I. Definition of the Subject and its Importance
Quantum mechanics is the branch of physics that describes how systems behave at
their most fundamental level. The theory of information processing studies how information
can be transferred and transformed. Quantum information science, then, is the
theory of communication and computation at the most fundamental physical level. Quantum
computers store and process information at the level of individual atoms. Quantum
communication systems transmit information on individual photons.
Over the past half century, the wires and logic gates in computers have halved in
size every year and a half, a phenomenon known as Moore’s law. If this exponential rate
of miniaturization continues, then the components of computers should reach the atomic
scale within a few decades. Even at current (2008) length scales of a little larger than one
hundred nanometers, quantum mechanics plays a crucial role in governing the behavior of
these wires and gates. As the sizes of computer components press down toward the atomic
scale, the theory of quantum information processing becomes increasingly important for
characterizing how computers operate. Similarly, as communication systems become more
powerful and efficient, the quantum mechanics of information transmission becomes the
key element in determining the limits of their power.
Miniaturization and the consequences of Moore’s law are not the primary reason for
studying quantum information, however. Quantum mechanics is weird: electrons, photons,
and atoms behave in strange and counterintuitive ways. A single electron can exist in
two places simultaneously. Photons and atoms can exhibit a bizarre form of correlation
called entanglement, a phenomenon that Einstein characterized as spukhafte Fernwirkung,
or ‘spooky action at a distance.’ Quantum weirdness extends to information processing.
Quantum bits can take on the values of 0 and 1 simultaneously. Entangled photons can
be used to teleport the states of matter from one place to another. The essential goal
of quantum information science is to determine how quantum weirdness can be used to
enhance the capabilities of computers and communication systems. For example, even a
moderately sized quantum computer, containing a few tens of thousands of bits, would be
able to factor large numbers and thereby break cryptographic systems that have until now
resisted the attacks of even the largest classical supercomputers . Quantum computers
could search databases faster than classical computers. Quantum communication systems
allow information to be transmitted in a manner whose security against eavesdropping is
guaranteed by the laws of physics.