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Characterisation and understanding of decoherence in the quantronium 'quantum bit circuit'

It is now established that several types of super-conducting circuits based on Josephson junctions are sufficiently quantum that simple manipulations of their quantum state can be performed. These quantum bit circuits are the basic building blocks of a quantum processor. The coherence time of the quantum state is an essential figure of merit, being related to the number of qubit operations that can be performed without error.

Despite significant advances in coherence times during recent years, with coherence times of order of a fraction of microsecond reached, decoherence due to the coupling between the quantum circuit and the degrees of freedom of the environment still severely hinders using these circuits for the development of a quantum processor, even with a small number of qubits. Thus the quantitative characterization and understanding of decoherence processes is presently a central issue for the development of qubit circuits.

In this result, we present experiments on the quantronium qubit, and we develop a general framework for their theoretical analysis. We show that a simple model for the spectral densities of the noise sources coupled to the qubit allows accounting for the experimental findings. The framework developed can be applied to all super-conducting qubits.

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