Decoherence in quantum bit circuits is presently a major limitation to their
use for quantum computing purposes. We present experiments, inspired from NMR,
that characterise decoherence in a particular superconducting quantum bit
circuit, the quantronium. We introduce a general framework for the analysis of
decoherence, based on the spectral densities of the noise sources coupled to
the qubit. Analysis of our measurements within this framework indicates a
simple model for the noise sources acting on the qubit. We discuss various
methods to fight decoherence.Comment: Long paper. 65 pages, 18 Figure
Coherent superpositions of quantum states have already been demonstrated in different superconducting circuits based on Josephson junctions. These circuits are now considered for implementing quantum bits. We report on experiments in which the state of a qubit circuit, the quantronium, is efficiently manipulated using methods inspired from nuclear magnetic resonance (NMR): multipulse sequences are used to perform arbitrary operations, to improve their accuracy, and to fight decoherence.
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