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.
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Host: Lobb
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