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.