title: Decoherence spectroscopy with individual two-level tunneling defects.
abstract: Recent progress with microfabricated quantum devices has revealed that an ubiquitous source of noise originates in tunneling material defects that give rise to a sparse bath of parasitic two-level systems (TLSs). For superconducting qubits, TLSs residing on electrode surfaces and in tunnel junctions account for a major part of decoherence and thus pose a serious roadblock to the realization of solid-state quantum processors. Here, we utilize a superconducting qubit to explore the quantum state evolution of coherently operated TLSs in order to shed new light on their individual properties and environmental interactions. We identify a frequency-dependence of TLS energy relaxation rates that can be explained by a coupling to phononic modes rather than by anticipated mutual TLS interactions. Most investigated TLSs are found to be free of pure dephasing at their energy degeneracy points, around which their Ramsey and spin-echo dephasing rates scale linearly and quadratically with asymmetry energy, respectively. We provide an explanation based on the standard tunneling model, and identify interaction with incoherent low-frequency (thermal) TLSs as the major mechanism of the pure dephasing in coherent high-frequency TLS.
authors.affiliation: Physikalisches Institut, Karlsruhe Institute of Technology (KIT), 76131 Karlsruhe, Germany.
authors.forename: Jürgen
authors.initials: J
authors.lastname: Lisenfeld
authors.affiliation: Physikalisches Institut, Karlsruhe Institute of Technology (KIT), 76131 Karlsruhe, Germany.
authors.forename: Alexander
authors.initials: A
authors.lastname: Bilmes
authors.affiliation: Department of Physics, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel.
authors.forename: Shlomi
authors.initials: S
authors.lastname: Matityahu
authors.affiliation: Institut für Theoretische Festkörperphysik, KIT, 76131 Karlsruhe, Germany.
authors.forename: Sebastian
authors.initials: S
authors.lastname: Zanker
authors.affiliation: Institut für Theoretische Festkörperphysik, KIT, 76131 Karlsruhe, Germany.
authors.forename: Michael
authors.initials: M
authors.lastname: Marthaler
authors.affiliation: Department of Physics, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel.
authors.forename: Moshe
authors.initials: M
authors.lastname: Schechter
authors.affiliation: Institut für Theoretische Festkörperphysik, KIT, 76131 Karlsruhe, Germany.
authors.forename: Gerd
authors.initials: G
authors.lastname: Schön
authors.affiliation: Institut für Theorie der Kondensierten Materie, KIT, 76131 Karlsruhe, Germany.|L. D. Landau Institute for Theoretical Physics RAS, Kosygina street 2, 119334 Moscow, Russia.
authors.forename: Alexander
authors.initials: A
authors.lastname: Shnirman
authors.affiliation: Physikalisches Institut, Karlsruhe Institute of Technology (KIT), 76131 Karlsruhe, Germany.
authors.forename: Georg
authors.initials: G
authors.lastname: Weiss
authors.affiliation: Physikalisches Institut, Karlsruhe Institute of Technology (KIT), 76131 Karlsruhe, Germany.|National University of Science and Technology MISIS, Leninsky prosp. 4, Moscow, 119049, Russia.|Russian Quantum Center, 100 Novaya St., Skolkovo, 143025 Moscow region, Russia.
authors.forename: Alexey V
authors.initials: AV
authors.lastname: Ustinov