Dephasing of a superconducting flux qubit*

In order to gain a better understanding of the origin of decoherence in superconducting flux qubits, we have measured the magnetic field dependence of the characteristic energy relaxation time $T_{1}$ and echo phase relaxation time $T_{2}^{echo}$ near the optimal operating point of a flux qubit. We have measured $T_{2}^{echo}$ by means of the phase cycling method. At the optimal point, we found the relation $T_{2}^{echo} \quad \sim 2T_{1}$. This means that the echo decay time is limited by the energy relaxation ($T_{1}$ process). Moving away from the optimal point, we observe a linear increase of the phase relaxation rate ($1/T_{2}^{echo})$ with the applied external magnetic flux. This behavior can be well explained by the influence of magnetic flux noise with a $1/f$ spectrum on the qubit. \begin{flushright} K.Kakuyanagi, et al., cond-mat/0609564 \end{flushright}