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Progress:

The QUIC theory group has continued to address issues pertaining to fault-tolerant storage and processing of quantum information, and to study the general features of entangled states and quantum nonlocality.

One of our main recent results is a much improved accuracy threshold for reliable storage of quantum information. John Preskill, undergraduate student Eric Dennis, graduate student Andrew Landahl, and QUIC visitor Peter Hoyer have investigated schemes for encoding quantum information that exploit topological principles. We have developed fault-tolerant procedures that can maintain an unknown quantum state with good fidelity indefinitely, as long as the probability of error per elementary qubit is less than about one percent. This new result is an improvement by about two orders of magnitude over the accuracy threshold found last year by Preskill and Daniel Gottesman (based on concatenated coding). So far, the new accuracy threshold applies only to storage of quantum information, not to universal quantum computation, but work is in progress to extend the analysis to derive an improved threshold for computation as well. Another goal for the coming year is to formulate new designs for intrinsically fault-tolerant quantum hardware that incorporate topological concepts.

Christopher Fuchs and Hideo Mabuchi, with a variety of (non-QUIC) collaborators, defined and studied a new measure of mixed-state quantum entanglement, the entanglement of assistance. This quantity characterizes how many maximally entangled qubit pairs can be distilled from a large number of copies of the state, provided that party who prepares the state cooperates fully with the distillers. The entanglement of assistance has the surprising property of superadditivity, and sheds light on the interpretation of various other entanglement measures. In related work, Fuchs and his collaborators discovered that there are sets of mutually orthogonal multi-partite product states that cannot be perfectly distinguished by local operations and classical communication. This discovery has illuminated the essence of quantum nonlocality, and has also clarified the inherently irreversible nature of the preparation of entangled states.

John Preskill taught a course on quantum information and quantum computation at Caltech this year. The lecture notes are available on the web here.

Papers:

"Fault-Tolerant Quantum Computation," J. Preskill, in Quantum Computation, edited by H.-K. Lo, S. Popescu, and T. Spiller.

"Robust Solutions to Hard Problems," J. Preskill, Nature 391, 631 (12 Feb. 1998).

"Topological Quantum Computation," R. W. Ogburn and J. Preskill to appear in Proceedings of the 1st NASA International Conference on Quantum Computing and Quantum Communications, edited by C. Williams and S. Gulati.

"Making Weirdness Work: Quantum Information and Computation," J. Preskill, in Proceedings of the 1998 IEEE Aerospace Conferance, edited by R. Profet.

Presentations:

"Fault-Tolerant Quantum Computation," Issues in Quantum Computation and Information, Novartis Foundation, London, UK, Nov. 7, 1997.

"Topological Quantum Computation," 1st NASA International Conference on Quantum Computing and Quantum Communications, Palm Springs, CA, Feb. 17, 1998.

"Putting Weirdness to Work: Quantum Information and Computation," March Meeting of the American Physical Society: Invited Session, Frontiers in Physics, Los Angeles, CA, Mar. 19, 1998.

"Making Weirdness Work: Quantum Information and Computation," IEEE Aerospace Conference, Plenary Address, Snowmass, CO, Mar. 23, 1998.

"Putting Weirdness to Work," Physics Colloquium, Harvey Mudd College, Claremont, CA, Apr. 7, 1998.

"Battling Decoherence: the Fault-Tolerant Quantum Computer," Caltech Physics Research Conference, Pasadena, CA, Apr. 16, 1998.

"Putting Weirdness to Work," Spring Meeting of the American Association of Physics Teachers, Pomona, CA, Apr, 25, 1988.

"Reliable Quantum Computing, ARO/NSA Kickoff Meeting: Demonstration of and Algorithmic Development for Quantum Computation, Bowie, MD, Jun. 10, 1998

"Battling Decoherence: the Fault-Tolerant Quantum Computer," Symposium on Gauge Theory, Cosmology, and Fundamental Physics, Imperial College, London, UK, July 10, 1998.