Preface to the First Edition

The Senate of the German Research Council (Deutsche Forschungsgemeinschaft) has set up a Focused Research Program "Quantum Information Processing" in July 1998. The Focused Program was jointly initiated by Th. Beth, G. Leuchs, W. Mathis, and W. Schleich. The present volume surveys the results of this work during the years 1999-2002.

The main thrive of this Focused Program is the research of the foundations of quantum information processing by means of controlled manipulation of entangled states. First experiments show that one can generate entangled states in a controlled way and manipulate them to a certain extent. This progress was made possible due to the recently developing unique worldwide synergy between theoretical and experimental physics, computer sciences, telecommunications and mathematics in the field of so called quantum information and physics of computation founded by Feynman.

The goal of this interdisciplinary Focused Program "Quantum Information Processing" is the systematic investigation of quantum systems aimed at their exact theoretical modelling and experimental manipulation, at tests of the foundations of quantum information and quantum physics and at the applications in the computer sciences, telecommunication, cryptography and high-precision measurement, quantum-error control and switching technologies. The main goal is meant to be not the examination of systems using quantum mechanical methods, as for example in the case of spectroscopy, but merely the controlled manipulation and exploitation of entangled states.

The development of some fields of modern physics leads to the possibility to isolate and to control quantum phenomena ever better. The transfer of the properties of a quantum state over a silica fibre, the high-precision backaction-evading, partially even interaction- free, measurements and the "computation" with quantum state are examples for that. All this is based on the principles of the state superposition and entanglement of various systems. The dream of a successful solution of exponentially difficult computational problems, like optimization or pattern identification could be thus fulfilled, despite of limited material hardware resources. According to general considerations these problems cannot be handled by means of classical computers with reasonable success chances.

Entangled states have played an important role in the attempt to understand quantum phenomena in Gedanken-experiments already at the beginning of quantum physics. This topic has surfaced again due to novel possibilities for the actual implementation of these Gedanken-experiments by means of modern physics. The new experiments on the foundations of physics provide more and more demanding tests of the quantum theory and the question of nonlocality. In the last 7 years new surprising effects were discovered in this field. A spectacular example of such effects is teleportation making use of the non-locality of entangled states to transfer the properties of a quantum state or the possibility of a truly exponential speed-up of quantum algorithms over classical algorithms.

Apart from this, there are some recent suggestions to employ the specific features of quantum information for technological applications. Such as quantum key distribution and the use of multipartite entanglement in quantum communication protocols. Recent discoveries on the information-theoretical side suggest that new surprising applications be provided by such technologies still to be invented - exceeding the state of today's Quantum Key Exchange apparatus.

One main goal is to construct a highly parallel computer which works according to a completely new paradigm and which in all probability can solve problems, which are not efficiently solvable with a conventional computer. Further applications open up in optical communication and in cryptographic key distribution. The exploitation of the entanglement to be studied here is developing into an important concept. It is the basis for the effects of acceleration and precision predicted by theory. It is hoped that the results and perception gained from the studies of pure demonstration systems in the frame of this program will help to increase the know-how required towards building future industrial products such as a quantum processing machine.

Delimitation from other Quantum Mechanical Problems: The further optimization of the up-to-date CMOS-Technology is expected to be completed in fifteen years at the latest. All the alternatives to CMOS-Technology (e.g. single electron transistor (SET), resonant tunneling devices (RTD), rapid single flux quantum devices (RSFQ), molecular nanoelectronics (ME) and spin-devices/magneto-electronics (SD)) recognizable today are based on quantum phenomena. However they do not make use of the entanglement principle. This eliminates the advantage of the exponential gain via scalable entangled pure quantum states provided by quantum information processing.

In a similar way the goals of the quantum information processing differ from the methods of NMR- and laser spectroscopy. The main goal of spectroscopy is to examine an unknown system (for example a molecule or a solid) making use of quantum mechanical phenomena. Quantum information processing in contrast abstracts first from the system itself. The subject of this Focused Program is the research and controlled exploitation of the fundamental quantum mechanical effects in pure, experimentally realizable systems, keeping in mind the technical feasibility. All the applications have been judged accordingly. General experimental or theoretical investigations to e.g. the foundations of quantum mechanics without direct connection are not part of the focused program.

The contributed chapters in this volume are grouped according to the following list of topics:

The most recent results of the Schwerpunktprogramm can be found by calling the home page http: //www.quiv.de.

On behalf of all participants of the Schwerpunktprogramm we gratefully acknowledge the long term funding of this research project by the Deutsche Forschungsgemeinschaft (DFG). We are especially indebted to Dr. A. Szillinsky, Dr. A. Engelke, Dr. K. Wefelmeier, and Dr. S. Krückeberg for their guidance during the development and the pursuit of this research programme. For the first renewal of the research projects there was an unforeseen large number of applications. We are most grateful to the Bundesministerium für Bildung und Forschung for helping with additional funding through its VDI-office and we especially thank Dr. M. Böltau for his support.

We express our sincere thanks to Gerlinde Gardavsky, Dr. Markus Grassl, Priv. Doz. Dr. Natalia Korolkova, Christoph Marquardt, Dr. Martin Rötteler, Jessica Schneider, and Robert Zeier for their help during the preparation of this volume.

Preface to the Second Edition

Since the publication of the first edition the results extensively described there have become established knowledge in Quantum Information Processing. Beyond this, new directions of research have evolved from amongst these areas. Thus in addition to revised chapters of the first edition, this second edition contains the additional new chapters 4, 12, 13, 24, 27-30 and 32.

The editors are pleased to note that the support for quantum information has been increasing during these last two years within Germany. The Deutsche Forschungsgemeinschaft is now funding a Graduiertenkolleg in Dortmund and a Sonderforschungsbereich in München. In addition several of the groups contributing to this volume received funding from the project "Quantum Information Highway A8" which is jointly sponsored by the Landesstiftung Baden-Würtemberg GmbH and the Bayerisches Staatsministerium für Forschung, Wissenschaft und Kunst. Our community is most grateful for this support and the present volume displays a cross section of the funded work.