Previous
Speakers

Biedenharn
Lectures

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.

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.

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.

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.

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.

September 8th, 3:30-5pm, RLM 5.104
Biedenharn Lecture 2: Physics Colloquium - The security of quantum cryptography

For most modern cryptographic protocols, information security is founded on the assumption that a computation that would break the protocol is too hard for an adversary to execute. But when quantum computers become readily available, such assumptions will need to be reevaluated, and much of classical cryptography will become obsolete. An alternative means of protecting information is via quantum cryptography, in which security is founded on fundamental principles of quantum physics, rather than on computational assumptions. I will explain how to prove the security of quantum key distribution, an important quantum protocol that (unlike large-scale quantum computation) is already feasible with existing technology.

September 13th, 2:00-3:30pm, RLM 11.204
Beidenharn Lecture 3: Special Seminar - Battling decoherence: the fault-tolerant quantum computer

To construct 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. I will explain the principles of fault-tolerant quantum computation, which can enable a properly designed quantum computer with imperfect components to achieve good reliability.

September 15th, 2:00-3:30pm, RLM 11.204
Biedenharn Lecture 4: Special Seminar - Topological quantum computation

The standard theory of fault-tolerant quantum computing shows that clever software design can overcome the deficiencies of noisy quantum hardware, as long as the noise is not too strong. In this lecture I will describe a different approach, in which the hardware itself is intrinsically resistant to noise. In a topological quantum computer, quantum information is encoded in the fusion spaces of nonabelian anyons in a two-dimensional medium, and can be manipulated robustly by guiding the anyons along specified trajectories. If the anyons have suitable properties, the topological computer can simulate efficiently an arbitrary quantum computation.