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QUANTUM COMPUTING
Jozef Gruska
QUANTUM WORLD CLASSICAL WORLD
Quantum computation is Classical computation is deterministic probabilistic, sequential highly (exponentially) parallel working with real probabilities working with complex amplitudes M one of the outcomes of quantum superposition is randomly picked up - all other results unitary E A of computation are irreversibly lost S E. quantum T. computation U (evolution) R E Only at this point do indeterminacy and probabilities M come in E N T quantum measurement has the effect of ‘‘magnifying’’ .. quantum events from quantum to classical level described by Schrodinger equation using entanglement as a computational resource ii iii iv
On the book web pages
http://mcgraw-hill.co.uk/gruska
one findes. 1. Basic information about the contents of the book. 2. Ordering and price information. 3. Second part of the Appendix. (A survey of basic concepts from complexity theory and models of computing. Additional exercises. Historical and bibliographical refrences.) 4. eps-versions of figures from the book. 5. Corrections. 6. Additions. v
To my parents for their love and care.
To my wife for her ever increasing care, support and patience.
To my children with best wishes for their future.
To my grandson with best wishes for quantum computing age. vi Contents
Contents v
Preface xiii
1FUNDAMENTALS 1 1.1WhyQuantumComputing...... 2 1.2PrehistoryofQuantumComputing...... 7 1.3FromRandomizedtoQuantumComputation...... 12 1.3.1 Probabilistic Turing machines ...... 12 1.3.2 QuantumTuringmachines...... 15 1.4HilbertSpaceBasics...... 19 1.4.1 Orthogonality, bases and subspaces ...... 23 1.4.2 Operators...... 25 1.4.3 Observablesandmeasurements...... 26 1.4.4 Tensor products in Hilbert spaces ...... 27 1.4.5 Mixedstatesanddensityoperators...... 29 1.5Experiments...... 32 1.5.1 Classicalexperiments...... 32 1.5.2 Quantum experiments—single particle interference ...... 33 1.5.3 Quantumexperiments—measurements...... 38 1.6QuantumPrinciples...... 40 1.6.1 Statesandamplitudes...... 40 1.6.2 Measurements—the projection approach ...... 43 1.6.3 Evolutionofquantumsystems...... 45 1.6.4 Compoundquantumsystems...... 48 1.6.5 Quantumtheoryinterpretations...... 49 1.7ClassicalReversibleGatesandComputing...... 49 1.7.1 Reversiblegates...... 50 1.7.2 ReversibleTuringmachines...... 53 1.7.3 Billiard ball model of (reversible) computing ...... 54
2ELEMENTS 57 2.1QuantumBitsandRegisters...... 58 2.1.1 Qubits...... 58 2.1.2 Two-qubitregisters...... 66 2.1.3 No-cloningtheorem...... 68 2.1.4 Quantumregisters...... 69
vii viii CONTENTS
2.2QuantumEntanglement...... 74 2.2.1 Entanglementofpurestates...... 74 2.2.2 Quantifyingentanglement...... 78 2.2.3 Substituting entanglement for communication ...... 79 2.3QuantumCircuits...... 81 2.3.1 Quantumgates...... 81 2.3.2 Measurementgates...... 88 2.3.3 Universalityofquantumgates...... 90 2.3.4 Arithmeticalcircuits...... 95 2.3.5 Quantumsuperoperatorcircuits...... 97
3 ALGORITHMS 101 3.1QuantumParallelismandSimpleAlgorithms...... 103 3.1.1 Deutsch’sproblem...... 105 3.1.2 TheDeutsch–Jozsapromiseproblem...... 107 3.1.3 Simon’sproblems...... 109 3.2Shor’sAlgorithms...... 112 3.2.1 Numbertheorybasics...... 112 3.2.2 QuantumFourierTransform...... 115 3.2.3 Shor’s factorization algorithm ...... 119 3.2.4 Shor’s discrete logarithm algorithm ...... 124 3.2.5 Thehiddensubgroupproblems...... 125 3.3 Quantum Searching and Counting ...... 127 3.3.1 Grover’ssearchalgorithm...... 128 3.3.2 G-BBHTsearchalgorithm...... 131 3.3.3 Minimum-finding algorithm ...... 133 3.3.4 Generalizations and modifications of search problems ...... 135 3.4 Methodologies to Design Quantum Algorithms ...... 137 3.4.1 Amplitude amplification–boosting search probabilities ...... 137 3.4.2 Amplitude amplification—speeding of the states searching ...... 139 3.4.3 Casestudies...... 139 3.5 Limitations of Quantum Algorithms ...... 140 3.5.1 No quantum speed-up for the parity function ...... 140 3.5.2 Framework for proving lower bounds ...... 143 3.5.3 Oracle calls limitation of quantum computing ...... 147
4 AUTOMATA 149 4.1QuantumFiniteAutomata...... 151 4.1.1 Modelsofclassicalfiniteautomata...... 151 4.1.2 One-way quantum finite automata ...... 152 4.1.3 1QFAversus1FA...... 155 4.1.4 Two-wayquantumfiniteautomata...... 157 4.1.5 2QFAversus1FA...... 160 4.2QuantumTuringMachines...... 164 4.2.1 One-tapequantumTuringmachines...... 164 4.2.2 Variationsonthebasicmodel...... 169 4.2.3 Are quantum Turing machines analogue or discrete? ...... 171 4.2.4 Programming techniques for quantum Turing machines ...... 174 4.3QuantumCellularAutomata...... 177 CONTENTS ix
4.3.1 Classical cellular automata ...... 177 4.3.2 One-dimensional quantum cellular automata ...... 180 4.3.3 Partitioned quantum one-dimensional cellular automata ...... 183 4.3.4 Quantum cellular automata versus quantum Turing machines . . . . . 185
5 COMPLEXITY 191 5.1UniversalQuantumTuringMachines ...... 192 5.1.1 Efficient implementation of unitary transformations ...... 192 5.1.2 Design of a universal quantum Turing machine ...... 196 5.2QuantumComputationalComplexity...... 199 5.2.1 Basic quantum versus classical complexity classes ...... 199 5.2.2 Relativizedquantumcomplexity...... 204 5.3QuantumCommunicationComplexity...... 207 5.3.1 Classical and quantum communication protocols and complexity . . . 208 5.3.2 Quantum communication versus computation complexity ...... 210 5.4 Computational Power of quantum non-linear mechanics ...... 212
6 CRYPTOGRAPHY 215 6.1Prologue...... 217 6.2QuantumKeyGeneration...... 218 6.2.1 Basic ideas of two parties quantum key generation ...... 218 6.2.2 SecurityissuesofQKGprotocols...... 220 6.2.3 QuantumkeygenerationprotocolsBB84andB92...... 222 6.2.4 Multipartykeygeneration...... 227 6.2.5 Entanglement-based QKG protocols ...... 228 6.2.6 Unconditional security of QKG∗ ...... 231 6.2.7 Experimental quantum cryptography ...... 234 6.3QuantumCryptographicProtocols...... 236 6.3.1 Quantum coin-flipping and bit commitment protocols ...... 238 6.3.2 Quantum oblivious transfer protocols ...... 241 6.3.3 Security of the quantum protocols ...... 243 6.3.4 Security limitations of the quantum cryptographic protocols ...... 246 6.3.5 Insecurity of quantum one-sided two-party computation protocols . . . 249 6.4 Quantum Teleportation and Superdense Coding ...... 250 6.4.1 Basicprinciples...... 250 6.4.2 Teleportationcircuit...... 252 6.4.3 Quantumsecretsharing...... 254 6.4.4 Superdensecoding...... 256
7 PROCESSORS 259 7.1EarlyQuantumComputersIdeas...... 261 7.1.1 Benioff’squantumcomputer...... 261 7.1.2 Feynman’s quantum computer ...... 261 7.1.3 Peres’quantumcomputer...... 262 7.1.4 Deutsch’squantumcomputer...... 263 7.2ImpactsofImperfections ...... 264 7.2.1 Internalimperfections...... 264 7.2.2 Decoherence...... 265 7.3 Quantum Computation and Memory Stabilization ...... 268 x CONTENTS
7.3.1 Thesymmetricspace...... 269 7.3.2 Stabilization by projection into the symmetric subspace ...... 270 7.4QuantumError-CorrectingCodes ...... 271 7.4.1 Classical error-detecting and -correctingcodes...... 272 7.4.2 Framework for quantum error-correcting codes ...... 278 7.4.3 Casestudies...... 284 7.4.4 Basic methods to design quantum error-correcting codes ...... 289 7.4.5 Stabilizercodes...... 292 7.5Fault-tolerantQuantumComputation...... 296 7.5.1 Fault-tolerant quantum error correction ...... 297 7.5.2 Fault-tolerant quantum gates ...... 300 7.5.3 Concatenatedcoding ...... 304 7.6ExperimentalQuantumProcessors-...... 306 7.6.1 Mainapproaches...... 307 7.6.2 Iontrap...... 308 7.6.3 CavityQED...... 310 7.6.4 Nuclear magnetic resonance (NMR) ...... 311 7.6.5 Otherpotentialtechnologies...... 313
8 INFORMATION 315 8.1QuantumEntropyandInformation...... 316 8.1.1 Basic concepts of classical information theory ...... 317 8.1.2 Quantum entropy and information ...... 318 8.2QuantumChannelsandDataCompression...... 320 8.2.1 Quantum sources, channels and transmissions ...... 321 8.2.2 Shannon’scodingtheorems...... 323 8.2.3 Schumacher’snoiselesscodingtheorem...... 325 8.2.4 Densequantumcoding...... 328 8.2.5 Quantum Noisy Channel Transmissions ...... 330 8.2.6 Capacities of erasure and depolarizing channels ...... 332 8.3QuantumEntanglement...... 332 8.3.1 Transformation and the partial order of entangled states...... 333 8.3.2 Entanglement purification/distillation ...... 333 8.3.3 Entanglement concentration and dilution ...... 336 8.3.4 Quantifyingentanglement...... 336 8.3.5 Boundentanglement...... 337 8.4 Quantum information processing principles and primitives ...... 338 8.4.1 Search for quantum information principles ...... 338 8.4.2 Quantum information processing primitives ...... 339
APPENDIX 341 9.1QuantumTheory ...... 341 9.1.1 Pre-historyofquantumtheory...... 342 9.1.2 Heisenberg’s uncertainty principle...... 345 9.1.3 Quantumtheoryversusphysicalreality...... 349 9.1.4 Quantummeasurements...... 350 9.1.5 Quantumparadoxes...... 352 9.1.6 Thequantumparadox...... 357 9.1.7 Interpretationsofquantumtheory...... 358 CONTENTS xi
9.1.8 Incompletenessofquantummechanics...... 363 9.2 Hilbert Space Framework for Quantum Computing ...... 364 9.2.1 Hilbertspaces...... 365 9.2.2 Linearoperators...... 368 9.2.3 Mixed states and density matrices ...... 371 9.2.4 Probabilities and observables ...... 375 9.2.5 Evolutionofquantumstates...... 376 9.2.6 Measurements...... 376 9.2.7 TensorproductsandHilbertspaces...... 377 9.2.8 Generalized measurements-POVmeasurements...... 378 9.3DeterministicandRandomizedComputing...... 381 9.3.1 Computingmodels...... 381 9.3.2 Randomizedcomputations...... 386 9.3.3 Complexityclasses...... 387 9.3.4 Computationaltheses...... 391 9.4Exercises...... 392 9.5 Historical and BibliographicalReferences...... 396
Bibliography 403 xii CONTENTS Preface xiii
PREFACE
Come forth into the light of things. LetNaturebeyourteacher. W. Wordsworth (1770–1850)