In measurements, einselection replaces quantum entanglement between the apparatus and the measured system with the classical correlation. Combination of einselection with dynamics leads to the idealizations of a point and of a classical trajectory. Einselection enforces classicality by imposing an effective ban on the vast majority of the Hilbert space, eliminating especially the flagrantly nonlocal ''Schrodinger-cat states.'' The classical structure of phase space emerges from the quantum Hilbert space in the appropriate macroscopic limit. They can retain correlations with the rest of the universe in spite of the environment.
This leads to environment-induced superselection or einselection, a quantum process associated with selective loss of information. Thus decoherence is caused by the interaction in which the environment in effect monitors certain observables of the system, destroying coherence between the pointer states corresponding to their eigenvalues. In the past two decades it has become increasingly clear that many (perhaps all) of the symptoms of classicality can be induced in quantum systems by their environments. The basic elements are accessible to experimental investigation with current technology.read more read lessĪbstract: as quantum engineering. Our methods exploit feedback from photo-detectors and are robust against errors from photon loss and detector inefficiency. Here we show that efficient quantum computation is possible using only beam splitters, phase shifters, single photon sources and photo-detectors. Until now, it suffered from the requirement for non-linear couplings between optical modes containing few photons. The proposal is appealing because of the ease with which photon interference can be observed. One of the earliest proposals for quantum computation is based on implementing a quantum bit with two optical modes containing one photon. One of the greatest challenges now is to implement the basic quantum-computational elements in a physical system and to demonstrate that they can be reliably and scalably controlled. read more read lessĪbstract: Quantum computers promise to increase greatly the efficiency of solving problems such as factoring large integers, combinatorial optimization and quantum physics simulation. The case of a complete basis Bibliography. Error correction: concrete procedures11.1. Error correction in the computation process: general principles10.1. Quantum codes (definitions and general properties)8.1. Computations with perturbations: the choice of a model §8. Polynomial quantum algorithm for the stabilizer problem §7.
Generalized quantum control and universal schemes §5. Construction of various operators from the elements of a basis4.3.
Boolean schemes and sequences of operations2.2.
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