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Quantum entanglement could connect drones for disaster relief, bypassing traditional networks (phys.org)
submitted 10 months ago by to c/qubits@mander.xyz
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Any time you use a device to communicate information—an email, a text message, any data transfer—the information in that transmission crosses the open internet, where it could be intercepted. Such communications are also reliant on internet connectivity, often including wireless signal on either or both ends of a transmission.

But what if two—or 10, or 100, or 1,000—entities could be connected in such a way that they could communicate information without any of those security or connectivity concerns?

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[–] 11 points 10 months ago* (last edited 10 months ago) (7 children)

We can't. Nonlocal effects in quantum mechanics are all random. The influences being random means you cannot send signals with it. This is proven by the no-communication theorem. Einstein's theory of relativity, despite common misconception, does not disallow faster than light effects. It only disallows them if they can be used to signal, because this superluminal signaling will create time paradoxes. Quantum mechanics has been unified with special relativity under the framework of quantum field theory, and the reason this unification is possible is because quantum nonlocal effects just so happen to never allow for time paradoxes because they cannot be used to transmit a signal as they are always random. The only way you could transmit a signal with them is if they turned out not to be always random, if there were "hidden variables" that could be used to predict the outcome of quantum events ahead of time, but there is currently no evidence such a thing exists, and if it did exist, it would either create time paradoxes or prove Einstein's theory of relativity wrong.

At least, according to current physics, it's impossible. If it were to be possible, it would have to overturn Einstein's theory of relativity and prove that theory to be wrong, and therefore quantum field theory would also have to be overturned. Of course, you can never know what big discovery might overturn all modern physics. But the point is that it would require a discovery that huge and monumental to entirely rewrite how we understand nature for this to be possible, so I wouldn't hang my hat on that occurring any time soon.

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  • [–] 2 points 10 months ago (6 children)

    Einstein's theory of relativity, despite common misconception, does not disallow faster than light effects. It only disallows them if they can be used to signal, because this superluminal signaling will create time paradoxes.

    In trying to imagine a phenomenon that cannot be used to signal, but I'm failing. Do you know of any examples?

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  • [–] 6 points 10 months ago* (last edited 10 months ago) (3 children)

    Not my area of expertise, but you can transfer a quantum state instantaneously across distance. That may sound like communication, but you cannot actually use that quantum information without an associated amount of classical information that decodes it.

    That classical information must travel through some standard means of transmission.

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  • [–] 2 points 10 months ago*

    Yes, but there isn't a consensus that the "quantum state" is actually a physical thing. It is not observable in a single experiment but only over an ensemble of experiments where you vary the measuring apparatus's settings over the ensemble. An ensemble is just a fancy way in physics of saying an idealized situation where the same experiment is conducted an infinite number of times such that it approaches the idealized statistical distribution of the results.

    The quantum state contains more information than the classical state, but if the quantum state is only observable over an ensemble of systems, then this shouldn't be surprising because an ensemble by its very nature is more complicated. An infinite number of experiments conducted where each contain different initial conditions obviously will contain more information than a single experiment will.

    There is thus a lot of debate in the literature over whether or not the quantum state is (1) a physical thing (called psi-ontic), (2) a statistical thing (called psi-epistemic), or (3) partially statistical and partially physical. There is a popsci misconception that the Copenhagen interpretation says that the quantum state is like a physical wave that spreads out until you look at it then undergoes a "collapse" into a single particle, but if you actually read the writings of people like Bohr and Heisenberg, they insisted on treating it as epistemic and not physical, and they never even talked about "collapse" at all because they did not assert particles spread out as quantum waves in the first place to need to "collapse" back into a particle.

    Of course, you can believe the quantum state is physical. That is a valid point of view. Psi-ontic interpretations are very common. But I just want to point out that there is a bit more nuance in the literature and not a consensus on it and so not every physicist would readily agree that something is actually being instantaneously teleported in quantum teleportation, although some others would agree to that statement.

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  • [–] 4 points 10 months ago* (1 child)

    The classic EPR-esque thought experiments is an illustration of this. Particle A and particle B take on values of either +1 or -1 at random, but are guaranteed to be opposite. They are sent to two different labs very far apart so Alice gets particle A and Bob gets particle B. Alice measures her particle and finds that, by happenstance, she gets +1, and now this must mean Bob's is influenced to become -1. You cannot use this to signal for two reasons. First, the values both of their particles take on is random and so the nonlocal influence is random, so no information can be encoded in the +1 vs -1. Second, Bob cannot even notice if Alice does this, because if he tries to measure his particle, he will just be the one who measures a random value that then fixes Alice's particle.

    This is nonlocal in Einstein's definition of locality, but since you cannot signal with it, it technically does not violate special relativity, so many contemporary physicists define locality in terms of being able to signal, so they would label this as local, even though there is literally instantaneous influence between the particles.

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