Quantum computers hold the promise of solving problems that would otherwise be intractable with conventional computers. Some prototypes of the simplest elements needed to build a quantum computer have already been implemented in the laboratory. The efforts now concentrate on combining these elements into scalable systems. In addition, alternative routes to creating large scale quantum computers are continuously being developed.This volume gives a cross-section of recent achievements in both the theory and the practical realization of quantum computing devices. Samuel L. Braunstein (Reader, University of Wales, Bangor, and editor of the book "Quantum Computing - Where do we want to go tomorrow") and Hoi-Kwong Lo (Chief Scientist, MagiQ Technologies, Inc., NY) invited experts across many disciplines involved in the development of quantum computers to review their proposals in a manner accessible to the non-expert.Breaking with tradition, this book not only contains proposals, but a set of independent expert evaluations of these ideas as well. As a by-product this volume facilitates a comparison between the widely varying disciplines covered, including: ion traps, cavity quantum electrodynamics, nuclear magnetic resonance, optical lattices, quantum dots, silicon systems, superconductivity and electrons on helium.