Decoherence in a superconducting quantum bit circuit

Decoherence in quantum bit circuits is presently a major limitation to their use for quantum computing purposes. We present experiments, inspired from NMR, that characterize decoherence in a particular super- conducting quantum bit circuit, the quantronium. We introduce a general framework for the analysis of deco- herence, 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.