Scientific progress in physics and mathematics has led to the development of efficient technology for communicating and distributing information in the 20th century. This technology forms one of the cornerstones of information society and our global economy, which are highly reliant on secure methods to transfer and distribute information quickly. To satisfy ever-increasing demands for speed and security, encryption and communication methods are constantly improved and there is an ongoing effort to develop corresponding technology further.

Classical information theory is not usually part of a physics undergraduate degree course. The theory assumes simple classical properties of physical systems and based on those a largely mathematical theory of information is established. The obtained results are independent of the chosen implementation and its physical details. However, one still has to acknowledge that information is physical, that is information carriers, senders, and receivers obey the laws of physics. The notion of quantum information comes about since it turns out that the rules of quantum mechanics violate some of the basic physics assumptions in classical information theory. The consequences of this are many, ranging from improved channel capacities, the possibility of physically secure communication protocols, to invalidating some beliefs about the security of classical communication protocols.

So far the approach followed in this book was to assume idealized physical systemswhich perfectly implement qubits and then demonstrate how quantum computing algorithms can be realized using perfect qubits. Error correction and fault tolerance were only mentioned briefly.