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Quantum physics, information theory, and computer science are among the
crowning intellectual achievements of the 20th century. Now, a new
synthesis of these themes is underway. The emerging field of quantum
information science is providing important insights into fundamental
issues at the interface of computation and physical science, and may guide
the way to revolutionary technological advances.
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**The quantum laws that govern atoms and other tiny objects differ radically
from the classical laws that govern our ordinary experience. In particular,
quantum information (information encoded in a quantum system) has weird
properties that contrast sharply with the familiar properties of classical
information. Physicists, who for many years have relished this weirdness,
have begun to recognize in recent years that we can put the weirdness to
work: There are tasks involving the acquisition, transmission, and
processing of information that are achievable in principle because Nature
is quantum mechanical, but that would be impossible in a less weird
classical world.
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**I will describe the properties of quantum bits (“qubits”), the
indivisible units of quantum information, and explain the essential ways
in which qubits differ from classical bits. For one thing, it is impossible
to read or copy the state of a qubit without disturbing it. This property
is the basis of “quantum cryptography,” wherein the privacy of secret
information can be founded on principles of fundamental physics.
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**Qubits can be “entangled” with one another. This means that the qubits
can exhibit subtle quantum correlations that have no classical analogue;
roughly speaking, when two qubits are entangled, their joint state is more
definite than the state of either qubit by itself. Because of quantum
entanglement, a vast amount of classical information would be needed to
describe completely the quantum state of just a few hundred qubits.
Therefore, a “quantum computer” operating on just a few hundred qubits
could perform tasks that ordinary digital computers could not possibly
emulate.
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**Constructing practical quantum computers will be tremendously challenging;
a particularly daunting difficulty is that quantum computers are far more
susceptible to making errors than conventional digital computers. But
newly developed principles of fault-tolerant quantum computation may
enable a properly designed quantum computer with imperfect components to
achieve good reliability.
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