Florent Ravaux

Tantalum-Encapsulated Niobium Superconducting Resonators: High Internal Quality Factor and Improved Temporal Stability via Surface Passivation

  1. Anas Alkhazaleh,
  2. Juan Villegas,
  3. Florent Ravaux,
  4. and Alexey Zharinov
Superconducting coplanar waveguide resonators are essential components in quantum processors, where their internal quality factor (Qi) constrains qubit coherence and readout fidelity.
In niobium devices, microwave losses at millikelvin temperatures are strongly influenced by two-level systems (TLS) associated with the complex NbOx surface oxide. To mitigate these losses, we investigate a surface-engineering approach in which Nb films are capped in situ with a thin tantalum layer to suppress Nb2O5 formation and replace the native NbOx interface with a Ta-based oxide. We fabricate Nb/Ta bilayer and reference Nb resonators on high-resistivity silicon using identical DC sputtering and wet etching conditions, and characterize their performance at millikelvin temperatures. Fresh Ta-encapsulated devices exhibit internal quality factors up to 2.4 x 10^6 in the near-single-photon regime, with power dependence consistent with reduced TLS-related loss at the metal-air interface. A control Nb device fabricated under the same process shows comparatively lower Q_TLS, consistent with the beneficial effect of the Ta capping layer. Furthermore, ageing tests performed on Nb/Ta resonators after six months reveal a moderate reduction in Q_TLS relative to their initial values, yet the performance remains superior to newly fabricated Nb-only devices. These results suggest that thin Ta encapsulation enhances interface quality and contributes to improved temporal stability while remaining compatible with Nb-based fabrication workflows.
arxiv pdf

Characterization of a Transmon Qubit in a 3D Cavity for Quantum Machine Learning and Photon Counting

  1. Alessandro D'Elia,
  2. Boulos Alfakes,
  3. Anas Alkhazaleh,
  4. Leonardo Banchi,
  5. Matteo Beretta,
  6. Stefano Carrazza,
  7. Fabio Chiarello,
  8. Daniele Di Gioacchino,
  9. Andrea Giachero,
  10. Felix Henrich,
  11. Alex Stephane Piedjou Komnang,
  12. Carlo Ligi,
  13. Giovanni Maccarrone,
  14. Massimo Macucci,
  15. Emanuele Palumbo,
  16. Andrea Pasquale,
  17. Luca Piersanti,
  18. Florent Ravaux,
  19. Alessio Rettaroli,
  20. Matteo Robbiati,
  21. Simone Tocci,
  22. and Claudio Gatti
In this paper we report the use of superconducting transmon qubit in a 3D cavity for quantum machine learning and photon counting applications. We first describe the realization and
characterization of a transmon qubit coupled to a 3D resonator, providing a detailed description of the simulation framework and of the experimental measurement of important parameters, like the dispersive shift and the qubit anharmonicity. We then report on a Quantum Machine Learning application implemented on the single-qubit device to fit the u-quark parton distribution function of the proton. In the final section of the manuscript we present a new microwave photon detection scheme based on two qubits coupled to the same 3D resonator. This could in principle decrease the dark count rate, favouring applications like axion dark matter searches.
arxiv pdf