Book excerpt: QUANTUM BLOCKCHAIN While addressing the security challenges and threats in blockchain, this book is also an introduction to quantum cryptography for engineering researchers and students in the realm of information security. Quantum cryptography is the science of exploiting quantum mechanical properties to perform cryptographic tasks. By utilizing unique quantum features of nature, quantum cryptography methods offer everlasting security. The applicability of quantum cryptography is explored in this book. It describes the state-of-the-art of quantum blockchain techniques and sketches how they can be implemented in standard communication infrastructure. Highlighting a wide range of topics such as quantum cryptography, quantum blockchain, post-quantum blockchain, and quantum blockchain in Industry 4.0, this book also provides the future research directions of quantum blockchain in terms of quantum resilience, data management, privacy issues, sustainability, scalability, and quantum blockchain interoperability. Above all, it explains the mathematical ideas that underpin the methods of post-quantum cryptography security. Readers will find in this book a comprehensiveness of the subject including: The key principles of quantum computation that solve the factoring issue. A discussion of a variety of potential post-quantum public-key encryption and digital signature techniques. Explanations of quantum blockchain in cybersecurity, healthcare, and Industry 4.0. Audience The book is for security analysts, data scientists, vulnerability analysts, professionals, academicians, researchers, industrialists, and students working in the fields of (quantum) blockchain, cybersecurity, cryptography, and artificial intelligence with regard to smart cities and Internet of Things.