"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.
The experiment was carried out at California Institute of Technology,
USA and involved physicists from Caltech, the University of Wales and
One of the future directions of the research in this direction in
Aarhus includes teleportation of macroscopic objects containing many
Quantum teleportation principles may also be used in the projects on
quantum computing and
quantum cryptography carried out at the Department of Physics and
Astronomy in collaboration with the Computer Science Department.
Aarhus members of the team include:
both from the
Quantum Optics Laboratory of the
Department of Physics, University of Aarhus.
More detailed explanation of
Non-classical Light Source.