Quantum information is concerned with using the special features of quantum physics for the processing and transmission of information. It should, however, be clearly understood that any physical object when analyzed at a deep enough level is a quantum object; as Rolf Landauer has succinctly stated, “A screwdriver is a quantum object.” In fact, the conduction properties of the metal blade of a screwdriver are ultimately due to the quantum properties of electron propagation in a crystalline medium, while the handle is an electrical insulator because the electrons in it are trapped in a disordered medium. It is again quantum mechanics which permits explanation of the fact that the blade, an electrical conductor, is also a thermal conductor, while the handle, an electrical insulator, is also a thermal insulator. To take an example more directly related to information theory, the behavior of the transistors etched on the chip inside your computer could not have been imagined by Bardeen, Brattain, and Shockley in 1947 were it not for their knowledge of quantum physics. Although your computer is not a quantum computer, it does function according to the principles of quantum mechanics!

This quantum behavior is also a collective behavior. Let us give two examples. First, if the value 0 of a bit is represented physically in a computer by an uncharged capacitor while the value 1 is represented by the same capacitor charged, the passage between the charged and uncharged states amounts to the displacement of 104 to 105 electrons.