Freiburg 2019 – wissenschaftliches Programm

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FM: Fall Meeting

FM 73: Quantum Sensing: Applications & Spectroscopy

FM 73.2: Talk

Donnerstag, 26. September 2019, 14:15–14:30, Aula

Probing Defects in Superconducting Qubits with Electric Fields — •Jürgen Lisenfeld¹, Alexander Bilmes¹, Anthony Megrant², Julian Kelly², Rami Barends², Paul Klimov², John M. Martinis², Georg Weiss¹, and Alexey V. Ustinov^1,3 — ¹Physikalisches Institut, Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany — ²Google Inc., Santa Barbara, USA — ³Russian Quantum Center, National University of Science and Technology MISIS, Moscow 119049, Russia

The coherence of superconducting quantum bits is severely reduced by atomic-scale material defects which provide a bath of parasitic two-state tunneling systems, so-called TLS. Here, we present a method to distinguish TLS contained in the tunnel barriers of the qubit's Josephson junctions from TLS residing on the surfaces of superconducting electrodes or the substrate. For this, we characterize the defect's responses to applied mechanical strain and electric DC-fields using the qubit as a sensor.

We find that 60% of the total dielectric loss in the investigated Xmon-qubit is due to defects on film interfaces and the substrate, while 40% comes from TLS in the tunnel barriers of Josephson junctions. Measurements of the TLS-qubit coupling strengths reveal that only about 3% of all defects are residing in the small qubit tunnel junctions, while most are contained in the large stray junctions that are used as wiring interconnects. The presented methods guide the way towards improved qubit coherence by indicating the critical circuit interfaces and fabrication steps that are promoting defect formation.