Large-amplitude driving of a superconducting artificial atom: Interferometry, cooling, and amplitude spectroscopy

Superconducting persistent-current qubits are quantum-coherent artificial atoms with multiple, tunable energy levels. In the presence of large-amplitude harmonic excitation, the qubit state can be driven through one or more of the constituent energy-level avoided crossings. The resulting Landau-Zener-Stueckelberg (LZS) transitions mediate a rich array of quantum-coherent phenomena. We review here three experimental works based on LZS transitions: Mach-Zehnder-type interferometry between repeated LZS transitions, microwave-induced cooling, and amplitude spectroscopy. These experiments exhibit a remarkable agreement with theory, and are extensible to other solid-state and atomic qubit modalities. We anticipate they will find application to qubit state-preparation and control methods for quantum information science and technology.Comment: 13 pages, 5 figure

Charge transport in normal metal–magnesiumdiboride junctions

The influence of the multiband nature of superconductivity in MgB2 on the charge transport properties of MgB2 tunnel junctions and normal metal–MgB2 heterostructures has been theoretically modeled and experimentally explored. The conductance as function of barrier height and angle is calculated. Planar c-axis tunnel junctions are realized in which two gaps are resolved in the tunnel spectrum. For junctions with additional conductance in the direction of the crystallographic a–b plane we observe a sharp resonance that can possibly be attributed to a Leggett mod

Temperature dependence measurements of the supercurrent-phase relationship in niobium nanobridges

The current-phase relationship has been measured as a function of temperature for niobium nanobridges with different widths. A deformation from Josephson-like sinusoidal characteristics at high temperatures to sawtooth shaped curves at intermediate and multivalued relationships at low temperatures was observed. Based on this, possible hysteresis in the current-voltage characteristics of niobium nanobridge superconducting quantum interference devices can be attributed to phase slippage