h would make it possible to explore Introduction databases with a quadratic increase in speed, as compared with any classical computer. As to Shor's factorization, it would represent the end of classical cryptography in global use today. It is, therefore, important to gain a basic understanding of both Grover and Shor QC algorithms, which is not a trivial task altogether! Such an understanding not only conveys a flavor of QC power and potentials (as due to the property of quantum parallelism), but it also brings an awareness of the high complexity of quantum-computing circuits, and thus raises true questions about practical hardware, or massive or parallel quantum-gates implementation. Quantum information theory really begins with Chapter 21, along with the intro- duction of von Neumann entropy, and related variants echoing the classical ones. With Chapters 22 and 23, the elegant analog of Shannon's channel source-coding and channel- capacity theorems, this time for quantum channels, is reached with the Holevo bound concept and the so-called HSW theorem. Chapter 24 is about quantum error correction, in which we learn that various types of single-qubit errors can be effectively and elegantly corrected with the nine-qubit Shor code or more powerfully with the equally elegant, but more universal seven-qubit CSS code. The book concludes with a hefty chapter dedicated to classical and quantum cryptog- raphy together. It is the author's observation and conviction that quantum cryptography cannot be safely approached (academically speaking) without a fair education and aware- ness of what cryptography, and overall, network security are all about. Indeed, there is a fallacy in believing in "absolute security" of one given ring in the security chain. Quan- tum cryptography, or more specifically as we have seen earlier, quantum key distribution (QKD), is only one constituent of the security issue, and contrary to common belief, it is itself exposed to several forms of potential attacks. Only with such a state of mind can c