Quantum Teleportation

"Unconditional Quantum Teleportation"
A.Furusawa, J.L.Sørensen, S.L.Braustein, C.A.Fuchs, H.J.Kimble, and E.S.Polzik
Science, October 1998 [Abstract]

Teleportation of various objects has become very popular thanks to Captain Kirk from Star Trek. It is, however, a formidable task to perform such an operation in reality.

The difficulty is not just due to the overwhelming amount of information which has to be transmitted in order to reconstruct an object at another site. The additional challenge is due to the so-called "non-cloning" theorem in quantum mechanics.

Biological cloning of the famous sheep Dolly has been possible due to the fact that the DNA molecule can reproduce itself in as many copies as needed. Cloning in quantum sense would require reading out the complete information about the quantum state of an object without destroying this state that is impossible in principle. In other words, any process of obtaining information about a quantum object somehow damages the object.

However, Nature allows us to transmit the information about the object to another site (without reading out this information!) where this quantum object can be reconstructed. This is probably what happens to Captain Kirk when he disappears at one place and re-appears at another. The state of the object at its initial site is destroyed, so that the "non-cloning" law is not broken.

In the experiment described in the October issue of Science the state of a beam of light, i.e. its amplitude and phase, has been teleported. The parties involved in the teleportation include: Alice (the sender), Bob (the receiver) and Victor (the verifier). Besides those three, another critical component is necessary for any implementation of quantum teleportation: the source of the Quantum Entanglement.

In our case this source is a crystal which emits two beams of light 1 and 2, the so-called Einstein-Podolsky-Rosen (EPR) beams. The EPR beams have a very special property: each of them taken separately is very noisy, however they are quantum correlated, so that, when superimposed one on the other, the quantum noise cancels out.

The teleportation process starts with Victor sending the beam V which he wants to teleport to Alice. Alice also gets one of the EPR beams, beam 1 and mixes the beams V and 1, so that the beam V is completely messed up by the noisy beam 1. Now Alice can read out the information about V+1 without getting any knowledge about V, and therefore the information about V is not damaged (see the non-cloning law above).

Alice now passes the classical signal containing V+1 along regular cables to Bob. The trick is that Bob also gets another EPR beam, beam 2 along the "quantum" optical channel. Bob now superimposes the V+1 signal on the beam 2 to "clean up" the quantum noise. Due to the EPR correlations between beams 1 and 2 their quantum noise cancels out and Bob recovers the teleported beam V!

This teleportation experiment is the first one where the verifier Victor can actually prove that the beam that Bob has received in the process of teleportation is a very good copy of the initial beam that Victor has given to Alice. The "fidelity" of the teleportation is better than any classical measure-send-receive process can achieve.