A.J. Dragt

Sub-gap leakage in Nb/AlO/sub x//Nb and Al/AlO/sub x//Al Josephson junctions

In an effort to determine the suitability of Josephson junctions for applications in quantum computation, we measured low-noise dc current-voltage characteristics of Nb/AlO/sub x//Nb and Al/AlO/sub x//Al junctions in the temperature range from 90 mK to 1 K. Nb-based samples were obtained from several different facilities with critical currents of a few /spl mu/A and critical current densities between 100 and 3000 A/cm/sup 2/. We fabricated the Al-based samples using double-angle evaporation and obtained critical currents of a few /spl mu/A with critical current densities of about 30 A/cm/sup 2/. We found that the sub-gap leakage current in the Nb-based samples does not depend on temperature in the range 90 mK to 1 K, whereas that for the Al-based samples follows the expected BCS behavior to about 150 mK. Our Al-based samples have a lower level of dissipation than Nb-based devices; however, both Al- and Nb-based samples achieved dissipation levels sufficiently low for some quantum computing applications.

Strong-Field Effects in the Rabi Oscillations of the Superconducting Phase Qubit

Rabi oscillations have been observed in many superconducting devices, and represent prototypical logic operations for quantum bits (qubits) in a quantum computer. We use a three-level multiphoton analysis to understand the behavior of the superconducting phase qubit (current-biased Josephson junction) at high microwave drive power. Analytical and numerical results for the ac Stark shift, single-photon Rabi frequency, and two-photon Rabi frequency are compared to measurements made on a dc SQUID phase qubit with Nb/AlOx/Nb tunnel junctions. Good agreement is found between theory and experiment.