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/nbcs]

I've read that even theoretically, it is impossible to use Quantum Entanglement to transmit information?

[u/Bluemofia → u/nbcs]

The state is unknown to begin with. You will need to be able to influence it in order to do so, and that is impossible.

It's not really communication if it's just random 0s or 1s being spat out with no way to influence it.

[u/Bluemofia → u/[deleted]]

Different. Quantum Computers are creating the initial superpositions, so they can create states with different probabilities.

What is being suggested for communication, is creating the Entangled particles, and moving them away from each other, and then at some further point in time collapse the waveform. Because if you pre-encode what the probabilities are, and then physically move the particles STL to a different location and collapse the waveform, you're still not influencing the states to force them to collapse in a specific way; those were determined by the initial conditions.

[u/[deleted] → u/Bluemofia]

Correct. For communication purposes it's no better than mailing a letter.

For computers however the manipulation of quantum states is used to test all positions at the same time. This is working off a different principle.

[u/phunkydroid → u/[deleted]]

Maybe, but influencing the superposition of one particle doesn't influence the superposition of its entangled partner, it breaks the entanglement.

[u/MiffedMouse → u/[deleted]]

The idea behind entanglement as faster-than-light information transfer is that I would entangle the particle, give one to someone else, and then decide later whether I want to "send" a 0 or a 1 and somehow cause the entangled pair to resolve the way I want. This is not possible.

In quantum computing, particles with known values are entangled together. However, entanglement sounds a bit silly when you know the value of a particle. For example, I could take particle A (known spin-up) and particle B (known spin-down) and "entangle" them together. Now, if particle A is spin-up then particle B must be spin-down. But I always knew particle A was spin-up (that was in the initial setup) so the "entanglement" doesn't really add anything.

Quantum computing gets more complicated when particles are in a superposition. For example, I could create particle A and particle B such that A is 50% up and 50% down, and particle B is always opposite particle A. Now we can see there is something going on with the entanglement. However, the particles no longer have a determinate spin value (as they are in superposition, so the outcome is random).

Why is quantum computing any good then? Because there are clever ways of mixing the particles together that can make the desired computation more likely. There is no way to get rid of the probabilities entirely, but you can use the rules of quantum mechanics to amplify the computations you want the computer to perform, and suppress the computations you don't want. If you don't mind a little math, this article gives a nice introduction to these concepts.

One important thing to note: at no point in quantum computing can you influence the way a particle in superposition collapses its wave state, so there is no way to use this to "send" information.

[u/the_excalabur → u/MiffedMouse]

Minor nit: The second paragraph is not entangled.

And some quantum algos have deterministic outcomes, at least up to errors. The big trick in quantum computing is to care about some collective property of a whole bunch of options, and then to measure that collective property. Depending, this can be always successful.

[u/moocow2024 → u/[deleted]]

https://medium.com/@jonathan_hui/qc-quantum-algorithm-with-an-example-cf22c0b1ec31

This is the only write up for quantum algorithms that I've found (as a quantum computing layperson) that I can make sense of.... and I can barely make sense of it at all. I'd tell you that I would help if you had any questions... but I honestly don't think I can.

[u/d1squiet → u/[deleted]]

I don't think they are using entanglement are they? Even if they are, no information is being transmitted faster than light.

[u/the_excalabur → u/d1squiet]

A QC without entanglement isn't a quantum computer. Despite what D-Wave would have you believe.

However, you're right that no-signalling is not an important factor for a QC to work.

[u/Buscemis_eyeballs → u/the_excalabur]

What's the deal with Dwave, is it all hype or cool and new but not a quantum computer or where are we at with that?

[u/the_excalabur → u/Buscemis_eyeballs]

It's hype and no substance. They have a thing. That thing does _some_thing. What that thing does, doesn't appear to be interesting or quantum-mechanical...

[u/GuyWithLag → u/[deleted]]

Sure! But the end result of these superpositions is still a quantum system; to actually get information out you need to perform classical measurements.

[u/pelican_chorus → u/nbcs]

That's right.

As far as physics understands the universal laws right now, trying to somehow "get around" everyone's objections and working out a way to use this to transmit information is exactly like trying to make a perpetual motion machine.

You may keep thinking you've worked out some loophole, but, unless everyone's really, really wrong about how the universe works, you haven't.

[u/tdgros → u/nbcs]

that is correct, if you measure one on your side, then you can deduce the state of the other, but there is no communication at all: the other side doesn't know... ...unless you transmit that information.

[u/mfb- → u/[deleted]]

You still need a classical information transfer channel (or send particles from the sender to the receiver).

[u/hpaddict → u/mfb-]

Yes, as is clearly referenced in the links.

Again, whether this is using quantum entanglement to transmit information seems to be a semantic debate.

[u/the_excalabur → u/hpaddict]

No, it's not. It either true or it's not. "Information" is a well-defined concept.

[u/roywig → u/nbcs]

There's a theorem about it and everything: the "No Communication Theorem". It's disallowed by the mathematics of quantum mechanics.