Quantum Information Processing

Quantum information processing, Science of—The theoretical, experimental and
technological areas covering the use of quantum mechanics for communication and
computation
—Kluwer Encyclopedia of Mathematics, Supplement II
Research conducted in the last few decades has established that quantum information,
or information based on quantum mechanics, has capabilities that exceed
those of traditional “classical” information. For example, in communication, quantum
information enables quantum cryptography, which is a method for communicating in
secret. Secrecy is guaranteed because eavesdropping attempts necessarily disturb the
exchanged quantum information without revealing the content of the communication.
In computation, quantum information enables efficient simulation of quantum physics, a
task for which general-purpose, efficient, classical algorithms are not known to exist.
Quantum information also leads to efficient algorithms for factoring large numbers, which
is believed to be difficult for classical computers. An efficient factoring algorithm would
break the security of commonly used public-key cryptographic codes used for authenticating
and securing Internet communications. Yet another application of quantum information
improves the efficiency with which unstructured search problems can be solved.
Quantum unstructured search may make it possible to solve significantly larger instances
of optimization problems, such as the scheduling and traveling salesman problems.
Because of the capabilities of quantum information, the science of quantum information
processing is now a prospering, interdisciplinary field focused on better understanding
the possibilities and limitations of the underlying theory, on developing new
applications of quantum information, and on physically realizing controllable quantum
devices. The purpose of this primer is to provide an elementary introduction to quantum
information processing (see Part II), and then to briefly explain how we hope to exploit
the advantages of quantum information (see Part III). These two sections can be read
independently. For reference, we have included a glossary of the main terms of quantum
information (see page 33).
When we use the word “information,” we generally think of the things we can talk
about, broadcast, write down, or otherwise record. Such records can exist in many
forms, such as sound waves, electrical signals in a telephone wire, characters on paper,
pit patterns on an optical disk, or magnetization on a computer hard disk. A crucial
property of information is that it is fungible: It can be represented in many different
physical forms and easily converted from one form to another without changing its
meaning. In this sense, information is independent of the devices used to represent it but
requires at least one physical representation in order to be useful.
We call the familiar information stored in today’s computers classical or deterministic
to distinguish it from quantum information. It is no accident that classical information is
the basis of all human knowledge. Any information passing through our senses is best
modeled by classical discrete or continuous information. Therefore, when considering
any other kind of information, we need to provide a method for extracting classically
meaningful information. We begin by recalling the basic ideas of classical information in
a way that illustrates the general procedure for building an information-processing theory.
Number 27 2002 Los Alamos Science 3