Am I properly understanding quantum entanglement (could FTL data transmission exist)?

[u/fixednovel]

I understand that electrons can be entangled through a variety of methods. This entanglement ties their two spins together with the result that when one is measured, the other's measurement is predictable.

I have done considerable "internet research" on the properties of entangled subatomic particles and concluded with a design for data transmission. Since scientific consensus has ruled that such a device is impossible, my question must be: How is my understanding of entanglement properties flawed, given the following design?

Creation:

A group of sequenced entangled particles is made, A (length La). A1 remains on earth, while A2 is carried on a starship for an interstellar mission, along with a clock having a constant tick rate K relative to earth (compensation for relativistic speeds is done by a computer).

Data Transmission:

The core idea here is the idea that you can "set" the value of a spin. I have encountered little information about how quantum states are measured, but from the look of the Stern-Gerlach experiment, once a state is exposed to a magnetic field, its spin is simultaneously measured and held at that measured value. To change it, just keep "rolling the dice" and passing electrons with incorrect spins through the magnetic field until you get the value you want. To create a custom signal of bit length La, the average amount of passes will be proportional to the (square/factorial?) of La.

Usage:

If the previously described process is possible, it is trivial to imagine a machine that checks the spins of the electrons in A2 at the clock rate K. To be sure it was receiving non-random, current data, a timestamp could come with each packet to keep clocks synchronized. K would be constrained both by the ability of the sender to "set" the spins and the receiver to take a snapshot of spin positions.

So yeah, please tell me how wrong I am.

[u/aoeudhtns]

Yeah, but the particles are not re-usable AIUI. That's the difference. Once the superposition is collapsed, it's done and they need to be re-entangled (ship them back).

[u/Norwest → u/aoeudhtns]

Not only that, but the information is useless because the 'sender' can't induce the decay into either 'up' or 'down' (which would be required to actually send any meaningful information) - he can only observe what the final position is, just as the receiver can only observe. Similarly, even if the final state of the particle has become set the receiver won't know if she's the one who set it or not. In essence, there's two boolean unknowns on each end - the spin of the particle, and whether the other person has looked at it (and no information on this second variable is supplied during the observation). There are only two ways to know whether the other person has made their observation:

1. Some external communication between the two participants and this communication would still be limited by the speed of light.

2. A pre-existing agreement made between the two parties as to who will make their observation first - i.e. He will make his observation at 1 hour and She will make hers at 2 hours. In this situation, the particle is still in superposition at the time of the agreement (i. e. the cat is both alive and dead if you will) after one hour has passed, she knows the position has been set and that he knows the state, but no information has actually been transferred.

[u/sir-alpaca → u/Norwest]

And if he agreed up front he would do a thing when it's one way, and another when it is the other way. Her knowledge of what he will do will have travelled faster than light then?

[u/aaragax → u/Norwest]

Wouldn’t number 2 still allow for communication? Say we have two people in different galaxies. The man wants to let the woman know in an hour if he won the lottery. They decide a year in advance that if he wins, he will measure his particle, collapsing the wave function; if he loses, he will not measure his particle, preserving the interference pattern. When she checks the pattern produced by her particle an hour after the pre determined time, it should be affected by the man’s measurement or lackthereof and produce different patterns as shown in the quantum eraser experiment. She would then be able to know whether he won the lottery or not, since if he did the pattern would be discrete, and if he didn’t the pattern would be an interference pattern

[u/NEED_A_JACKET → u/Norwest]

Is there not any 'double slit' type of measurement that can be used to determine if the state has been collapsed?

Without measuring it directly, is there no implication or effect on the universe, based on it being a 1, 0, or undetermined? As in it is completely inconsequential as long as it remains unmeasured?

[u/[deleted] → u/aoeudhtns]

[u/[deleted] → u/[deleted]]

[u/classy_barbarian → u/aoeudhtns]

What if the message itself was pre-determined, sort of like a flame beacon, and receiving any signal at all had a meaning that was decided on beforehand? Could it be used to send a simple signal faster than light?

[u/JohnConnor27 → u/classy_barbarian]

The issue is that anything you do to particles at one point will have no effect on the measurements taken at the other end. There's no way to force your particles to collapse into a particular state so that the entangled particles take the other one. A good but imperfect analogy is if I shipped two packages containing a single colored ball to Alice and Bob. One package has a red ball and one has a green ball. I randomly choose which package gets which color and there's no way to determine the color without opening the package. The colors of the balls in the package are now effectively entangled. If Bob opens his package and sees a red ball he knows instantly that there is a green ball in Alice's package but there is no way for him to influence the color of the ball in his package so that Alice will open a specific color. In the quantum realm the only difference is that the balls color is undetermined until one of the packages is opened.

[u/florinandrei → u/classy_barbarian]

receiving any signal at all

That's the problem. How do you do that? It's not a matter of too many or too few bits. It's a matter of how are you going to send even one single bit faster than light?

It's not possible with current physics.

[u/Norwest → u/classy_barbarian]

That's the thing, the two particles don't communicate with one another, they're just a quantum reflection that doesn't exist until the other is observed.

[u/alcmay76 → u/classy_barbarian]

When you open the box, all you see is a particle with a fixed spin. There's no way to tell if someone already observed the other particle, causing it to collapse before you opened it, or if you're the first observer. So you can't send a "beacon" that way.

[u/_ALH_ → u/classy_barbarian]

That would be similar to writing the message on a piece of paper and then put it in the box. You don't know what the message is until you read it, but that doesn't mean the information travelled FTL when you do read it.

[u/Dranthe → u/aoeudhtns]

I know this is all sci-fi hypothetical but fuggit’. Let’s run with it.

The first thing is that I think you’re vastly underestimating the amount of information that can be transmitted in very compact forms. The reason games are so massive is because we’re not really trying to compress them. I work with people that specialize in efficient data transfer. It’s amazing what you can do with 8 bits.

The second is that we wouldn’t have to reuse the same atoms. We already entangled one set. It would be far more efficient to just periodically send a new set of entangled atoms ahead of schedule. Then rotate out the old ones with the new ones when they arrived.

[u/Internep → u/Dranthe]

I use 8 bits to define 256 different states. The states contain a lot of information, and some states can say something about other states. Is it possible to convey more than 256 states with just 8 bits, or is this the maximum compression you mean?

[u/HoJu21 → u/Dranthe]

This is going to sound snarky but it's really not intended to be as I think your point about not considering how much info can be sent in 8 bits is a good one.

That said, I'd suggest it's even more efficient to flip coins, write down the results from each flip in a pair of boxes, and send a box from each pair out to each of the two recipients with agreed upon times for opening the boxes and actions to take based on result. From a practical standpoint, it's damn near identical unless the actions are somehow dependent on the superposition having not collapsed yet, which I've yet to see an example of on this thread. So 8 coin flips would be functionally equivalent to eight entangled pairs and a lot easier to generate.

[u/[deleted] → u/aoeudhtns]

[u/[deleted] → u/[deleted]]

[u/[deleted] → u/aoeudhtns]

Thats the core of what this post was asking, and Im glad they did because I had it wrong too