Teleportation (in the sense of somehow scanning an object to extract all the information required to reassemble it at a later stage) was long thought to be impossible in quantum mechanics as it was seen to violate Heisenberg's Uncertainty Principle. The Uncertainty Principle states that it is impossible to know both the position and momentum of an object exactly - as attempts to improve the accuracy to which one quantity is known, will decrease the accuracy to which the other may be known. As a result, it would be impossible to 'scan' all the details of a quantum system.
Entanglement The present concept of quantum teleportation, however, is based on an unusual feature of quantum mechanics - entanglement. Entangled particles always behave in a very peculiar way. If an experiment is done to measure a property of one particle (such as a photon) the result will be completely random within a range of possible results. However, as soon as this result is obtained, the corresponding property of its entangled 'partner' will automatically and instantaneously correlate according to that of the first particle - regardless of how far apart they are.
EPR Type Experiments In recent experiments (known as EPR type experiments due to the work of Einstein, Podolsky and Rosen), physicists have confirmed this outcome of quantum mechanical theory by testing the polarization of entangled photons ('packets of light') using detectors at such large distances that no standard 'communication' could have occurred between the photons.
Basic Teleportation Process One essential part of entanglement lies in the fact that the results of the test applied to each photon are entirely random and cannot be controlled - hence, it does not provide a means for faster than light communication. It is possible, though, to make use of entanglement to bring about quantum teleportation and avoid conflict with the Uncertainty Principle. While there are a variety of quantum teleportation techniques, the most common is the single photon pair experiment (as studied by teams at Innsbruck - C.H.Bennett et al; and Vienna - A. Zeilinger et al) which operates as follows.
The process begins with two entangled photons, A and B, with A placed at the sending station and B at the receiving station. The photon, C, selected to be teleported is then jointly measured with A. This measurement reveaks how to transform B so that it has the same quantum state as C, without actually making known what the original quantum state of C actually was. In addition, since B is entangled with A, B will instantaneously assume the same new quantum state as A. If the sending station then sends the necessary transformation information to the receiving station, the receiving station will be able to make the necessary transformation and will turn B into a precise replica of C.
The basic single photon pair quantum teleportation process. (Image from IBM Quantum Teleportation Web Site). Note : in this diagram A is actually being teleported, not C as appears in the experiment I have described.
Outcomes of Process The beauty of this process is that it does not violate Heisenberg's Uncertainty Principle since there is never any attempt to try to measure the exact quantum state of C, rather the approach is to measure how C differs from A - a somewhat subtle distinction with significant consequences. It is important to note two aspects of this process, however. Firstly, although B adjusts to assume the new quantum state of A instantaneously, the information regarding the necessary transformation process to create C can only be sent at, or below, the speed of light (via conventional means such as telephone or fax). Thus, quantum teleportation does not actually lead to instantaneous transportation of the photons involved.
Secondly, quantum teleportation requires that the original quantum state of particle C be destroyed during the measurement with A. Multiple replicas of an object, therefore, are unable to be created by quantum teleportation.
The ultimate outcome of this is that quantum teleportation is a reasonable prospect, given the present level of understanding of the 'laws of physics' - although practical considerations (namely that the complexity of the operations involved increases exponentially with the size of the object to be teleported) will not allow the teleportation of macroscopic objects for some time to come, if at all.
Finally, it is important to note that quantum teleportation is not the only proposed method of teleportation being examined in physics. An alternative method is that of teleportation via a 'wormhole' linking us to distant parts of the galaxy. Although it sounds improbable, serious theoretical studies have recently been conducted in this area. Unfortunately, such teleportation would probably require extremely large amounts of 'negative energy', although (as with quantum teleportation) it is a theoretical possibility.