Loss mechanisms in superconducting thin film microwave resonators

  1. Jan Goetz,
  2. Frank Deppe,
  3. Max Haeberlein,
  4. Friedrich Wulschner,
  5. Christoph W. Zollitsch,
  6. Sebastian Meier,
  7. Michael Fischer,
  8. Peter Eder,
  9. Edwar Xie,
  10. Kirill G. Fedorov,
  11. Edwin P. Menzel,
  12. Achim Marx,
  13. and Rudolf Gross
We present a systematic analysis of the internal losses of superconducting coplanar waveguide microwave resonators based on niobium thin films on silicon substrates. At millikelvin
temperatures and low power, we find that the characteristic saturation power of two-level state (TLS) losses shows a pronounced temperature dependence. Furthermore, TLS losses can also be introduced by Nb/Al interfaces in the center conductor, when the interfaces are not positioned at current nodes of the resonator. In addition, we confirm that TLS losses can be reduced by proper surface treatment. For resonators including Al, quasiparticle losses become relevant above \SI{200}{\milli\kelvin}. Finally, we investigate how losses generated by eddy currents in the conductive material on the backside of the substrate can be minimized by using thick enough substrates or metals with high conductivity on the substrate backside.

Spin-boson model with an engineered reservoir in circuit quantum electrodynamics

  1. Max Haeberlein,
  2. Frank Deppe,
  3. Andreas Kurcz,
  4. Jan Goetz,
  5. Alexander Baust,
  6. Peter Eder,
  7. Kirill Fedorov,
  8. Michael Fischer,
  9. Edwin P. Menzel,
  10. Manuel J. Schwarz,
  11. Friedrich Wulschner,
  12. Edwar Xie,
  13. Ling Zhong,
  14. Enrique Solano,
  15. Achim Marx,
  16. Juan José García-Ripoll,
  17. and Rudolf Gross
A superconducting qubit coupled to an open transmission line represents an implementation of the spin-boson model with a broadband environment. We show that this environment can be
engineered by introducing partial reflectors into the transmission line, allowing to shape the spectral function, J({\omega}), of the spin-boson model. The spectral function can be accessed by measuring the resonance fluorescence of the qubit, which provides information on both the engineered environment and the coupling between qubit and transmission line. The spectral function of a transmission line without partial reflectors is found to be Ohmic over a wide frequency range, whereas a peaked spectral density is found for the shaped environment. Our work lays the ground for future quantum simulations of other, more involved, impurity models with superconducting circuits.