It's not clear, for example, what you mean by the term "superposition" and what relevance it has to entanglement.

The whole point of entanglement is that the two particles are not independent. In fact, you cannot even describe the particles individually. You can only describe them as a two-particle system, upon which various measurements must correlate. For example, measurements of spin must be correlated in order to conserve the total spin of the system.

DrChinese
Gold Member
What I am understanding here is that the superpositons of both particles are independent (and random) relative to each other (so no mathematical relationship in terms of superposition relative to each other exists) at all times until detection, and hence, when Particle A is detected, it necessarily at that point of detection, [and only then], influences Particle B. That influence, over a very long distance, is thus the 'spooky influence at a distance that Einstein talked about.
A few points to add to what Nugatory and PeroK have already said:

1. Bell's Theorem is needed to understand why the entangled particle pair must be considered at different angles (rather than the same ones).

2. There is no sense - OTHER THAN BY ASSUMPTION - that you can say the first particle measured A affects the second measured B. You would be completely justified in saying B affects A. No one knows what actually happens inside the entangled system. For example: 2 particles can even be entangled AFTER they are measured. (I know this is counter intuitive, but it has been accomplished in the lab in a process called entanglement swapping. But that is a subject for a different thread. )

• entropy1, ScienceMike33 and PeroK
Thank you Dr. Chinese. Being trained and educated in medicine (and not grad physics), I have only recently come to know and be fascinated by quantum entanglement. I am seeking to understand in a crystal clear fashion, the steps involved in Quantum entanglement and more so, the proof (or refutation) of it through detectors, as I believe that the implications of QE for our society and future are enourmous (Quantum computing, etc). In fact his century may come to be known as the Quantum century. So thank you and indeed I will study Bell's Theorem.

The problem appears when we choose to measure the spins of the two particles on different axes.

Say we measure the spin of one particle along one axis, and then measure the spin of the other particle along another one at an angle ##\theta## relative to the first. The probability that both particles will be aligned along their axis is predicted by quantum mechanics and experimentally confirmed to be ##\sin^2\frac{\theta}{2}## (try this for ##\theta=0## and ##\theta=\pi## to verify that it says that if one particle is spin-up in a given direction the other one will always be spin-down in that direction).

In 1965 John Bell proved that no relationship between the spins when the pair is created can produce this ##\sin^2## probability for all values of ##\theta##. That's Bell's theorem, and you will find much detailed explanation at https://www.drchinese.com/Bells_Theorem.htm (maintained by our own @DrChinese).
Thank you Nugatory,
Appreciate your help. I will indeed study Bell's Theorem...

A few points to add to what Nugatory and PeroK have already said:

2. There is no sense - OTHER THAN BY ASSUMPTION - that you can say the first particle measured A affects the second measured B. You would be completely justified in saying B affects A. No one knows what actually happens inside the entangled system. For example: 2 particles can even be entangled AFTER they are measured. (I know this is counter intuitive, but it has been accomplished in the lab in a process called entanglement swapping. But that is a subject for a different thread. )
I think it is a very good assumption that the first particle measured affects the second particle measured. The only way you can say the second particle measured affects the first particle measured is if you believe the all knowing universe knows how all future events will unfold and lets the first particle to be measured in on this future knowledge.

Can this assumption be tested? Could you route the 2nd photon to be measured back to where the first photon was measured and then change how you will measure the 2nd photon based on what you know about the first photon. Would this experiment definitively answer this?

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Also,
a question is: have these detection experiments involved varying conditions such as application of an external magnetic field (e.g Zeeman effect) on the entangled electrons (or just the 1st electron?). Could such an application affect the measurements /outcome?

Nugatory
Mentor
I think it is a very good assumption that the first particle measured affects the second particle measured.
That assumption fails dismally if the two measurements are spacelike-separated because then some observers find that the second measurement happens before the first (this isn't a quantum mechanics thing, it's relativity of simultaneity from special relativity). Thus we have two coexisting, equally valid, and experimentally indistinguishable interpretations of the experimental data: particle A was measured first and that determined the result of the measurement of B; and particle B was measured first and that determined the result of the measurement of A.

And yes, Bell experiments have been done with spacelike-separated measurements.

DrChinese
Gold Member
I think it is a very good assumption that the first particle measured affects the second particle measured. ...

Can this assumption be tested? Could you route the 2nd photon to be measured back to where the first photon was measured and then change how you will measure the 2nd photon based on what you know about the first photon. Would this experiment definitively answer this?
Besides what Nugatory says: QM does not make a prediction related to time ordering of the measurements. Everything is the same when you say A affects B as when you say B affects A. There could even be some mutual interaction, although that is equally speculative.

That assumption fails dismally if the two measurements are spacelike-separated because then some observers find that the second measurement happens before the first (this isn't a quantum mechanics thing, it's relativity of simultaneity from special relativity). Thus we have two coexisting, equally valid, and experimentally indistinguishable interpretations of the experimental data: particle A was measured first and that determined the result of the measurement of B; and particle B was measured first and that determined the result of the measurement of A.
The assumption does not fail like you describe. First we don't have a combined theory of relativity and quantum mechanics so your point is already on thin ice. Second this assumption requires FTL (instance effect) and so it is clearly a violation of the speed of light. So again your statement is misguided and at best you are arguing, having already assumed the other assumption.

And yes, Bell experiments have been done with spacelike-separated measurements.
I very much understand the EPR experiment like that done by Alain Aspect in 1983 with spacelike-seperated measurements. And it is exactly the type of experiment these assumptions are based on, which I assume all of the experienced members like your self understand. In this experiment the polarizers are randomly oriented after the entangled photons are in flight so that it is impossible for any influence at the speed of light to have communicated between the measurements. Therefore you are left with a choice of 2 assumptions that don't violate reality. The first assumption (the one that I prefer and called good) is that there is instant FTL collapse (or call it shared state until the first measurement); and it is this first measurement that affects both photons. The alternative assumption would have to be that the first measurement knows ahead of time on how all other events in the universe will unfold in order to know what will be measured (or not) at some future point in space outside of the second measured photon's light cone. Assuming you have not left reality or you think there is some experimental flaw we missed, those seem to be the choices we are left with.

Nugatory
Mentor
The assumption does not fail like you describe. First we don't have a combined theory of relativity and quantum mechanics
We do - that's what quantum field theory is. What's missing is a complete unification of quantum mechanics and general relativity, which doesn't matter here.
Second this assumption requires FTL (instance effect) and so it is clearly a violation of the speed of light.
That's pretty much the point. The only way the first measurement can affect the second one is if the influence travels faster than light. Therefore the assumption that the first measurement affects the second is a "good assumption" (your words) only if you are willing to accept a violation of the speed of light.

Which it seems you are....
The first assumption (the one that I prefer and called good) is that there is instant FTL collapse (or call it shared state until the first measurement); and it is this first measurement that affects both photons.
....and that brings in all the logical problems that come with FTL. Which one is "first"?

We do - that's what quantum field theory is. What's missing is a complete unification of quantum mechanics and general relativity, which doesn't matter here.That's pretty much the point. The only way the first measurement can affect the second one is if the influence travels faster than light. Therefore the assumption that the first measurement affects the second is a "good assumption" (your words) only if you are willing to accept a violation of the speed of light.
Great, I think we understand each other.

Which it seems you are........and that brings in all the logical problems that come with FTL. Which one is "first"?
I am not aware of any logical problems with the FTL influence with respect to entanglement. Could you bring up one or two? Or post a link if you have one? I certainly would like to know why you or others don't think FTL influence in entanglement is a good assumption. Thanks.

Lord Jestocost
Gold Member
I am not aware of any logical problems with the FTL influence with respect to entanglement.
Quantum theory doesn’t need an assumption of FTL influence. The assumption is thus superfluous – from a theoretical point of view.

• entropy1
timmdeeg
Gold Member
1) Entanglement only lasts until the system is measured. Any measurement of the system breaks the entanglement.

2) Quantum particles do not have a definite value of spin until they are measured. A measurement (or detector) does not so much change the spin of an electron, as the spin is the result of the measurement.
So if both particles are measured at the same instant of time their quantum states are correlated. If B is measured later, is its quantum state still correlated to that of A (measured earlier)? If so, wouldn't this mean that the quantum state of B being no more entangled (due to the first measurement) has a definite value before measurement though. Or has the quantum state of B no well defined value after A was measured even though B's quantum state had a well defined value at the time A was measured?

PeroK
Homework Helper
Gold Member
So if both particles are measured at the same instant of time their quantum states are correlated. If B is measured later, is its quantum state still correlated to that of A (measured earlier)? If so, wouldn't this mean that the quantum state of B being no more entangled (due to the first measurement) has a definite value before measurement though. Or has the quantum state of B no well defined value after A was measured even though B's quantum state had a well defined value at the time A was measured?
You are confusing states with measurement values. Before measurement neither particle has an individual state; the two particle system has a state. The first measurement of either particle is a measurement of that two-particle state. These first measurements are correlated. Subsequent measurements are not correlated.

The state of each particle after measurement depends on the measurement made.

In the simple case where the total spin of the system is 0 and spin is measured about the same axis, then correlation implies equal and opposite spins. But, in the case of the Bell theorem, the measurements are about different axes and the correlation is subtler: in the sense that an experiment can distinguish between local hidden variables and quantum probability amplitudes.

timmdeeg
Gold Member
You are confusing states with measurement values. Before measurement neither particle has an individual state; the two particle system has a state. The first measurement of either particle is a measurement of that two-particle state. These first measurements are correlated. Subsequent measurements are not correlated.

The state of each particle after measurement depends on the measurement made.

In the simple case where the total spin of the system is 0 and spin is measured about the same axis, then correlation implies equal and opposite spins. But, in the case of the Bell theorem, the measurements are about different axes and the correlation is subtler: in the sense that an experiment can distinguish between local hidden variables and quantum probability amplitudes.
Ah, got it thanks