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 (T₁) and echo phase relaxation time (T₂^echo) near the optimal operating point of a flux qubit. We have measured T₂^echo by means of the phase cycling method. At the optimal point, we found the relation T₂^echo≈2T₁. This means that the echo decay time is limited by the energy relaxation (T₁ process). Moving away from the optimal point, we observe a linear increase of the phase relaxation rate (1/T₂^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.