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Quantum Entanglement Question

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Tue, 2012-09-11 04:50

Erenthia

Quantum Entanglement Question

This is a question I've been trying to find an answer to for several years now. (I only just now realized this might just be the perfect place to ask) As I understand it, QE communication is not currently thought to be able to facilitate FTL communication because while you can measure the spin of an entangled atom, the information about which atoms you measured must still be transferred at subluminal speed. On top of that, I've heard that there are elaborate "no signalling" theorems and most importantly, causality violations that QE communications can create. So the question I have is this: does that imply that it is impossible - even in principle - to control spin? For instance: Let's say Alice and Bob entangle 2 pairs of atoms and then travel to distant locations. They also carry atomic clocks and have prearranged times where they enter into zero acceleration/zero g reference frames. Bob then starts to alter the spins on his atoms. When he wants to send a 0 he causes each of his pair to have the same spin and when he wants to send a 1 he causes them to have opposite spins. This let's Bob send Alice a binary data stream instantly over any distance. If this is so then how are quantum computers ever supposed to work? I'm severely out of my depth here, but at least I'm trying :-)

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The end really is coming. What comes after that is anyone's guess.

Tue, 2012-09-11 07:32

Arenamontanus Arenamontanus's picture

Erenthia wrote:

Erenthia wrote:
As I understand it, QE communication is not currently thought to be able to facilitate FTL communication because while you can measure the spin of an entangled atom, the information about which atoms you measured must still be transferred at subluminal speed. On top of that, I've heard that there are elaborate "no signalling" theorems and most importantly, causality violations that QE communications can create.

FTL communication using QE is entirely made-up for EP. It is one of those parts I regard as optional, since it is not *needed* for anything important in the setting.

Quote:
So the question I have is this: does that imply that it is impossible - even in principle - to control spin? For instance: Let's say Alice and Bob entangle 2 pairs of atoms and then travel to distant locations. They also carry atomic clocks and have prearranged times where they enter into zero acceleration/zero g reference frames. Bob then starts to alter the spins on his atoms. When he wants to send a 0 he causes each of his pair to have the same spin and when he wants to send a 1 he causes them to have opposite spins. This let's Bob send Alice a binary data stream instantly over any distance.

When Bob alters the spin of the atom, the entanglement breaks. So he is free to set the spins to anything he likes, it is just that it has absolutely no effect on Alices spins. You can use entanglement for quantum encryption: when you *measure* the spin, the other spin will be compatible. But anything harsher than just looking at the spin breaks the entanglement, and even looking uses it all up. (Then there is the problem with "instant": relativity theory shows that there is nothing like "simultaneous". Different observers will see Alice and Bob do their spin-manipulations and measurements in different orders. FTL communication implies that you can send messages back in time.)

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Tue, 2012-09-11 18:59

Erenthia

Now when bob alters the spin

Now when bob alters the spin on his atom and breaks entanglement is there any way for Alice to know about it other than classical channels?

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The end really is coming. What comes after that is anyone's guess.

Wed, 2012-09-12 08:06

Arenamontanus Arenamontanus's picture

Erenthia wrote:Now when bob

Erenthia wrote:
Now when bob alters the spin on his atom and breaks entanglement is there any way for Alice to know about it other than classical channels?

Not from the quantum mechanics I know.

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Wed, 2012-09-12 16:00

Decivre

Arenamontanus wrote:When Bob

Arenamontanus wrote:
When Bob alters the spin of the atom, the entanglement breaks. So he is free to set the spins to anything he likes, it is just that it has absolutely no effect on Alices spins. You can use entanglement for quantum encryption: when you *measure* the spin, the other spin will be compatible. But anything harsher than just looking at the spin breaks the entanglement, and even looking uses it all up.

I always thought that observation itself is a form of spin manipulation. The question then becomes how much can you affect a particle before entanglement is broken. Is there a specific amount of impact that another particle can have before entanglement breaks? Do we know the limit? Couldn't we simply try to keep our manipulations below the threshold?

Arenamontanus wrote:
(Then there is the problem with "instant": relativity theory shows that there is nothing like "simultaneous". Different observers will see Alice and Bob do their spin-manipulations and measurements in different orders. FTL communication implies that you can send messages back in time.)

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Wed, 2012-09-12 18:36

Erenthia

Has there been any work done

Has there been any work done on entangling more than two atoms?

—

The end really is coming. What comes after that is anyone's guess.

Wed, 2012-09-12 18:39

Arenamontanus Arenamontanus's picture

Decivre wrote:Arenamontanus

Decivre wrote:
Arenamontanus wrote:
You can use entanglement for quantum encryption: when you *measure* the spin, the other spin will be compatible. But anything harsher than just looking at the spin breaks the entanglement, and even looking uses it all up.
I always thought that observation itself is a form of spin manipulation. The question then becomes how much can you affect a particle before entanglement is broken. Is there a specific amount of impact that another particle can have before entanglement breaks? Do we know the limit? Couldn't we simply try to keep our manipulations below the threshold?

Normally a measurement in quantum mechanics removes all entanglement for that qubit. Technically, it moves an observable to an eigenstate of the measurement operator. However, there are subtle complications like the https://en.wikipedia.org/wiki/W_state that retains some entanglement: one qubit is used up, but the other two qubits are still entangled. But there *are* mindbending partial measurements: http://arxiv.org/pdf/1105.2021v1.pdf Here you do certain operations that might not give you any information, yet change the quantum state. Among other things they allow you to entangle two particles, each with another entangled partner, to each other by messing with their partners ("splicing qubits"). It looks like you can get arbitrarily close to zero entanglement, yet amplify it through partial measures back up to 1 again! Unfortunately, any real measurement that produces a result you can see breaks the whole thing.

Arenamontanus wrote:
(Then there is the problem with "instant": relativity theory shows that there is nothing like "simultaneous". Different observers will see Alice and Bob do their spin-manipulations and measurements in different orders. FTL communication implies that you can send messages back in time.)

Of course different observers will see them do their spin manipulations in different order. There is a speed limit on light. The same is true for sound; If we sync up our voices and yell something simultaneously from a distance, observers hear our voices in different order based on position. This does not disprove simultaneity so much as it disproves the accurate observation of simultaneity. One has to be equidistant from the two events to determine whether simultaneity occurs.[/quote] It is worse than that. If you calculate when an event actually occurred from your measurements by backdating it by the time the signal took to reach you, you will still get a discrepancy from the measurements of other observers. In relativity, you can stand equidistant from Alice and Bob and yet see one of them send or receive before the other (if they are moving relative to you).

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Wed, 2012-09-12 19:22

Erenthia

on a side note, I'd really

on a side note, I'd really like to get started learning about quantum mechanics. I've looked at some of the online stuff like the MIT opensourceware, but it didn't seem like a very good introduction. I have a fairly strong math background, but very little when it comes to physics.

—

The end really is coming. What comes after that is anyone's guess.

Wed, 2012-09-12 20:21

Arenamontanus Arenamontanus's picture

I think I really "got" it

I think I really "got" it (insofar I have understood anything) when I found an axiomatic explanation that did not try to explain quantum mechanics in terms of physics. Most textbooks give some kind of historical overview, listing weird experiments that don't make much sense, and then try to map the physics onto a mathematical theory that come out of nowhere. Instead it stated the mathematical axioms of the theory, spent time exploring the things they implied, and *then* explained how it all mapped to physics. It made far more sense that way. From a mathematical point of view most of the theory is fairly simple - linear algebra, some boundary value differential equations, a bit of group theory (depending on what aspect you like to use). What it *means*... ah, that is another matter.

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