# Quantum Teleportation: An Invention Based On Quantum Mechanics

Teleportation is science fiction terminology, of when something disappears on one place and reappears on another without travelling the distance between.
If you would like to teleport a particle you would of course first measure it to see in what state it’s in, but as we’ve seen above, a measurement disturbs the particle you measure in such a way so that it might not even be the same particle before and after the measurement, since the measurement would disturb and change it’s different states. But it seems that quantum mechanics itself offers a solution to it’s own limitations.

Alice and Bob have one particle each. And these particles are entangled, they are EPR particles so they stand in a special relationship to eachother; e.g. they might have the opposite spin to one another. The way in which they are related to each other is called their Bell state. This pair will work as the transmitter.
Alice then has a original particle, which she wants to teleport to Bob.
Alice then measures the relationship between her EPR particle and the particle which she wants to teleport. E.g she might find out that they also have opposite spin of eachother but she doesn’t get to know their actuall spin, this is a Bell state. The important thing is that she doesn’t measure the particle directly themselves, so she doesn’t disturb them. However Alice original particle still gets destroyed, this is because in the Bell state measurement, Alice EPR particle and her original particle also gets entangled, which means that the original particle looses it’s own state and instead it’s state now depends on Alice EPR particles and vice versa. EPR particles have no identity of their own any more but are now dependant of eachother.

She then sends the data recovered from the measurement to Bob. Bob can now, using the information Alice sent him, change his EPR particles state into the exact same state as the one Alice original particle had, since he now knows:

• A: In what relationship his and Alice’s EPR particles stands to eachother.
• B: In what relationship Alice’s EPR particle and the particle which she wanted to teleport stands to eachother.

This gives Bob the information he needs to change his EPR particle to Alice original particle. Say that Alice and Bobs EPR particles have the opposite spin of eachother, and then Alice finds out that the particle which she wanted to teleport and her EPR particle had the same spin. Then, because of the different relation between the particles, Bob would know that Alice’s original particle had the opposite spin of his EPR particle, so he simply invert it’s spin to the opposite and then he’s got a copy of Alice original particle(note that to invert the spin to the opposite doesn’t require him to know it’s exact spin so he doesn’t disturb the particle and also note that it’s not only the spin that can be teleported). It might also be so that the particle will be teleported directly without Bob having to do anything. If Alice and Bobs EPR particles have opposite spins and Alice original particle had the opposite spin of her EPR particle then no change to Bobs particle had to be made since they would have the same spin immediately, but you still had to send a message to see it that really was the case. It is also possible to teleport super positions of states.

But in the teleportation neither Alice nor Bob(or anyone else) can ever know exactly what was transferred in the teleportation, the only thing that they can know is that the correct state was teleported, since if they tried to measure the state they would mess it up. A problem with the experiment is to keep Alice and Bobs EPR particles isolated, because any kind of disturbance will destroy the EPR connection.

The important thing is that there’s no real faster then light teleportation, there still has to be send a message(the one from Alice containing her particles Bell states) with slower then light means to perform the particle teleportation, and there already has to be two particles at the different places where you are and where you want to teleport your particle to, since no matter gets teleported only the particles state. And also that the original particle gets destroyed is important, since else you would have two particles with the same properties so you could measure one thing on the other and another thing on the other and in this way circumvent the uncertainty principal and know all the properties of the particle, this is something called the non-cloning theorem.

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