Quantum Information
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Quantum Information Luiz Davidovich
A
s ever greater numbers of bits are crammed into smaller and smaller volumes to increase the memory and computing capacity of digital computers, we must consider the ultimate end of such cramming. What happens when bits get so small that they consist of a single atom? At this scale, quantum effects become significant, and the life of a bit becomes much more complex. Instead of existing in either a 0 or a 1 state, a quantum bit, or “qubit,” can exist in both states at the same time—a concept called “superposition.” While such an indeterminate state would seem to rule out the use of qubits for practical computing purposes, scientists have shown that quantum computers consisting of only a few hundred atoms could perform massive parallel computations of great significance to the fields of cryptography, database searching, and modeling of complex real-world systems such as an ensemble of atoms undergoing a phase transition. Practical algorithms already exist to take advantage of quantum computing systems; what remains is to solve the problems of assembling a working quantum computer. The following articles by Luiz Davidovich and Bruce E. Kane shed light upon the world of quantum information from two different angles. Davidovich discusses the theory of quantum computing in detail to show the possibilities of the technology, including encryption and teleportation of data. Through insightful explanations of basic concepts and lively examples of encrypted communications, he gives us an overview of the field. Kane, on the other hand, looks at the materials challenges involved in implementing a quantum computing device. He shows that while solid-state qubits based on doped silicon are theoretically possible, their implementation may prove to be extremely challenging. The inherent variability of devices made by solid-state processing techniques may prevent the positioning of a single atom of phosphorus on a silicon lattice with the atomic-level precision necessary for quantum computation. Still, micropositioning or self-assembly techniques that have yet to be developed may solve this problem. Together, these two articles provide the theoretical and practical bases for understanding possible materials solutions to the challenge of quantum computing. —Eds.
MRS BULLETIN • VOLUME 30 • FEBRUARY 2005
Abstract The following article is based on the plenary address by Luiz Davidovich (Federal University of Rio de Janeiro), presented on April 14, 2004, at the 2004 MRS Spring Meeting in San Francisco. The field of quantum information is a discipline that aims to investigate methods for characterizing, transmitting, storing, compressing, and computationally utilizing the information carried by quantum states. It owes its rapid development over the last few years to several factors: the ability, developed in several laboratories, to control and measure simple microscopic systems; the discovery of fast quantum algorithms; and the recognition that Moore’s law will soon lead to the single-atom limit of
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