Christian Kübel

High Impedance Granular Aluminum Ring Resonators

  1. Mahya Khorramshahi,
  2. Martin Spiecker,
  3. Patrick Paluch,
  4. Simon Geisert,
  5. Nicolas Gosling,
  6. Nicolas Zapata,
  7. Lucas Brauch,
  8. Christian Kübel,
  9. Simone Dehm,
  10. Ralph Krupke,
  11. Wolfgang Wernsdorfer,
  12. Ioan M. Pop,
  13. and Thomas Reisinger
Superconducting inductors with impedance surpassing the resistance quantum, i.e., superinductors, are important for quantum technologies because they enable the development of protected
qubits, enhance coupling to systems with small electric dipole moments, and facilitate the study of phase-slip physics. We demonstrate superinductors with densely packed meandered traces of granular aluminum (grAl) with inductances up to 4μH, achieving impedances exceeding 100kΩ in the 4−8GHz range. Ring resonators made with grAl meandered superinductors exhibit quality factors on the order of 105 in the single-photon regime and low non-linearity on the order of tens of Hz. Depending on the grAl resistivity, at 10Hz, we measure frequency noise spectral densities in the range of 102 to 103Hz/Hz‾‾‾√. In some devices, in the single-photon regime, we observe a positive Kerr coefficient of unknown origin. Using more complex fabrication, the devices could be released from the substrate, either freestanding or suspended on a membrane, thereby further improving their impedance by a factor of three.
arxiv pdf

High quality superconducting tantalum resonators with beta phase defects

  1. Ritika Dhundhwal,
  2. Haoran Duan,
  3. Lucas Brauch,
  4. Soroush Arabi,
  5. Dirk Fuchs,
  6. Amir-Abbas Haghighirad,
  7. Alexander Welle,
  8. Florentine Scharwaechter,
  9. Sudip Pal,
  10. Marc Scheffler,
  11. José Palomo,
  12. Zaki Leghtas,
  13. Anil Murani,
  14. Horst Hahn,
  15. Jasmin Aghassi-Hagmann,
  16. Christian Kübel,
  17. Wulf Wulfhekel,
  18. Ioan M. Pop,
  19. and Thomas Reisinger
For practical superconducting quantum processors, orders of magnitude improvement in coherence is required, motivating efforts to optimize hardware design and explore new materials.
Among the latter, the coherence of superconducting transmon qubits has been shown to improve by forming the qubit capacitor pads from α-tantalum, avoiding the meta-stable β-phase that forms when depositing tantalum at room temperature, and has been previously identified to be a source of microwave losses. In this work, we show lumped element resonators containing β-phase tantalum in the form of inclusions near the metal-substrate interface with internal quality factors (Qi) up to (5.0±2.5)×106 in the single photon regime. They outperform resonators with no sign of the β-phase in x-ray diffraction and thermal quasi-particle loss. Our results indicate that small concentrations of β-phase can be beneficial, enhancing critical magnetic fields and potentially, for improving coherence in tantalum based superconducting circuits.
arxiv pdf