Quantum information

Quantum Information is whatever can be transmitted by using systems obeying Quantum Theory as carriers. It is thus not any newly discovered "stuff", but has been implicit in Quantum Theory all along. In fact, some approaches to the foundations of quantum theory (e.g., Ludwig's from the 60ies [Lu83]) are based on the view that Quantum Theory is precisely about the kind of influence transported from a preparing device (the "transmitter") to a measuring device (the "receiver"). What is new in the recent work on quantum information theory is that this view is taken seriously in a quantitative way. The basic questions of this theory are taken from classical information theory: how much "quantum information" is carried by any given system or transmission channel, how much is stored in a storage device, how can such information be coded and decoded efficiently etc.
Classical Information Theory usually abstracts completely from the physical nature of the carriers of information. This is sensible because information can be converted easily and essentially without loss between different carriers such as magnetized patches on a disk, currents in a wire, electromagnetic waves and printed paper. However, this convertibility no longer holds for microscopic particles as described by Quantum Theory.
A hypothetical process of faithful translation of Quantum Information into Classical Information and back has come to be known as Teleportation or, more precisely, as "classical teleportation". It would consist of a device, which makes a measurement on the input quantum particles, producing a classical value as outcome (In this context "measurement" is just another word for the conversion from quantum to classical information.) The measured data are then transmitted classically to a preparing device, producing the output quantum particles, taking the classical input into account in some arbitrary way. The crucial condition for successful teleportation is then that no statistical measurement (involving arbitrarily prepared input particles and arbitrary measurements at the output) can distinguish this device from a device doing nothing at all. One can show rather easily from the structure of Quantum Theory and the characterization of general quantum devices ("channels") that such a device cannot exist. But this means that Quantum Information has to be taken seriously as a quantity in its own right. Many questions from Classical Information Theory can also be posed in this new context, but for the moment many of them are still awaiting rigorous answers. Trying to find these answers will help deciding the feasibility of Quantum Computers, and other practical applications, but it also opens new perspectives on the foundations of quantum mechanics itself.

Teleportation

"Teleportation" is used here merely as a technical term for "transmission of quantum information on a classical channel". Although this usage was inspired [BB95] by Science Fiction technology and although some colleagues found it convenient to harp on such overtones in their advertising, the connections with Star Trek beamers are really very slight, if only for reasons of scale (see e.g. [Br95]).
Teleportation with purely classical means is impossible, which is precisely the observation making the theory of Quantum Information a new branch of Information Theory. This makes it all the more surprising that by using the quantum mechanical correlations (entanglement) in an auxiliary pair of systems, a classical channel may be upgraded to do precisely this "impossible task". This new process, first published in a paper by Bennett et al. [BB93], is referred to as "entanglement enhanced (or quantum) teleportation" or sometimes just plain "teleportation". It has become one of the basic operations of Quantum Information Theory. First experimental realizations also exist [BP97].
 
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