# Entanglement has me stumped - Help? (urgent)

Hi there. I have spent most of today reading and researching entanglement. Most website say something along the lines of if one photon has been observed to have a spin state of up, the entangled pair will automatically become spin down. They also say that if you change of interfere with one photon, the other is affected. What I don't understand is how the other is affected. For example does it mimic what happens to the first photon?

Hi there. I have spent most of today reading and researching entanglement. Most website say something along the lines of if one photon has been observed to have a spin state of up, the entangled pair will automatically become spin down. They also say that if you change of interfere with one photon, the other is affected. What I don't understand is how the other is affected. For example does it mimic what happens to the first photon?
We really only know the partner photon has been affected by measuring it; before measurement it is really hard to say if it acquires the same polarization.

I believe spin up and spin down is attributed to electrons, not photons.

In regards to photons, you have four bell-states in which the entangled photons could be found, so by finding one photon measured |H> polarization doesn't mean the other photon will be found |V> (the opposite of |H>), unless you know that the photons were entangled as one of two applicable bell-states.

So because the two entangled photons are linked, is it a theoretical possibility to transfer data between them? After all photons are what compose EM waves, which we use to transfer data around the world. So if we created two EM waves (from pairs of entangled photons) and separated them, and manipulated the first to carry certain information, could that information be transferred to the second wave?

Nugatory
Mentor
So because the two entangled photons are linked, is it a theoretical possibility to transfer data between them?

It is not possible. Let's say I measure one member of the pair on a particular axis and find a particular polarization. I then know what the result of a measurement on the same axis of the other member of the pair will be... But that's all I know. I don't know whether that other measurement has already been made, will be made in the future, or will never be made. All I can say is that if the other guy and I get together after the fact and compare notes we'll see that his results and mine are correlated. So until we get together and compare, the only information I have is my random results and the only information he has is his random results.

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Nugatory
Mentor
up, the entangled pair will automatically become spin down.

Strictly speaking, that would be entangled spin one-half particles - entangled photons have polarizations not spins and they differ by 90 degrees not the up/down 180 degrees spin difference. But that's beside the point here as the principle is the same - it only matters when you're calculating with the results from a specific experiment of one type or the other.

It is not possible. Let's say I measure one member of the pair on a particular axis and find a particular polarization. I then know what the result of a measurement on the same axis of the other member of the pair will be... But that's all I know. I don't know whether that other measurement has already been made, will be made in the future, or will never be made. All I can say is that if the other guy and I get together after the fact and compare notes we'll see that his results and mine are correlated. So until we get together and compare, the only information I have is my random results and the only information he has is his random results.

Then why the need for entanglement. What purpose does it serve in this context.

for example
If I know from google that the empire state building is 1454 feet tall.
Then I could just as well say I (bob) using my measuring apparatus (google) , am entangled with Alice in Manhattan with her measuring apparatus, who is going to measure its height(empire state) pretty soon an send me the info.

Does this mean we are entangled before the measurement , and our results are then correlated when we both confirm the height?

or am I confusing myself here , has decoherence played a part here?

Staff Emeritus
2021 Award
Why is this urgent?

DrChinese
Gold Member
So because the two entangled photons are linked, is it a theoretical possibility to transfer data between them? After all photons are what compose EM waves, which we use to transfer data around the world. So if we created two EM waves (from pairs of entangled photons) and separated them, and manipulated the first to carry certain information, could that information be transferred to the second wave?

Welcome to PhysicsForums!

Entanglement does not work like that. There is NO information to be transferred unless you call random outcomes information. Whenever you measure the polarization, for example, of an entangled photon, you get a random result. Alice sees randomness, and so does Bob (the usually observer names for entangled particle pairs).

BTW: photons are the same thing as electromagnetic waves, which is what light is.

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vanhees71
Nugatory
Mentor
Then why the need for entanglement. What purpose does it serve in this context.

for example
If I know from google that the empire state building is 1454 feet tall.
Then I could just as well say I (bob) using my measuring apparatus (google) , am entangled with Alice in Manhattan with her measuring apparatus, who is going to measure its height(empire state) pretty soon an send me the info.

Does this mean we are entangled before the measurement , and our results are then correlated when we both confirm the height?
No, you and alice aren't entangled at all. You're both measuring the same thing so not surprisingly you get the same result. To get to the weirdness of entanglement you have to go through two steps. First, let's try a different classical thought experiment (google for "Bertlmann's socks"):

We take a pair of gloves, one left-handed and one right-handed, we put each in its own box, and we mail one box to Bob and the other to Alice. If Bob opens his box and finds a left-handed glove, he immediately knows that Alice will find a right-handed glove when and if she opens her box. There's nothing mysterious about this; we prepared the contents of the two boxes so that it had to turn out that way. That's pretty much how people expected entanglement to work: I measured one spin, you measure the other, we assume it's because our particles were created with opposite spins so the result is unsurprising.

But now let's add two more attributes: some of the pairs of gloves are leather and some are wool, and some pairs are brown while others are black. Furthermore, we have the rule that Bob and Alice can only measure and record one attribute for each pair: If Bob finds a brown glove he knowns Alice's glove is also brown, but when they compare notes they find that Alice has looked at the handedness instead and found her glove to be left-handed. They will naturally conclude that the whole experiment started with a pair of brown gloves of unknown material and Bob got the right-handed one.

And here is where Bell's theorem comes in and entanglement gets weird. If they do a large number of runs... and they find that the number of left-handed brown gloves of unknown material that Bob receives (some of them Bob will have observed to be left-handed and we infer the color from Alice's result, some of them Bob will have observed to be brown and we infer the handedness from Alice's result) will be greater than the number of left-handed wool gloves of unknown color plus the number of brown leather gloves of unknown handedness.... then what? That's what quantum mechanics and Bell's inequality predicts, and experiment confirms. With subatomic entangled particles, you cannot assume that they acquired their properties when they were created.

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sanpkl and vanhees71
bhobba
Mentor
Then why the need for entanglement. What purpose does it serve in this context.

Entanglement has no classical analogue - it is uniquely quantum. In fact a deep analysis shows there are basically only two ways to model physical systems probabilistically - standard probability theory and Quantum Mechanics.

The interesting thing is what separates the two is entanglement:
http://arxiv.org/abs/0911.0695

Thanks
Bill

BTW: photons are the same thing as electromagnetic waves, which is what light is.

So are EM waves one photon , or more?

Entanglement does not work like that. There is NO information to be transferred unless you call random outcomes information. Whenever you measure the polarization, for example, of an entangled photon, you get a random result. Alice sees randomness, and so does Bob (the usually observer names for entangled particle pairs).

But what if you previously allocated value (say binary 0 and 1) to the polarisations, could you not by changing the polarisation of one photon, affect the other and hence communicate?

Nugatory
Mentor
But what if you previously allocated value (say binary 0 and 1) to the polarisations, could you not by changing the polarisation of one photon, affect the other and hence communicate?
No. See my post #4.

Remember, each observer only gets one measurement and then the entanglement is broken, and we don't even get to control the result of that measurement.

So I measure my particle and get a value. I have no idea whether I got that value because you have already measured your particle or because I've made the first measurement on the pair. So all I have is a random bit... No information passed from you to me.

We take a pair of gloves, one left-handed and one right-handed, we put each in its own box, and we mail one box to Bob and the other to Alice. If Bob opens his box and finds a left-handed glove, he immediately knows that Alice will find a right-handed glove when and if she opens her box. There's nothing mysterious about this; we prepared the contents of the two boxes so that it had to turn out that way. That's pretty much how people expected entanglement to work: I measured one spin, you measure the other, we assume it's because our particles were created with opposite spins so the result is unsurprising.

But now let's add two more attributes: some of the pairs of gloves are leather and some are wool, and some pairs are brown while others are black. Furthermore, we have the rule that Bob and Alice can only measure and record one attribute for each pair: If Bob finds a brown glove he knowns Alice's glove is also brown, but when they compare notes they find that Alice has looked at the handedness instead and found her glove to be left-handed. They will naturally conclude that the whole experiment started with a pair of brown gloves of unknown material and Bob got the right-handed one.

Thanks Nugatory , for that analogy. It really brings the point through quiet clearly.

I have wrestled with this weirdness for the past few weeks , trying to make sense of it.
I have been/read through numerous in depth discussions in this forum, regarding this.
Looking for loopholes and simpler classical explanations.
There just are none. QM is truly ground breaking.

I found another great lecture that helped me understand QM a little better

for those that have difficulty to make the transition, its a great start.
And it gets the point through quiet descriptively.

If Anybody knows where to get the next lectures , please let me know.
I have some deeper unanswered questions which were raised in this lecture, and most probably answered in the following ones.

Nugatory
Mentor
So are EM waves one photon , or more?

Neither :(

Even ordinary quantum mechanics isn't much help here; you need quantum field theory to form the right model. But loosely speaking, we can say that any EM radiation you've encountered is in a superposition of many different states with different numbers of photons.

Fortunately none of this matters for purposes of understanding photon entanglement. It is a consideration when you're actually designing a real experiment, as it is surprisingly difficult to produce single-photon sources (google for "parametric down-level conversion" to see how it is most easily done).

bhobba and vanhees71
Thank you! I really appreciate your help :)

stevendaryl
Staff Emeritus
Entanglement has no classical analogue - it is uniquely quantum.

I wouldn't say that it has no classical analog (you spell it your way, I'll spell it my way).

Entanglement occurs whenever the state of a composite system consisting of two spatially separated subsystems fails to factor into a product of states for each subsystem. In classical probability, if you view the "state" as a probability distribution, you can certainly have the situation where the joint probability distribution for two subsystems fails to be factorable into a product of subsystem probability distributions.

Quantum-mechanically, if $|\Psi\rangle$ is the state of the composite system, then the state is factorable if it can be written as:
$|\Psi\rangle = |\psi\rangle \otimes |\phi\rangle$ where $|\psi\rangle$ is the state of the first subsystem, and $|\phi\rangle$ is the state of the second subsystem. If the state cannot be written this way, then the two subsystems are "entangled".

In classical probability theory, if you have one system whose complete state is described by a parameter $\alpha$, and another system whose complete state is described by a parameter $\beta$, then the probability distribution $P(\alpha, \beta)$ for the composite system is factorable if it can be written in the form: $P(\alpha, \beta) = F(\alpha) \times G(\beta)$. When that is not the case, people don't use the word "entangled", but it seems analogous.

stevendaryl
Staff Emeritus
I wouldn't say that it has no classical analog (you spell it your way, I'll spell it my way).

Entanglement occurs whenever the state of a composite system consisting of two spatially separated subsystems fails to factor into a product of states for each subsystem. In classical probability, if you view the "state" as a probability distribution, you can certainly have the situation where the joint probability distribution for two subsystems fails to be factorable into a product of subsystem probability distributions.

Quantum-mechanically, if $|\Psi\rangle$ is the state of the composite system, then the state is factorable if it can be written as:
$|\Psi\rangle = |\psi\rangle \otimes |\phi\rangle$ where $|\psi\rangle$ is the state of the first subsystem, and $|\phi\rangle$ is the state of the second subsystem. If the state cannot be written this way, then the two subsystems are "entangled".

In classical probability theory, if you have one system whose complete state is described by a parameter $\alpha$, and another system whose complete state is described by a parameter $\beta$, then the probability distribution $P(\alpha, \beta)$ for the composite system is factorable if it can be written in the form: $P(\alpha, \beta) = F(\alpha) \times G(\beta)$. When that is not the case, people don't use the word "entangled", but it seems analogous.

What is different about QM is that entanglement (lack of factorability) can happen for pure states, while classically, it only happens for mixed states.

from the post below we get:

LB > LW + BLe

L - left handed
B - brown
W - wool
Le - leather

from the viewpoint of classical mechanics

- why is the above equation inconsistent with it's understanding of reality? or prediction

But what if you previously allocated value (say binary 0 and 1) to the polarisations, could you not by changing the polarisation of one photon, affect the other and hence communicate?
You cannot change the polarization to your choice.

Also you don't really know if the
No, you and alice aren't entangled at all. You're both measuring the same thing so not surprisingly you get the same result. To get to the weirdness of entanglement you have to go through two steps. First, let's try a different classical thought experiment (google for "Bertlmann's socks"):

We take a pair of gloves, one left-handed and one right-handed, we put each in its own box, and we mail one box to Bob and the other to Alice. If Bob opens his box and finds a left-handed glove, he immediately knows that Alice will find a right-handed glove when and if she opens her box. There's nothing mysterious about this; we prepared the contents of the two boxes so that it had to turn out that way. That's pretty much how people expected entanglement to work: I measured one spin, you measure the other, we assume it's because our particles were created with opposite spins so the result is unsurprising.

But now let's add two more attributes: some of the pairs of gloves are leather and some are wool, and some pairs are brown while others are black. Furthermore, we have the rule that Bob and Alice can only measure and record one attribute for each pair: If Bob finds a brown glove he knowns Alice's glove is also brown, but when they compare notes they find that Alice has looked at the handedness instead and found her glove to be left-handed. They will naturally conclude that the whole experiment started with a pair of brown gloves of unknown material and Bob got the right-handed one.

And here is where Bell's theorem comes in and entanglement gets weird. If they do a large number of runs... and they find that the number of left-handed brown gloves of unknown material that Bob receives (some of them Bob will have observed to be left-handed and we infer the color from Alice's result, some of them Bob will have observed to be brown and we infer the handedness from Alice's result) will be greater than the number of left-handed wool gloves of unknown color plus the number of brown leather gloves of unknown handedness.... then what? That's what quantum mechanics and Bell's inequality predicts, and experiment confirms. With subatomic entangled particles, you cannot assume that they acquired their properties when they were created.

bhobba
Mentor
from the post below we get:

LB > LW + BLe

L - left handed
B - brown
W - wool
Le - leather

Come again - to be blunt that's utter nonsense.

from the viewpoint of classical mechanics- why is the above equation inconsistent with it's understanding of reality? or prediction

Bells inequality follows from QM and is different to what naive reality says.
http://www.drchinese.com/David/Bell_Theorem_Easy_Math.htm

Thanks
Bill

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Nugatory
Mentor
from the post below we get:

LB > LW + BLe

L - left handed
B - brown
W - wool
Le - leather

from the viewpoint of classical mechanics

- why is the above equation inconsistent with it's understanding of reality? or prediction

Every LB must be either a LW or a BLe... So how can there be more LB than LW and BLe combined?

sanpkl
Every LB must be either a LW or a BLe... So how can there be more LB than LW and BLe combined?

Why can an LB not be a LLe?

It is because Alice/Bob have already measured as LW and BLe?

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Nugatory
Mentor
Why can an LB not be a LLe?

It is because Alice/Bob have already measured as LW and BLe?

Yes. Remember, they each get only one measurement, so you have to read "LB" as "a left-handed brown glove of unknown material". If whoever had decided to measure the color had instead chosen to measure the material instead, that particular pair of gloves would have been counted as either leather or wool gloves of unknown color.

sanpkl
I got the Bell's theorem, once again. And this time in a easier, to retain, way.

I can now check it off my to do list. ...;)

Thanks Nugatory.

bhobba
This is basically the whole reason of why "Quantum Entanglement" is sometimes called "Spooky Action At a Distance". It is because there is a type of superluminal force/action going on that is interacting with the other distant particle. If one is affected, the other is instantly altered to be perfectly opposite to it. This is the interesting part, "instantly altered", there is no time delay before the other is affected, it is peculiar. For instance, if I have managed to entangle two non-rotating electrons and have sent them 100,000 light years away from each other (somehow) and all of a sudden induce a rotation upon one of them (let's say up), the other is instantly affected and will have an opposite rotation (down) induces upon it. I hope this helps :D.