This chapter addresses quantum information transmission and processing. Quantum noise is dominant in long-haul transmission lines, even for strong signals. Amplifier noise is avoided by using a noise-free amplifier. Injecting a strongly squeezed state into an unused port of a coupler reduces branching noise. The next section explains the no-cloning theorem and the theory and experimental evidence for (imperfect) quantum cloning machines. The use of single photons for secure quantum key distribution in cryptography is then discussed. Entanglement as a quantum resource first appears in the explanation of quantum dense coding and the inverse process of quantum teleportation. The chapter ends with a brief discussion of quantum computing, including quantum parallelism, quantum logic gates, and quantum circuits. A survey of experiments in quantum computing is followed by a study of proposals for combining linear optics with local measurements to construct quantum computers.
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