Joint JQI/Condensed Matter Physics Seminar

2 p.m., Thursday, May 3, 2007
Room 1201, Physics Building

 Two-Level Systems in Superconducting Circuit Resonators

Kevin Osborn

(Laboratory for Physical Sciences)

Abstract:  Superconducting qubits currently suffer from short coherence times. A couple of years ago wiring dielectrics used in the circuit fabrication of superconducting phase qubits were a suspect in limiting decoherence times. To uncover this, microwave resonators were used to measure the dissipation in amorphous dielectrics at stored resonator energies down to a single photon. It was found that the nonlinear loss tangent of the dielectric was consistent with a continuum of parasitic two-level systems. Phase qubits also exhibit splittings due to the interaction of the qubit with discrete two-level systems. To study these splittings further, we have recently fabricated a high-Q Josephson junction resonator that allows us to tune the resonance frequency. We measure the Josephson junction resonator down to the single photon limit and observe discrete two-level systems, similar to those in the qubit, that are attributed to defects in the Josephson junction. At higher photon numbers the resonator non-linearity becomes apparent and a bifurcation of dynamic states occurs, which has further applications in superconducting quantum computing.

Bio: Kevin Osborn received his doctorate in physics from the University of Illinois at Urbana-Champaign in 2001 under the advisement of Professor Dale Van Harlingen, with a thesis on critical fluctuations of the superfluid density in high-temperature superconducting films. He then went to the National Institute of Standards and Technology in Boulder, Colorado and completed two postdoctoral projects. In the first project he measured individual electrons tunneling onto InGaAs quantum dots using Al single-electron transistors. Then in 2004 he received a National Research Council Postdoctoral Award and stayed at NIST-Boulder to investigate decoherence mechanisms caused by two-level systems in superconducting phase qubits with microwave resonators. In January of 2007, Kevin came to the Laboratory for Physical Sciences at the University of Maryland to lead research on superconducting circuit resonators for quantum computing.

Host:  Lobb
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