This book aims at giving an introduction to the principles and techniques of quantum cryptography, including secret-key distillation, as well as some more advanced topics. As quantum cryptography is now becoming a practical reality with products available commercially, it is important to focus not only on the theory of quantum cryptography but also on practical issues. For instance, what kind of security does quantum cryptography offer? How can the raw key produced by quantum cryptography be efficiently processed to obtain a usable secret key? What can safely be done with this key? Many challenges remain before these questions can be answered in their full generality. Yet quantum cryptography is mature enough to make these questions relevant and worth discussing.

The content of this book is based on my Ph.D. thesis [174], which initially focused on continuous-variable quantum cryptography protocols. When I decided to write this book, it was essential to include discrete-variable protocols so as to make its coverage more balanced. In all cases, the continuous and discrete-variable protocols share many aspects in common, which makes it interesting to discuss about them both in the same manuscript.

Quantum cryptography is a multi-disciplinary subject and, in this respect, it may interest readers with different backgrounds. Cryptography, quantum physics and information theory are all necessary ingredients to make quantum cryptography work. The introductory material in each of these fields should make the book self-contained. If necessary, references are given for further readings.

Structure of this book

The structure of this book is depicted in Fig. 0.1. Chapter 1 offers an overview of quantum cryptography and secret-key distillation.