PMG Phd Thesis FINAL
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Practical Free-Space Quantum Key Distribution Philip Michael Gorman B.Sc. (Hons), MInstP. Thesis submitted for the degree of Doctor of Philosophy Department of Physics Heriot-Watt University December 2010 The copyright in this thesis is owned by the author. Any quotation from the thesis or use of any of the information contained in it must acknowledge this thesis as the source of the quotation or information. ABSTRACT Within the last two decades, the world has seen an exponential increase in the quantity of data traffic exchanged electronically. Currently, the widespread use of classical encryption technology provides tolerable levels of security for data in day to day life. However, with one somewhat impractical exception these technologies are based on mathematical complexity and have never been proven to be secure. Significant advances in mathematics or new computer architectures could render these technologies obsolete in a very short timescale. By contrast, Quantum Key Distribution (or Quantum Cryptography as it is sometimes called) offers a theoretically secure method of cryptographic key generation and exchange which is guaranteed by physical laws. Moreover, the technique is capable of eavesdropper detection during the key exchange process. Much research and development work has been undertaken but most of this work has concentrated on the use of optical fibres as the transmission medium for the quantum channel. This thesis discusses the requirements, theoretical basis and practical development of a compact, free-space transmission quantum key distribution system from inception to system tests. Experiments conducted over several distances are outlined which verify the feasibility of quantum key distribution operating continuously over ranges from metres to intercity distances and finally to global reach via the use of satellites. i Acknowledgements I would like to extend my thanks to my supervisor, Professor Gerald Buller for his encouragement, advice and help during the preparation of this thesis. Equal thanks should also be extended to my industrial supervisors, firstly Professor John G. Rarity, who got me into this in the first place, and Dr. David A. Huckridge who graciously assented to finish where JGR left off. I would also like to thank the single photon optics team at Malvern, David Benton, David Taylor, Ewan Finlayson and last, but by no means least, the magnificent Paul Tapster. I didn’t realise one could do such good work while laughing so much. Acknowledgement should also be made to the Lasers and Photonics group at Malvern, Rebecca Wilson, David Orchard, Andrew Turner, Kevin Ridley, Terry Shepherd and the rest. They are an inexhaustible supply of know-how and advice. I should also like to acknowledge support of QinetiQ and the UKMoD for funding the work over a decade. Finally, I would like to thank my parents, Jean and Frank (& Glen), my brothers, Andrew and Matthew and my fantastic wife, Loll. ii ACADEMIC REGISTRY Research Thesis Submission Name : Philip Michael Gorman School/PGI: School of Engineering and Physics Version: (i.e. First, Final Degree Sought (Award Doctor of Philosophy (Physics) Resubmission, Final) and Subject area) Declaration In accordance with the appropriate regulations I hereby submit my thesis and I declare that: 1) the thesis embodies the results of my own work and has been composed by myself 2) where appropriate, I have made acknowledgement of the work of others and have made reference to work carried out in collaboration with other persons 3) the thesis is the correct version of the thesis for submission and is the same version as any electronic versions submitted*. 4) my thesis for the award referred to, deposited in the Heriot-Watt University Library, should be made available for loan or photocopying and be available via the Institutional Repository, subject to such conditions as the Librarian may require 5) I understand that as a student of the University I am required to abide by the Regulations of the University and to conform to its discipline. * Please note that it is the responsibility of the candidate to ensure that the correct version of the thesis is submitted. Signature of Date: Candidate : Submission Submitted By (name in capitals): Signature of Individual Submitting: Date Submitted: For Completion in Academic Registry Received in the Academic Registry by (name in capitals): Method of Submission (Handed in to Academic Registry; posted through internal/external mail): E-thesis Submitted (mandatory for final theses from January 2009) Signature: Date: iii TABLE OF CONTENTS ABSTRACT.......................................................................................................................i Acknowledgements...........................................................................................................ii TABLE OF CONTENTS.................................................................................................iv List of relevant publications by the author.....................................................................viii Chapter 1 - Introduction....................................................................................................1 1.1 Introduction..........................................................................................................1 1.2 Classical cryptography.........................................................................................1 1.3 Security and cryptography...................................................................................4 1.4 Outline of this thesis ............................................................................................5 1.5 Chapter 1 references ............................................................................................7 Chapter 2 – An historical review of QKD Technology ....................................................9 2.1 Introduction..........................................................................................................9 2.2 Foundations..........................................................................................................9 2.3 The birth of QKD...............................................................................................10 2.4 Childhood, 1992 to 1996 ...................................................................................13 2.5 QKD reaches puberty, 1997 to 2003 .................................................................19 2.6 QKD comes of Age, 2004 to 2009 ....................................................................27 2.7 Summary............................................................................................................37 2.8 The future...........................................................................................................37 2.9 Chapter 2 references ..........................................................................................40 Chapter 3 – Some considerations for practical QKD systems ........................................54 3.1 Introduction........................................................................................................54 3.2 The physical basis for QKD...............................................................................54 3.2.1 The postulates of quantum mechanics ........................................................55 3.2.2 Application to QKD....................................................................................60 3.3 The BB84 key exchange protocol......................................................................62 3.3.1 Introduction.................................................................................................62 3.3.2 Protocol operations......................................................................................63 3.3.3 Error correction...........................................................................................66 3.3.4 Privacy amplification ..................................................................................68 3.4 Security of QKD................................................................................................70 3.4.1 The eavesdropping model ...........................................................................70 3.4.2 Practical single photon sources...................................................................71 iv 3.4.3 Why 0.1 photons per pulse?........................................................................73 3.5 The transmission channel...................................................................................75 3.5.1 Introduction.................................................................................................75 3.5.2 Optical fibre transmission media ................................................................76 3.5.3 Free-space optical transmission ..................................................................79 3.5.4 Atmospheric turbulence ..............................................................................87 3.5.5 Beam effects................................................................................................89 3.5.6 Beam spreading and wander .......................................................................90 3.6 Random number generation...............................................................................99 3.7 Single photon detection ...................................................................................101 3.7.1 Introduction...............................................................................................101