1 Scientific RepoRts | 6:23786 | DOI: 10.1038/srep23786 www.nature.com/scientificreports Decoherence spectroscopy with individual two-level tunneling defects Jürgen Lisenfeld 1 , Alexander Bilmes 1 , shlomi Matityahu 2 , sebastian Zanker 3 , Michael Marthaler 3 , Moshe schechter 2 , Gerd schön 3 , Alexander shnirman 4,5 , Georg Weiss 1 & Alexey V. Ustinov 1,6,7 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. Although the existence of two-level tunneling systems in amorphous materials has been known for decades, they have attracted much renewed interest afer their detrimental efect on the performance of microfabricated quan- tum devices was discovered. Tere is evidence that TLSs reside in surface oxides of thin-flm circuit electrodes 1 , at disordered interfaces 2 , and in the tunnel barrier of Josephson junctions 3 . Since TLSs possess both electric and elastic dipole moments by which they couple to their environment, they generate noise in various devices rang- ing from microwave resonators and kinetic inductance photon detectors 4 through single-electron transistors 5 to even nanomechanical resonators 6 . In state-of-the-art superconducting qubits, interaction with individual TLSs constitutes a major decoherence mechanism, where they give rise to fuctuations in time 7 and frequency 8 of qubit relaxation rates. On the other hand, this strong interaction turns qubits into versatile tools for studying the distri- bution of TLS 9,10 , their physical origin 11 and mutual interactions 12 as well as their quantum dynamics 13 . Te omnipresence of TLSs interference is contrasted by a notable lack of certainty regarding the microscopic nature of the tunneling entity 14 . Figure 1a illustrates some proposed models of TLS formation in the amorphous tunnel barrier of a Josephson junction: the tunnelling of individual or small groups of atoms between two confg- urations 15,16 , displacements of dangling bonds, and hydrogen defects 17 . Near the interface with superconducting electrodes, TLSs may also arise from bound electron/hole Andreev states 18 or Kondo-fuctuators 19 . In this work, we present frst direct measurements of the decoherence rates of individual TLSs in dependence of their strain-tuned internal asymmetry energy parameter. Our experiment provides unprecedented information about the spectrum of the environment to which a TLS couples and the nature of this coupling. 1 Physikalisches Institut, Karlsruhe Institute of Technology (KIT), 76131 Karlsruhe, Germany. 2 Department of Physics, Ben-Gurion University of the Negev, Beer Sheva 84105, Israel. 3 institut für theoretische festkörperphysik, Kit, 76131 Karlsruhe, Germany. 4 Institut für Theorie der Kondensierten Materie, KIT, 76131 Karlsruhe, Germany. 5 L. D. Landau Institute for Theoretical Physics RAS, Kosygina street 2, 119334 Moscow, Russia. 6 National University of Science and Technology MISIS, Leninsky prosp. 4, Moscow, 119049, Russia. 7 Russian Quantum Center, 100 Novaya St., Skolkovo, 143025 Moscow region, Russia. Correspondence and requests for materials should be addressed to J.L. (email: Juergen.Lisenfeld@kit.edu) received: 07 January 2016 accepted: 14 March 2016 Published: 31 March 2016 opeN