Synopsis
There is a particular moment that recurs in every course I have taught on this subject. A student who has followed the algebra faithfully for six weeks — who can expand a Bell state, compute a partial trace, and recite the no-cloning theorem — puts down her pen and asks the question that the algebra never answers: but what actually stops Eve? Not what forbids her in the formalism, but what physically prevents a determined adversary with a fibre splice, a cryogenic detector and a generous budget from copying the light that passes down the cable.
This book is my attempt at a complete answer. It is a long answer, because the honest one has three parts. The first part is physical: measurement disturbs, non-orthogonal states cannot be distinguished with certainty, and unknown quantum states cannot be duplicated. The second part is information-theoretic: those physical facts can be converted, by a carefully designed protocol and a great deal of classical post-processing, into a quantitative bound on how many bits an adversary can hold about a key. The third part is engineering, and it is the part textbooks usually skip: real sources emit Poissonian pulses rather than single photons, real detectors click in the dark, real fibre depolarises, real satellites move, and every one of these imperfections opens a door that the idealised security proof assumed was shut.
Most existing treatments do one of these three well and gesture at the other two. Books written by physicists develop the formalism beautifully and then present BB84 as a two-page epilogue. Books written by cryptographers analyse security definitions with great care but treat the optical channel as an abstract symbol. Engineering handbooks catalogue components without explaining why the protocol needs them. A student who reads only one of these emerges able to talk about quantum communication but unable to design, evaluate or attack a system. My aim has been to write a single volume in which the Hilbert-space algebra of Chapter 2, the Holevo bound of Chapter 5, the fibre attenuation coefficient of Chapter 7 and the finite-key correction of Chapter 13 all visibly serve the same purpose.
What this book covers
Part I develops the foundations: the state space of a qubit, measurement and open-system dynamics, entanglement and nonlocality, and the elements of quantum information theory that determine what a channel can carry. Part II turns to the physical layer — how qubits are actually encoded in light, what sources and detectors exist, and how optical fibre, the atmosphere and free space degrade a quantum signal. Part III is the core of the book: teleportation and superdense coding, the prepare-and-measure and entanglement-based families of key distribution, measurement-device-independent and twin-field protocols, security proofs and finite-key analysis, practical attacks, and the classical post-processing pipeline without which no raw key becomes a secret key. Part IV looks outward to repeaters, memories, network architectures, satellite links, standardisation and the wider post-quantum landscape, and closes with a chapter of extended projects.
Five appendices supply the mathematical and engineering background that the main text assumes, together with worked simulation code, hints for selected exercises, and a glossary.
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