Characterization of superconducting structures designed for qubit realizations

We implement the impedance measurement technique in order to characterize superconducting structures designed for applications in quantum computing. We report an experimental study of the circuit consisting of three small Josephson junctions incorporated in a low-inductance superconducting loop. Measurements of the circuit response to an applied weak ac bias as a function of the external magnetic flux phi(e) yield complete information on the properties of the circuit. We found that the system displays two metastable states. From experimental data, we have determined the magnetic field dependent Josephson energy U(phi(e)) of these states, the potential barrier DeltaU between these states, as well as the difference of the critical currents of the junction in the loop. (C) 2002 American Institute of Physics.