Imagining the EPR paradox/quantum entanglement

[tommmyyyy]

TL;DR

I frequently have problems imagining the EPR paradoxon, hence how I can imagine entangled states. Could someone explain me the error in my thoughts?

Dear community,

I frequently have problems imagining the EPR paradoxon, hence how I can imagine entangled states.

Let's say we have the Bell state of the basis /0> and /1>: Ψ = 1/√2 (\00>+\11>), where the first element of the ket belong to particle/qubit 1 and the second element to particle/qubit 2. After separating the two particles and measuring the state of particle 1, the superposition collapses and even though the distance, the outcome of measuring particle 2 is determined. EPR called this a paradoxon and that there is no analog to the macroscropic world/classical physics.

Well, it is hard to understand what's so strange about entangled states. If I understand it right, the system exists in a superposition although they are separated in space. But the superposition cannot be separated into a Kronecker/Tensor product. This is what entanglement means, right?

But how about the following thought:

BOX 1: dead cat \0> or living Cat \1>--Connection--BOX2: Poison that evaporates into BOX 1 \0> or no poison \1>

BOX1 in Tokyo.......Separation........BOX2 in New York

Let's open BOX2 now. There is no poisson. Hence, the Cat is alive. This also can be described as \00>+\11> and the outcomes of the "measurement", investigation here, are correlated.

I cannot see a loss of causality neither locality. Because the Boxes had a causal and local connection previously.

Wouldn't it be the same in the quantum world? In the system's perspective there is no superposition, but the superposition is only our mathematical description in order to define all eventualities.

Isn't superposition just a statistical concept in order to include all eventualities? But this doesn't mean that the physically the states are really in a superposition. Am I wrong?

The "spooky interaction at distance" is not spooky if the two entities have had interaction before separation. Logically the outcomes of investigations are correlated.

Where's my error? Why am I not baffled by quantum entanglement? Why do I think "so what"?

I feel somehow like the author of this paper:

https://arxiv.org/pdf/quant-ph/9805074.pdf

Kind regards!

 

[PeroK]

Summary:: I frequently have problems imagining the EPR paradoxon, hence how I can imagine entangled states. Could someone explain me the error in my thoughts?

Dear community,

I frequently have problems imagining the EPR paradoxon, hence how I can imagine entangled states.

Let's say we have the Bell state of the basis /0> and /1>: Ψ = 1/√2 (\00>+\11>), where the first element of the ket belong to particle/qubit 1 and the second element to particle/qubit 2. After separating the two particles and measuring the state of particle 1, the superposition collapses and even though the distance, the outcome of measuring particle 2 is determined. EPR called this a paradoxon and that there is no analog to the macroscropic world/classical physics.

Well, it is hard to understand what's so strange about entangled states. If I understand it right, the system exists in a superposition although they are separated in space. But the superposition cannot be separated into a Kronecker/Tensor product. This is what entanglement means, right?

But how about the following thought:

BOX 1: dead cat \0> or living Cat \1>--Connection--BOX2: Poison that evaporates into BOX 1 \0> or no poison \1>

BOX1 in Tokyo.......Separation........BOX2 in New York

Let's open BOX2 now. There is no poisson. Hence, the Cat is alive. This also can be described as \00>+\11> and the outcomes of the "measurement", investigation here, are correlated.

I cannot see a loss of causality neither locality. Because the Boxes had a causal and local connection previously.

Wouldn't it be the same in the quantum world? In the system's perspective there is no superposition, but the superposition is only our mathematical description in order to define all eventualities.

Isn't superposition just a statistical concept in order to include all eventualities? But this doesn't mean that the physically the states are really in a superposition. Am I wrong?

The "spooky interaction at distance" is not spooky if the two entities have had interaction before separation. Logically the outcomes of investigations are correlated.

Where's my error? Why am I not baffled by quantum entanglement? Why do I think "so what"?

I feel somehow like the author of this paper:

https://arxiv.org/pdf/quant-ph/9805074.pdf

Kind regards!

I didn't follow how Schrödinger's cat got into the EPR paradox.

The paper you reference looks dodgy to me.

The problem with quantum entanglement, as opposed to classical entanglement, is that quantum objects do not have well-deined dynamic properties until measured. The separated pair of particles cannot be said to have a definite spin until they are measured. But, if they are measured they alos have opposite spins.

The paradox, therefore, is that if the spin of a particle is only determined by nature upon measurement, then how does nature ensure that the two particles always have opposite spin?

If that doesn't give you something to think about, then you haven't understood it.

 

[bobob]

First, skip the entire live/dead cat thing. That is a terrible way to try to demonstrate anything in quantum mechanics, since trying to define a cat to fit the model is impossible not to mention that alive and dead aren't eigenstates of anything, if for no other reason, that neither of those terms even have a universally agreed upon medical definition.

You aren't baffled for several reasons, mostly because you don't have the concept of entanglement pinned down well enough to understand where the questions arise. The questions arise because in the typical epr experiment, the point is to make the two measurements simultaneously. What that means in special (or general) relativity, is that the measurements are spacelike separated and that you can perform Lorentz transformations to different frames in which either measurement occurs before the other. In other words, the spacelike separation means there is no way to time order the two measurements to justify saying one measurement caused the other.

 

[tommmyyyy]

I didn't follow how Schrödinger's cat got into the EPR paradox.

The paper you reference looks dodgy to me.

The problem with quantum entanglement, as opposed to classical entanglement, is that quantum objects do not have well-deined dynamic properties until measured. The separated pair of particles cannot be said to have a definite spin until they are measured. But, if they are measured they alos have opposite spins.

The paradox, therefore, is that if the spin of a particle is only determined by nature upon measurement, then how does nature ensure that the two particles always have opposite spin?

If that doesn't give you something to think about, then you haven't understood it.

Thank you PeroK!

The paper looks dodgy to me too. But I am experiencing trouble imagining the "myth" on it. And I guess you are right: If you don't understand quantum mechanics in terms of "why", you are on the right way, if you have a "so what" moment, you should go back and see if you made a mistake. This is why I asked.

Why Schrödinger's cat? - Well, actually Schrödinger's thought experiment was somehow a reaction to the EPR publication:

https://en.wikipedia.org/wiki/Schrödinger's_cat

http://www.fisicafundamental.net/relicario/doc/SituationinderQuantenmechanik_Schrodinger.pdf

§5: "Are variables really washed-out" (literally, I guess "hidden" is the better choice to translate, unfortunatelly I don't find an English translation of it.)

The crux of my question is: Is the spin only determined by nature by measuring it or is it already determined, but we cannot know it before measuring? If the first one is the case, I understand the paradox.

 

[PeroK]

The crux of my question is: Is the spin only determined by nature by measuring it or is it already determined, but we cannot know it before measuring? If the first one is the case, I understand the paradox.

That's the crux of QM and so-called hidden variable theories. The failure of Bell's theorem rules out local hidden variables.

The spin of a particle is not well-defined until we measure it. It's not something that we simply don't know. The's the crux of the Bohr/Einstein debates etc.

 

[RUTA]

Summary:: I frequently have problems imagining the EPR paradoxon, hence how I can imagine entangled states. Could someone explain me the error in my thoughts?

Dear community,

I frequently have problems imagining the EPR paradoxon, hence how I can imagine entangled states.

Let's say we have the Bell state of the basis /0> and /1>: Ψ = 1/√2 (\00>+\11>), where the first element of the ket belong to particle/qubit 1 and the second element to particle/qubit 2. After separating the two particles and measuring the state of particle 1, the superposition collapses and even though the distance, the outcome of measuring particle 2 is determined. EPR called this a paradoxon and that there is no analog to the macroscropic world/classical physics.

Well, it is hard to understand what's so strange about entangled states. If I understand it right, the system exists in a superposition although they are separated in space. But the superposition cannot be separated into a Kronecker/Tensor product. This is what entanglement means, right?

But how about the following thought:

BOX 1: dead cat \0> or living Cat \1>--Connection--BOX2: Poison that evaporates into BOX 1 \0> or no poison \1>

BOX1 in Tokyo.......Separation........BOX2 in New York

Let's open BOX2 now. There is no poisson. Hence, the Cat is alive. This also can be described as \00>+\11> and the outcomes of the "measurement", investigation here, are correlated.

I cannot see a loss of causality neither locality. Because the Boxes had a causal and local connection previously.

Wouldn't it be the same in the quantum world? In the system's perspective there is no superposition, but the superposition is only our mathematical description in order to define all eventualities.

Isn't superposition just a statistical concept in order to include all eventualities? But this doesn't mean that the physically the states are really in a superposition. Am I wrong?

The "spooky interaction at distance" is not spooky if the two entities have had interaction before separation. Logically the outcomes of investigations are correlated.

Where's my error? Why am I not baffled by quantum entanglement? Why do I think "so what"?

I feel somehow like the author of this paper:

https://arxiv.org/pdf/quant-ph/9805074.pdf

Kind regards!

Instruction sets (“instruction kits” in your link) work fine for explaining Bell state outcomes when Alice and Bob make the same measurement (as in your cat example). The mystery obtains when Alice and Bob make different measurements. Instruction sets predict no correlation but the Bell (triplet) state in your example predicts a negative correlation. Here is a paper that you should be able to follow https://arxiv.org/abs/1809.08231

 

[DrChinese]

Why am I not baffled by quantum entanglement? Why do I think "so what"?

I feel somehow like the author of this paper:

https://arxiv.org/pdf/quant-ph/9805074.pdf

Unfortunately, this paper is just hand-waving to completely ignore the EPR and Bell results. It doesn't matter if we are talking "cakes" or "recipes", to use the paper's analogy.

EPR implies/tells us that any spin measurement must be predetermined (if you assume locality). That is because any single spin measurement can be predicted with certainty, so it is an "element of reality". The collection of such possible predictions is "realism".

On the other hand: Bell shows us the contradiction with there being ANY such instruction set (recipe) or hidden data (cake). There cannot be such instructions+data that yields the quantum mechanical expectation value UNLESS you include the other entangled particle's measurement setting as a parameter. That would violate the locality assumption.

You can hand select the values at different angle settings for pairs of entangled particles. No matter how you try, there are no value sets that reproduce the stats. Try at angle settings 0 degrees, 120 degrees, and 240 degrees and you will quickly see the problem. There is no recipe that works. To make the stats work out, you need to know BOTH particles' measurement settings.