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.

High cooperativity between a phosphorus donor spin ensemble and a microwave resonator

  1. Christoph W. Zollitsch,
  2. Kai Mueller,
  3. David P. Franke,
  4. Sebastian T. B. Goennenwein,
  5. Martin S. Brandt,
  6. Rudolf Gross,
  7. and Hans Huebl
We investigate the coupling of an ensemble of phosphorus donors in an isotopically purified 28Si host lattice interacting with a superconducting coplanar waveguide resonator. The microwave
transmission spectrum of the resonator shows a normal mode splitting characteristic for high cooperativity. The evaluated collective coupling strength geff is of the same order as the loss rate of the spin system γ, indicating the onset of strong coupling. We develop a statistical model to describe the influence of temperature on the coupling strength from 50mK to 3.5K and find a scaling of the coupling strength with the square root of the number of thermally polarized spins.