Long coherence time in a superconducting persistent-current qubit

We report relaxation times $T_1$ in excess of 10 $\mu$s in aluminum persistent-current qubits (flux qubits), read out by a switching dc SQUID. At the sweet spot in flux bias, spin-echo refocusing gives a relaxation-limited coherence time $T_2=2T_1 $. The free-induction decay time constant $T_2^*$ reaches 2.5 $\mu$s. Detuning the quantization axis away from this optimal point, the much-increased sensitivity to flux noise enhances the phase-decay rate. We confirm the Gaussian phase decay indicative of flux noise with a nearly $1/f$ spectral density, as well as the magnitude of the noise reported in ref.\@ [1]. Contrary to dephasing, the relaxation rate has weak dependence on quantization axis (flux-bias). Finding the microscopic mechanism for $T_1$ relaxation remains one of the most important topics in superconducting qubit research. \newline[1] Yoshihara \emph{et al.}, PRL \textbf{97}, 167001 (2006)