Kavli Institute For Theoretical Physics
The KITP Public Lecture Series

Putting Weirdness to Work: Quantum Information Science

sponsored by Friends of KITP

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 emerg- ing 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.

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 en- coded 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. I will describe the properties of quantum bits ("qubits"), the indivisible units of quantum infor- mation, 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.

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 en- tangled, 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.

Constructing practical quantum computers will be tremendously challenging; a particularly daunting difficulty is that quantum computers are far more susceptible to making errors than con- ventional digital computers. But newly developed principles of fault-tolerant quantum computa- tion may enable a properly designed quantum computer with imperfect components to achieve good reliability.

Biographical sketch:
JOHN PRESKILL received the A.B. degree in physics from Princeton University in 1975, and the Ph.D. degree in physics from Harvard University in 1980. In 1983, he joined the faculty of the California Institute of Technology, where he is now the John D. MacArthur Professor of Theoretical Physics, Director of the Institute for Quantum Information, and Director of the Center for the Physics of Information.

Prof. Preskill is a two-time recipient of the Associated Students of Caltech Teaching Award. He has been the Lorentz Lecturer at the University of Leiden, the Rouse Ball Lecturer at the University of Cambridge, the Biedenharn Lecturer at the University of Texas at Austin, and the Loeb Lecturer at Harvard University. His research interests include elementary particles, the very early universe, black holes, quantum information, and quantum computation.