## The Physics of Quantum Information: Quantum Cryptography, Quantum Teleportation, Quantum ComputationDirk Bouwmeester, Artur K. Ekert, Anton Zeilinger Information is stored, transmitted and processed by physical means. Thus, the concept of information and computation can be formulated in the con text of a physical theory and the study of information requires ultimately experimentation. This sentence, innocuous at first glance, leads to non-trivial consequences. Following Moore's law, about every 18 months microprocessors double their speed and, it seems, the only way to make them significantly faster is to make them smaller. In the not too distant future they will reach the point where the logic gates are so small that they consist of only a few atoms each. Then quantum-mechanical effects will become important. Thus, if computers are to continue to become faster (and therefore smaller), new, quantum technology must replace or supplement what we have now. But it turns out that such technology can offer much more than smaller and faster microprocessors. Several recent theoretical results have shown that quantum effects may be harnessed to provide qualitatively new modes of communication and computation, in some cases much more powerful than their classical counterparts. This new quantum technology is being born in many laboratories. The last two decades have witnessed experiments in which single quantum particles of different kinds were controlled and manipulated with an unprecedented preci sion. Many "gedanken" experiments, so famous in the early days of quantum mechanics, have been carried out. |

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### Contents

Quantum Cryptography | 15 |

Quantum Dense Coding | 49 |

Concepts of Quantum Computation | 93 |

Copyright | |

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Alice and Bob Alice's amplitude atom basis beam beamsplitter Bell Bell-state bits Bob's classical computer CNOT coherent coincidence components cooling correlations corresponding coupling decoherence density matrix described detection detectors detuning down-conversion encoded ensemble entangled photons entanglement purification entanglement swapping environment EPR pair error operators example excited experimental experiments exponentially fibre fidelity field frequency function Hadamard Hamiltonian implement initial input interaction interferometer ion trap key distribution laser linear matrix maximally entangled measurement mode motion NMR quantum computers obtained optical orthogonal oscillator output performed phase shift photon Phys physical possible probability problem procedure protocol pulse quantum algorithm quantum channel quantum computer quantum cryptography quantum error correction quantum gates quantum information quantum key distribution quantum mechanics quantum system quantum teleportation qubit Rabi realise result rotation scheme Sect shown in Fig sideband signal spin string superposition syndrome transform transition transmission trapped ions two-particle unitary vibrational