J. Lee

Direct Measurement of Microwave Loss in Nb Films for Superconducting Qubits

  1. B. Abdisatarov,
  2. D. Bafia,
  3. A. Murthy,
  4. G. Eremeev,
  5. H. E. Elsayed-Ali,
  6. J. Lee,
  7. A. Netepenko,
  8. C. P. A. Carlos,
  9. S. Leith,
  10. G. J. Rosaz,
  11. A. Romanenko,
  12. and A. Grassellino
Niobium films are a key component in modern two-dimensional superconducting qubits, yet their contribution to the total qubit decay rate is not fully understood. The presence of different
layers of materials and interfaces makes it difficult to identify the dominant loss channels in present two-dimensional qubit designs. In this paper we present the first study which directly correlates measurements of RF losses in such films to material parameters by investigating a high-power impulse magnetron sputtered (HiPIMS) film atop a three-dimensional niobium superconducting radiofrequency (SRF) resonator. By using a 3D SRF structure, we are able to isolate the niobium film loss from other contributions. Our findings indicate that microwave dissipation in the HiPIMS-prepared niobium films, within the quantum regime, resembles that of record-high intrinsic quality factor of bulk niobium SRF cavities, with lifetimes extending into seconds. Microstructure and impurity level of the niobium film do not significantly affect the losses. These results set the scale of microwave losses in niobium films and show that niobium losses do not dominate the observed coherence times in present two-dimensional superconducting qubit designs, instead highlighting the dominant role of the dielectric oxide in limiting the performance. We can also set a bound for when niobium film losses will become a limitation for qubit lifetimes.
arxiv pdf

High quality superconducting Nb co-planar resonators on sapphire substrate

  1. S. Zhu,
  2. F. Crisa,
  3. M. Bal,
  4. A. A. Murthy,
  5. J. Lee,
  6. Z. Sung,
  7. A. Lunin,
  8. D. Frolov,
  9. R. Pilipenko,
  10. D. Bafia,
  11. A. Mitra,
  12. A. Romanenko,
  13. and A. Grassellino
We present measurements and simulations of superconducting Nb co-planar waveguide resonators on sapphire substrate down to millikelvin temperature range with different readout powers.
In the high temperature regime, we demonstrate that the Nb film residual surface resistance is comparable to that observed in the ultra-high quality, bulk Nb 3D superconducting radio frequency cavities while the resonator quality is dominated by the BCS thermally excited quasiparticles. At low temperature both the resonator quality factor and frequency can be well explained using the two-level system models. Through the energy participation ratio simulations, we find that the two-level system loss tangent is ∼10−2, which agrees quite well with similar studies performed on the Nb 3D cavities.
arxiv pdf