ABSTRACT
Quantum physics offers the world of communications both opportunities and threats. Opportunities, from the fundamental nature of indivisible quanta and our inability to copy them, and threats because of the enormous potential of quantum computing provided the decoherence obstacle can be overcome so that large-scale quantum registers can be manipulated. This entry concentrates on secure key distribution (QKD) for cryptography as it represents the most developed quantum technology. In QKD, classical information is transmitted using a quantum resource. However, quantum information itself can also be transmitted: This is the basis of quantum teleportation. This area has developed into one of the major growth fields of modern physics and is likely to generate new insights, new applications, and new technologies.
