# I Quantum teleportation

1. Jul 18, 2017

### Trollfaz

Quantum teleportation exploits entanglement but I don't really know how it works. And I heard that the no cloning theorem says that the original state must be destroyed. Wouldn't it violate the principle of conservation of energy mass?

2. Jul 18, 2017

### David Olivier

I don't think there is any such violation. "Destroying" the original state doesn't mean destroying its matter. It means changing the state of the original system, in such a way that it cannot be measured again with the same result. On the receiving end, say, on Mars, the copy is made from matter on Mars; the telportation just means that that matter is put into a state that mirrors the original state of the matter on Earth.

What is transmitted from Earth to Mars is information, not matter.

3. Jul 18, 2017

### DrChinese

Quantum teleportation is directionless. Nothing is transmitted from point A to B any more than from B to A. At least, not as far as any experiment can differentiate.

4. Jul 18, 2017

### Strilanc

Quantum teleportation is really really similar to a classical encryption method called the one-time pad. That's one way to understand what teleportation is doing.

Another way to understand what teleportation is doing is by starting with a quantum circuit that swaps two qubits and making trivially correct changes to the circuit until you're left with teleportation.

The measurement is what "destroys" the sender's "copy" of the state. But this is a bit misleading; even if you omitted the measurement, the teleportation circuit would simply replace the sender's qubit's state with the state $|0\rangle + |1\rangle$. There's never actually a true copy of the state.

No, it definitely has a direction. The sender is the one doing the Bell basis measurement and broadcasting the results, and the receiver is the one applying a fixup operation based on the outcome of the measurement.

The entanglement used by teleportation is directionless, though. An EPR pair can be used to send a qubit in either direction. Also you could easily create a (more complicated and requiring more entanglement) two-way teleportation that was completely symmetric and swapped a qubit at A for a qubit at B.

Last edited: Jul 18, 2017
5. Jul 18, 2017

### Hi123

The no cloning theorem states that the original matter must be altered, not destroyed.

6. Jul 18, 2017

### DrChinese

No argument, and nice reference example. I misspoke.