José Palomo

Hyperinductance based on stacked Josephson junctions

  1. Paul Manset,
  2. José Palomo,
  3. Aurélien Schmitt,
  4. Kyrylo Gerashchenko,
  5. Rémi Rousseau,
  6. Himanshu Patange,
  7. Patrick Abgrall,
  8. Emmanuel Flurin,
  9. Samuel Deléglise,
  10. Thibaut Jacqmin,
  11. and Léo Balembois
Superinductances are superconducting circuit elements that combine a large inductance with a low parasitic capacitance to ground, resulting in a characteristic impedance exceeding the
resistance quantum RQ=h/(2e)2≃6.45kΩ. In recent years, these components have become key enablers for emerging quantum circuit architectures. However, achieving high characteristic impedance while maintaining scalability and fabrication robustness remains a major challenge. In this work, we present two fabrication techniques for realizing superinductances based on vertically stacked Josephson junctions. Using a multi-angle Manhattan (MAM) process and a zero-angle (ZA) evaporation technique — in which junction stacks are connected pairwise using airbridges — we fabricate one-dimensional chains of stacks that act as high-impedance superconducting transmission lines. Two-tone microwave spectroscopy reveals the expected n‾√ scaling of the impedance with the number of junctions per stack. The chain fabricated using the ZA process, with nine junctions per stack, achieves a characteristic impedance of ∼16kΩ, a total inductance of 5.9μH, and a maximum frequency-dependent impedance of 50kΩ at 1.4 GHz. Our results establish junction stacking as a scalable, robust, and flexible platform for next-generation quantum circuits requiring ultra-high impedance environments.
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