Quantum Information

Quantum Information





Preamble











Information is represented, stored, processed, transmitted and readout by physical systems: “Information is physical,” as Rolf Landauer has summarized. Until recently, information has largely been thought of in classical terms, quan- tum mechanics having played a supportive role in designing the equipment to store and process it. With the tremendous progress in semiconductor technol- ogy and ever shrinking size of microelectronics, presently a single transistor in a PC processor is as small as ? 60 nm. According to Moore’s low, every 18 months computer chips double in density and power. If this trend continues during the next 15–20 years, we’ll have a single transistor represented by a sin- gle atom or molecule. Then quantum mechanical e?ects will begin playing an important role. This has in part motivated the birth of a new ?eld—Quantum Information Theory—based on quantum principles, which extends and gener- alizes classical information theory. Quantum information theory is currently attracting enormous interest in view of its fundamental nature and its poten- tially revolutionary applications to computation and secure communication.
This Part of the book is devoted to quantum information and computation. In Chap. 7 we brie?y outline the basic concepts of classical computation. This will prove useful in the description of the fundamental building blocks of quantum information in Chap. 8 and the principles of quantum computation in Chap. 9. We then conclude this Part and the whole book by Chap. 10, where we outline several representative quantum optical systems for quantum information processing.


7

Elements of Classical Computation











Computation is data processing. Computers process the input data, following a certain set of instructions called program, to proceed toward the result of computation contained in the output data. One distinguishes analog and digital computation. In analog computation, the computer basically imitates the physical process being simulated. In digital computation that most of us use on a daily basis, one “digitalizes” the parameters of the system to be modeled or analyzed and feeds this digital data into the computer input. The computation is then digital data processing.
In this chapter we review the basic concepts of classical computation. This is necessary in order to make the discussion on the theory of quantum informa- tion and computation more transparent. As will be seen in the chapters that follow, there are many parallels and analogies one can draw between classi- cal and quantum computers, but there are also striking di?erences associated with the superposition principle that results in quantum parallelism.