Quantifying Trapped Magnetic Vortex Losses in Niobium Resonators at mK Temperatures

  1. D. Bafia,
  2. B. Abdisatarov,
  3. R. Pilipenko,
  4. Y. Lu,
  5. G. Eremeev,
  6. A. Romanenko,
  7. and A. Grassellino
Trapped magnetic vortices in niobium can introduce microwave losses in superconducting devices, affecting both niobium-based qubits and resonators. While our group has extensively studied
this problem at temperatures above 1~K, in this study we quantify for the first time the losses driven by magnetic vortices for niobium-based quantum devices operating down to millikelvin temperature, and in the low photon counts regime. By cooling a single interface system a 3-D niobium superconducting cavity in a dilution refrigerator through the superconducting transition temperature in controlled levels of magnetic fields, we isolate the flux-induced losses and quantify the added surface resistance per unit of trapped magnetic flux. Our findings indicate that magnetic flux introduces approximately 2~nΩ/mG at 10~mK and at 6~GHz in high RRR niobium. We find that the decay rate of a 6~GHz niobium cavity at 10~mK which contains a native niobium pentoxide will be dominated by the TLS oxide losses until vortices begin to impact T1 for trapped magnetic field (Btrap) levels of >100~mG. In the absence of the niobium pentoxide, Btrap=~10~mG limits Q0∼~10\textsuperscript{10}, or T1∼~350~ms, highlighting the importance of magnetic shielding and magnetic hygiene in enabling T1>~1~s. We observe that the flux-induced resistance decreases with temperature-yet remains largely field-independent, qualitatively explained by thermal activation of vortices in the flux-flow regime. We present a phenomenological model which captures the salient experimental observations. Scaling our findings to typical transmon qubit dimensions suggests that these 2-D structures could be robust against vortex dissipation up to several hundreds~mG. We are directly addressing vortex losses in transmon qubits made with low RRR Nb films in a separate experimental study.

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.