Quasiparticle dynamics in granular aluminum close to the superconductor to insulator transition

  1. Lukas Grünhaupt,
  2. Nataliya Maleeva,
  3. Sebastian T. Skacel,
  4. Martino Calvo,
  5. Florence Levy-Bertrand,
  6. Alexey V. Ustinov,
  7. Hannes Rotzinger,
  8. Alessandro Monfardini,
  9. Gianluigi Catelani,
  10. and Ioan M. Pop
Superconducting high kinetic inductance elements constitute a valuable resource for quantum circuit design and millimeter-wave detection. Granular aluminum (GrAl) in the superconducting regime is a particularly interesting material since it has already shown a kinetic inductance in the range of nH/◻ and its deposition is compatible with conventional Al/AlOx/Al Josephson junction fabrication. We characterize microwave resonators fabricated from GrAl with a room temperature resistivity of 4×103μΩ⋅cm, which is a factor of 3 below the superconductor to insulator transition, showing a kinetic inductance fraction close to unity. The measured internal quality factors are on the order of Qi=105 in the single photon regime, and we demonstrate that non-equilibrium quasiparticles (QP) constitute the dominant loss mechanism. We extract QP relaxation times in the range of 1 s and we observe QP bursts every ∼20 s. The current level of coherence of GrAl resonators makes them attractive for integration in quantum devices, while it also evidences the need to reduce the density of non-equilibrium QPs.
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