Only limited by the speed of light, quantum teleportation could make communications nearly instantaneous.
Isn’t the quantum communication (if it were possible) supposed to be actually instantaneous, not just “nearly instantaneous”?
The process works by harnessing quantum entanglement, a technique in which two particles are linked, regardless of the distance between them. Instead of particles physically traveling to deliver information, entangled particles exchange information over great distances — without physically carrying it.
As I understand it, even if the particles exchange information with each other, there is no way to send new information using entangled particles.
Isn’t the quantum communication (if it were possible) supposed to be actually instantaneous, not just “nearly instantaneous”?
There is no instantaneous information transfer (“nonlocality”) in quantum mechanics. You can prove this with the No-communication Theorem. Quantum theory is a statistical theory, so predictions are made in terms of probabilities, and the No-communication Theorem is a relativity simple proof that no physical interaction with a particle in an entangled pair can alter the probabilities of the other particle it is entangled with.
(It’s actually a bit more broad than this as it shows that no interaction with a particle in an entangled pair can alter the reduced density matrix of the other particle it is entangled with. The density matrix captures more than probabilities, but also the ability for the particle to exhibit interference effects.)
The speed of light limit is a fundamental property of special relativity, and if quantum theory violated this limit then it would be incompatible with special relativity. Yet, it is compatible with it and the two have been unified under the framework of quantum field theory.
There are two main confusions as to why people falsely think there is anything nonlocal in quantum theory, stemming from Bell’s theorem and the EPR paradox. I tried to briefly summarize these two in this article here. But to even more briefly summarize…
People falsely think Bell’s theorem proves there is “nonlocality” but it only proves there is nonlocality if you were to replace quantum theory with a hidden variable theory. It is important to stress that quantum theory is not a hidden variable theory and so there is nothing nonlocal about it and Bell’s theorem just is not applicable.
The EPR paradox is more of a philosophical argument that equates eigenstates to the ontology of the system, which such an equation leads to the appearance of nonlocal action, but this is just because the assumption is a bad one. Relational quantum mechanics, for example, uses a different assumption about the relationship between the mathematics and the ontology of the system and does not run into this.
