Wolfgang Wernsdorfer

Onset of phase diffusion in high kinetic inductance granular aluminum micro-SQUIDs

  1. Felix Friedrich,
  2. Patrick Winkel,
  3. Kiril Borisov,
  4. Hannes Seeger,
  5. Christoph Sürgers,
  6. Ioan M. Pop,
  7. and Wolfgang Wernsdorfer
Superconducting granular aluminum is attracting increasing interest due to its high kinetic inductance and low dissipation, favoring its use in kinetic inductance particle detectors,
superconducting resonators or quantum bits. We perform switching current measurements on DC-SQUIDs, obtained by introducing two identical geometric constrictions in granular aluminum rings of various normal-state resistivities in the range from ρn=250μΩcm to 5550μΩcm. The relative high kinetic inductance of the SQUID loop, in the range of tens of nH, leads to a suppression of the modulation in the measured switching current versus magnetic flux, accompanied by a distortion towards a triangular shape. We observe a change in the temperature dependence of the switching current histograms with increasing normal-state film resistivity. This behavior suggests the onset of a diffusive motion of the superconducting phase across the constrictions in the two-dimensional washboard potential of the SQUIDs, which could be caused by a change of the local electromagnetic environment of films with increasing normal-state resistivities.
arxiv pdf

Phonon traps reduce the quasiparticle density in superconducting circuits

  1. Fabio Henriques,
  2. Francesco Valenti,
  3. Thibault Charpentier,
  4. Marc Lagoin,
  5. Clement Gouriou,
  6. Maria Martínez,
  7. Laura Cardani,
  8. Lukas Grünhaupt,
  9. Daria Gusenkova,
  10. Julian Ferrero,
  11. Sebastian T. Skacel,
  12. Wolfgang Wernsdorfer,
  13. Alexey V. Ustinov,
  14. Gianluigi Catelani,
  15. Oliver Sander,
  16. and Ioan M. Pop
Out of equilibrium quasiparticles (QPs) are one of the main sources of decoherence in superconducting quantum circuits, and are particularly detrimental in devices with high kinetic
inductance, such as high impedance resonators, qubits, and detectors. Despite significant progress in the understanding of QP dynamics, pinpointing their origin and decreasing their density remain outstanding tasks. The cyclic process of recombination and generation of QPs implies the exchange of phonons between the superconducting thin film and the underlying substrate. Reducing the number of substrate phonons with frequencies exceeding the spectral gap of the superconductor should result in a reduction of QPs. Indeed, we demonstrate that surrounding high impedance resonators made of granular aluminum (grAl) with lower gapped thin film aluminum islands increases the internal quality factors of the resonators in the single photon regime, suppresses the noise, and reduces the rate of observed QP bursts. The aluminum islands are positioned far enough from the resonators to be electromagnetically decoupled, thus not changing the resonator frequency, nor the loading. We therefore attribute the improvements observed in grAl resonators to phonon trapping at frequencies close to the spectral gap of aluminum, well below the grAl gap.
arxiv pdf