Quantum Computing and Communications Springer London Berlin Heidelberg New York Barcelona Hong Kong Milan Paris Santa Clara Singapore Tokyo Michael Brooks (Ed.) Quantum Computing and Communications " Springer Michael Brooks, DPhil, BSc (Hons) ISBN-13:978-1-85233-091-0 Springer-Verlag London Berlin Heidelberg British Library Cataloguing in Publication Data Quantum computing and communications 1. Quantum computers 2. Telecommunication -Technological innovations 1. Brooks, Michael 621.3'9'81 ISBN-13:978-1-85233-091-0 Library of Congress Cataloging-in-Publication Data Brooks, Michael,1970- Quantum computing and communications 1 Michael Brooks. p. cm. ISBN-13:978-1-85233-091-0 e-ISBN-13:978-1-4471-0839-9 DOl: 10.1007/978-1-4471-0839-9 1. Quantum computers. 2. Optical communications. 1. Title. QA76.889.B76 1999 99-19975 004.1-dc21 CIP Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms of licences issued by the Copyright Licensing Agency. Enquiries concerning reproduction outside those terms should be sent to the publishers. © Springer-Verlag London Limited 1999 The use of registered names, trademarks etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant laws and regulations and therefore free for general use. The publisher makes no representation, express or implied, with regard to the accuracy of the information contained in this book and cannot accept any legal responsibility or liability for any errors or omissions that may be made. Typesetting: Camera ready by editor Printed and bound by the Athenreum Press Ltd., Gateshead, Tyne & Wear 34/3830-543210 Printed on acid-free paper Preface We have, in the last few years, radically improved our grasp of the quantum world. Not just intellectually, either: our ability to manipulate real quantum systems has grown in equal measure with our understanding of their fundamental behavior. These two shoots - the intellectual and the practical harnessing of the quantum world - have sprung up at a time when a third shoot - information processing - has also been experiencing explosive growth. These three shoots are now becoming intertwined. Twisted together, our understanding of information processing, quantum theory and practical quantum control make for a strong new growth with enormous potential. One must always be careful about using the word 'revolutionary' too readily. It is, however, difficult to find another word to describe the developments that have been taking place during the second half of the 1990s. In 1986 Richard Feynman, the visionary professor of physics, made a very interesting remark: " ... we are going to be even more ridiculous later and consider bits written on one atom instead of the present 1011 atoms. Such nonsense is very entertaining to professors like me." It is exceptionally unfortunate that Feynman did not live to see this 'nonsense' fully transformed into reality. He, more than anybody, would enjoy the fact that it is now possible to write information onto an atom, or indeed an ion or a photon. Furthermore, theorists and experimentalists have shown that this information can be processed and transmitted in ways that allow a seemingly absurd degree of power and control over the information. It is now possible to use one quantum particle to influence another particle that it has never met It is possible to transmit information encoded in a single photon through the air and, on detection, to verify whether that information has been read by anyone else. Experiments are just beginning to string together quantum bits of information that promise massively parallel computing power, far beyond anything that classical machines can manage. In short, we are on the verge of the quantum information revolution. vi Preface There could have been no better time for the European Commission to fund a Pathfinder Project in Quantum Computing and Communications* to look into this subject. This Project, which was carried out with the financial support of the Commission, within the frame of the Esprit LTR Working Group 27126 QCEPP, facilitated the gathering and organization of a large amount of useful material about the field; I have freely and extensively used this material in this book. It is reproduced here with the kind permission of the Commission, but it does not necessarily reflect the views of the Commission. It has been a great privilege for me to spend the last year being so closely involved with the Pathfinder Project. Some of the members of the Project's Working Group have laid the theoretical or experimental foundations in important areas of this field; I have taken great pleasure in working alongside them, and am grateful for all their assistance in drafting parts of this volume. As a journalist I often have to skim the surface of a subject, taking in its essence, but with little time to consider its implications or to examine its fundamental basis. My involvement with the Pathfinder Project has enabled me to investigate this fascinating area to my heart's content, often learning directly from those who have originated the concepts. I am grateful to the European Commission, and to all of the Working Group for their input and advice. However, I take full responsibility for any errors or omissions that have crept into this volume. Finally, I would like to acknowledge the important role played by Brian Oakley, Chairman of the Pathfinder Project, and Charles Ross, its Honorary Secretary. Without their seemingly boundless energy and enthusiasm, it is likely that little of this material would ever have been gathered. Michael Brooks, January 1999 * The term 'Quantum Information Processing' (QIP) is also used to describe Quantum Computing and Communications throughout this book. Contents Section I A Wide Perspective Chapter 1 Introduction Michael Brooks .......................................................................... 3 1.1 Exploiting the Quantum World ...................................... 3 1.2 Historical Background .................................................... 5 1.3 Worldwide Efforts in QCC ............................................. 6 Chapter 2 The Fundamentals of Quantum Information Michael Brooks .......................................................................... 9 Chapter 3 Quantum Computer Science Michael Brooks........................................................................... 17 3.1 Introduction ..................................................................... 17 3.2 Algorithms and the Complexity Problem........................ 18 3.3 The Quantum Computation Answer. ............................... 19 3.4 Quantum Algorithms ....................................................... 20 3.5 Quantum Logic Gates and Networks ............................... 21 3.6 Obstacles .......................................................................... 23 3.7 A Workable Solution: Quantum Error Correction ........... 23 3.8 Conclusions ..................................................................... 24 Chapter 4 Experimental Realizations Michael Brooks.......... ................. ................................. 27 4.l Introduction ...... ..... .... ..... ....... ........ .............. ..... ... .... ... ..... 27 4.2 Trapped Ions .......... .... ... ...... ......... ...... ..... ..... ..... ..... ..... ..... 27 4.3 Nuclear Magnetic Resonance ....... ... ........ ..... .............. ..... 29 4.4 Cavity Quantum Electrodynamics ..... .... ....... ...... ...... ....... 30 4.5 Quantum Dots .................................................................. 31 Chapter 5 Optical Technologies for Quantum Computing and Communications Michael Brooks ....... ......... ... ... .... ..... ............... ........ ......... 33 5.1 Introduction ...... ......... ... ...... ...... ......... ... ........ ......... ... ....... 33 5.2 Using the Quantum Nature ofLight... ............................. 34 5.2.1 Potential ........................................................................... 34 5.2.2 Problems.......................................................................... 34 viii Quantum Computing and Communications 5.3 Quantum Noise in Optical Communications ................... 35 5.4 Generic Technologies in Quantum Communications ...... 35 5.4.1 Nonlinear Optics .............................................................. 36 5.4.2 Cavity Quantum Electrodynamics .............................. ..... 36 5.5 Operations Performed on Optical Signals ....................... 37 5.5.1 Signal Generation ............................................................ 37 5.5.2 Detection .......................................................................... 38 5.5.3 Attenuation ...................................................................... 38 5.5.4 Distribution ...................................................................... 39 5.5.5 Amplification................................................................... 39 5.6 Conclusions: Towards the Second Generation ................ 40 Chapter 6 Applications Michael Brooks..................... ...................................................... 43 6.1 Introduction ..................................................................... 43 6.2 Emerging Technologies ................................................... 44 6.2.1 Quantum Cryptography................................................... 44 6.2.2 Quantum Repeaters.......................................................... 44 6.2.3 Quantum Simulators ........................................................ 45 6.2.4 Metrology and Few Photon Applications ........................ 46 6.3 Conclusions: Measuring Progress .................................... 47 Chapter 7 A Note on the Question of Scaling: Decoherence and Error Correction Michael Brooks........................................................................... 49 Section II Personal Perspectives Chapter 8 Solid State Quantum Computation: Prospects, Proposals, and Prejudices Bruce Kane, University ofN ew South Wales, Sydney, Australia 53 Chapter 9 Information is Physical, But Slippery Rolf Landauer, IBM, Yorktown Heights ...................................... 59 Chapter 10 Nanocircuitry, Defect Tolerance and Quantum Computing: Architectural and Manufacturing Considerations R. Stanley Williams, Quantum Structures Research Initiative, Hewlett-Packard Laboratories................................................... 63 Chapter 11 Quantum Computing and NMR Jonathan A. Jones, University of Oxford .................................... 71 Chapter 12 Quantum Networks and Quantum Algorithms Vlatko Vedral, University of Oxford ........................................... 79 Contents ix Chapter 13 Quantum Cryptography Richard Hughes, Physics Division, Los Alamos National Laboratory........................ ....................... 87 Section III A Perspective for the Future Chapter 14 Realizing the Potential of Quantum Information Processing Michael Brooks........................................................................... 97 14.1 Prospects for Quantum Computing ................................. 97 14.2 Prospects for Special Applications .................................. 98 14.2.1 Quantum Simulation ........................................................ 98 14.2.2 Limited Qubit and Reduced-Noise High Precision Applications ........................ ........ ............ 98 14.2.3 Secure Communications .................................................. 99 14.3 Meeting The Needs of the QIP Field ............................... 99 14.3.1 The Need for Academic Focus ........................................ 100 14.3.2 The Need for Industrial Focus ......................................... 100 14.3.3 The Need for Awareness ................................................. 101 Chapter 15 The Role of Europe Michael Brooks ........................................................................... 103 15.1 The Pioneering Stage ....................................................... 103 15.2 Today ............................................................................... 104 15.2.1 Multi-Disciplinary or Trans-Disciplinary Nature of the Community ................................................ 104 15.2.2 The Numbers Involved .................................................... 104 15.2.3 The Geographical Spread ................................................ 105 15.2.4 The Industrial Scene ........................................................ 105 15.2.5 Summary of the European Scene ..................................... 106 Chapter 16 Quantum Computing and Communications: A View from the USA Colin P. Williams, Jet Propulsion Laboratory, California Institute of Technology .............................................. 107 16.1 Introduction ..................................................................... 107 16.2 What Works Well ............................................................ 108 16.3 What Does Not Work So Well ........................................ 108 16.4 NASA/JPL ....................................................................... I09 16.5 Lessons Learned from Experience with NASA ............... III 16.6 Opportunities for Europe ................................................. 111 16.7 Commercialization ........................................................... 112 16.8 Recommendations ........................................................... 113 16.8.1 Programmatic Recommendations .................................... 113 16.8.2 Technology Solutions ...................................................... 113 16.8.3 Quantum Computer Science ............................................ 113 16.8.4 Education and Training ................................................... 114 x Quantum Computing and Communications Section IV Reference materials Chapter 17 Quantum Information Processing: A Brief Overview of Recent Advances Antonella Karlson, StarLab, Riverland nv .................................. 119 17.1 Introduction ..................................................................... 119 17.2 The Underlying Physical System. .................................... 120 17.2.1 Quantum Bits and Quantum Superpositions .................... 120 17.2.2 Quantum Gates ................................................................ 120 17.2.3 Quantum Parallelism ....................................................... 121 17.3 Fundamentals of Quantum Information .......................... 121 17.3.1 Entanglement ................................................................... 121 17.3.2 Quantum Dense Coding ................................................... 122 17.3.3 Quantum Teleportation .................................................... 123 17.4 Quantum Cryptography ................................................... 123 17 04.1 Standard Cryptosystems .................................................. 123 17 04.2 Quantum Key Distribution .............................................. 124 17.5 Quantum Computing ....................................................... 125 17.5.1 Quantum Algorithms ....................................................... 125 17.5.2 Grover's Search Algorithm .............................................. 126 17.5.3 Period Finding and Shor's Factorization Algorithm .................................................. 127 17.5 A Minimum Requirements for any Quantum System to Be a Quantum Computer ................................ 128 17.5.5 Simulation of Other Quantum Systems ........................... 128 17.6 Quantum Decoherence .................................................... 129 17.6.1 What is Decoherence? ..................................................... 129 17.6.2 Quantum Error Correction ............................................... 130 17.7 Experimental Realizations ............................................... 131 17.7.1 Systems with Few Degrees of Freedom .......................... 131 17.7.2 Macroscopic Systems ...................................................... 132 17.8 Conclusions ..................................................................... 132 Chapter 18 Categories and Definitions ....................................................... 137 18.1 Defmitions and Glossary of Terms .................................. 137 18.1.1 Defmition of Quantum Computing and Communications ....................................................... 137 18.1.2 Sub-Sector Definitions .................................................... 13 8 18.1.3 Some Relevant Terms ...................................................... 140 18.2 The Disciplines that Contribute to Quantum Information Processing .................................... 142 Appendix A The Pathfinder Project ............................................................. 145 A.l Pathfmder Activities ........................................................ 145 A.l.l Newsletters ...................................................................... 145 A.I.2 Database of Active Individuals and Organizations .......... 146 A.I.3 Taxonomy of the Subject... .............................................. l46 A.IA Helsinki Conference ........................................................ 146