A Quick Introduction to Quantum Information

Quantum information is an exciting new paradigm for information science which makes use of the counterintuitive concept of quantum superpositions of information. This new concept raises the possibility of capabilities for information transmission, storage, and manipulation that are simply impossible with conventional information technologies. In the past few years there have been advances in the experimental study of the foundations of quantum mechanics, photonics, and atomic physics that have made accessible these novel uses of quantum states. Moreover, important applications of quantum mechanical concepts to information security and information assurance have been identified, and so this field has recently undergone a dramatic revolution from an essentially academic subject to one with an enormous potential to revolutionize computer science. The realization of these new information science concepts requires the ability to "engineer" quantum mechanical (coherent) states of several particles which have hitherto only been used in quite limited forms for testing the foundations of quantum mechanics.

The foundations of information science were laid out during and shortly after World War II and tacitly assumed that information, whether in the form of ink on paper or voltages in a microprocessor, would be represented by processes obeying classical physics. However, in the early 1980s Richard Feynman and Charles Bennett (among others) began to investigate the generalization to information represented by quantum physical processes. That is, they considered the representation of binary numbers by orthogonal quantum states (|0> or |1>) of some suitable two-level quantum system. (The representation of a single bit of information in this form has come to be known as a "qubit.") Examples of physical systems that permit such a qubit representation are ubiquitous: vertical and horizontal photon polarization states; single-photon interference states in which a photon can emerge from one or the other exit ports of a Mach-Zehnder interferometer; and the electronic ground and first (metastable) excited state of a (trapped) ion, to list only three. From this pioneering work it has been shown that quantum mechanics opens up powerful new methods for information storage, transmission and manipulation because of the superposition principle, the indivisibility of quanta and the peculiarities of measurement in quantum mechanics.

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