Kiril Borisov

Implementation of a transmon qubit using superconducting granular aluminum

  1. Patrick Winkel,
  2. Kiril Borisov,
  3. Lukas Grünhaupt,
  4. Dennis Rieger,
  5. Martin Spiecker,
  6. Francesco Valenti,
  7. Alexey V. Ustinov,
  8. Wolfgang Wernsdorfer,
  9. and Ioan M. Pop
The high kinetic inductance offered by granular aluminum (grAl) has recently been employed for linear inductors in superconducting high-impedance qubits and kinetic inductance detectors.
Due to its large critical current density compared to typical Josephson junctions, its resilience to external magnetic fields, and its low dissipation, grAl may also provide a robust source of non-linearity for strongly driven quantum circuits, topological superconductivity, and hybrid systems. Having said that, can the grAl non-linearity be sufficient to build a qubit? Here we show that a small grAl volume (10×200×500nm3) shunted by a thin film aluminum capacitor results in a microwave oscillator with anharmonicity α two orders of magnitude larger than its spectral linewidth Γ01, effectively forming a transmon qubit. With increasing drive power, we observe several multi-photon transitions starting from the ground state, from which we extract α=2π×4.48MHz. Resonance fluorescence measurements of the |0>→|1> transition yield an intrinsic qubit linewidth γ=2π×10kHz, corresponding to a lifetime of 16μs. This linewidth remains below 2π×150kHz for in-plane magnetic fields up to ∼70mT.
arxiv pdf

Non-degenerate parametric amplifiers based on dispersion engineered Josephson junction arrays

  1. Patrick Winkel,
  2. Ivan Takmakov,
  3. Dennis Rieger,
  4. Luca Planat,
  5. Wiebke Hasch-Guichard,
  6. Lukas Grünhaupt,
  7. Nataliya Maleeva,
  8. Farshad Foroughi,
  9. Fabio Henriques,
  10. Kiril Borisov,
  11. Julian Ferrero,
  12. Alexey V. Ustinov,
  13. Wolfgang Wernsdorfer,
  14. Nicolas Roch,
  15. and Ioan M. Pop
Determining the state of a qubit on a timescale much shorter than its relaxation time is an essential requirement for quantum information processing. With the aid of a new type of non-degenerate
parametric amplifier, we demonstrate the continuous detection of quantum jumps of a transmon qubit with 90% fidelity in state discrimination. Entirely fabricated with standard two-step optical lithography techniques, this type of parametric amplifier consists of a dispersion engineered Josephson junction (JJ) array. By using long arrays, containing 103 JJs, we can obtain amplification at multiple eigenmodes with frequencies below 10 GHz, which is the typical range for qubit readout. Moreover, by introducing a moderate flux tunability of each mode, employing superconducting quantum interference device (SQUID) junctions, a single amplifier device could potentially cover the entire frequency band between 1 and 10 GHz.
arxiv pdf

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