Decoherence of a Josephson Quantum Bit during its Free Evolution: The Quantronium

Relaxation and coherence times of the quantronium, a Josephson Quantum Bit (qubit), as a function of its operating point have been measured. Several coherence times are obtained from complementary techniques such as resonance linewidth, Ramsey fringes, detuning pulse, or spin-echo measurements. We find we can explain their variations by a simple model involving modified l/f charge and phase noise spectral densities.