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I've been working through the Brilliant Course on Quantum Computing, and I've reached the lesson on Quantum Teleportation. After completing the lesson and reading several questions and answers here:

  1. quantum teleportation,
  2. Is quantum teleportation really teleportation?,

I'm still struggling to understand its purpose.

If I understand the idea correctly, Quantum State Teleportation works in the following way:

A qubit represents infinite information because it can be in infinite states. Because of this, it's impossible to communicate a qubit state in a purely classical way.

However, a state can be "teleported" by entangling two qubits, A and B, and giving one to Alice and one to Bob. Then Alice can entangle a further qubit, C, with A via a Bell State measurement and send the results to Bob classically.

That allows Bob to apply his own measurement that recreates the state Alice's qubit C was originally in before the Bell State measurement.

Apparently, the reason to do this is to avoid thermal decoherence. That makes sense, but surely the entangled qubits A and B are just as susceptible to thermal decoherence?

If that's the case why not send C directly?

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  • $\begingroup$ "why not send C directly?" - it's like saying why would you 3D print anything. Suppose a colonist on Mars is in need of a new tool that someone on Earth just designed. You can beam some data, and they can 3D print it there, or you can send cargo on a rocket. One of those is cheaper. $\endgroup$ Jul 4, 2023 at 21:03
  • $\begingroup$ @FilipMilovanović Okay, but if I'm only sending a single qubit in either situation, what's the difference? Can you explain an application to me? I don't understand why it would be useful! How fast is the quantum channel the original entangled qubit went along? Why would I need to teleport state? Your analogy is okay, but you can produce a tool with an extremely small amount of information. It seems the qubit example requires you to send a "tool" anyway, so why not send the original tool? $\endgroup$
    – Connor
    Jul 5, 2023 at 6:32

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One scenario is that the qubit C is of superconductor type, which is not good for transmitting long distance, and qubit B is of photon type and is good for long distance transmission. Hope this answer helps.

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One possible use is to separate sending a qubit from sending information.

For example, suppose you were building a quantum computer with multiple nodes. You could populate the nodes with entangled qubits before run-time. That way, when it becomes necessary to transmit quantum information during run-time you could simply send classical bits.

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