Simon Geisert

Low crosstalk modular flip-chip architecture for coupled superconducting qubits

  1. Sören Ihssen,
  2. Simon Geisert,
  3. Gabriel Jauma,
  4. Patrick Winkel,
  5. Martin Spiecker,
  6. Nicolas Zapata,
  7. Nicolas Gosling,
  8. Patrick Paluch,
  9. Manuel Pino,
  10. Thomas Reisinger,
  11. Wolfgang Wernsdorfer,
  12. Juan Jose Garcia-Ripoll,
  13. and Ioan M. Pop
We present a flip-chip architecture for an array of coupled superconducting qubits, in which circuit components reside inside individual microwave enclosures. In contrast to other flip-chip
approaches, the qubit chips in our architecture are electrically floating, which guarantees a simple, fully modular assembly of capacitively coupled circuit components such as qubit, control, and coupling structures, as well as reduced crosstalk between the components. We validate the concept with a chain of three nearest neighbor coupled generalized flux qubits in which the center qubit acts as a frequency-tunable coupler. Using this coupler, we demonstrate a transverse coupling on/off ratio ≈ 50, zz-crosstalk ≈ 0.7 kHz between resonant qubits and isolation between the qubit enclosures > 60 dB.
arxiv pdf

Pure kinetic inductance coupling for cQED with flux qubits

  1. Simon Geisert,
  2. Sören Ihssen,
  3. Patrick Winkel,
  4. Martin Spiecker,
  5. Mathieu Fechant,
  6. Patrick Paluch,
  7. Nicolas Gosling,
  8. Nicolas Zapata,
  9. Simon Günzler,
  10. Dennis Rieger,
  11. Denis Bénâtre,
  12. Thomas Reisinger,
  13. Wolfgang Wernsdorfer,
  14. and Ioan M. Pop
We demonstrate a qubit-readout architecture where the dispersive coupling is entirely mediated by a kinetic inductance. This allows us to engineer the dispersive shift of the readout
resonator independent of the qubit and resonator capacitances. We validate the pure kinetic coupling concept and demonstrate various generalized flux qubit regimes from plasmon to fluxon, with dispersive shifts ranging from 60 kHz to 2 MHz at the half-flux quantum sweet spot. We achieve readout performances comparable to conventional architectures with quantum state preparation fidelities of 99.7 % and 92.7 % for the ground and excited states, respectively, and below 0.1 % leakage to non-computational states.
arxiv pdf

Observation of Josephson Harmonics in Tunnel Junctions

  1. Dennis Willsch,
  2. Dennis Rieger,
  3. Patrick Winkel,
  4. Madita Willsch,
  5. Christian Dickel,
  6. Jonas Krause,
  7. Yoichi Ando,
  8. Raphaël Lescanne,
  9. Zaki Leghtas,
  10. Nicholas T. Bronn,
  11. Pratiti Deb,
  12. Olivia Lanes,
  13. Zlatko K. Minev,
  14. Benedikt Dennig,
  15. Simon Geisert,
  16. Simon Günzler,
  17. Sören Ihssen,
  18. Patrick Paluch,
  19. Thomas Reisinger,
  20. Roudy Hanna,
  21. Jin Hee Bae,
  22. Peter Schüffelgen,
  23. Detlev Grützmacher,
  24. Luiza Buimaga-Iarinca,
  25. Cristian Morari,
  26. Wolfgang Wernsdorfer,
  27. David P. DiVincenzo,
  28. Kristel Michielsen,
  29. Gianluigi Catelani,
  30. and Ioan M. Pop
An accurate understanding of the Josephson effect is the keystone of quantum information processing with superconducting hardware. Here we show that the celebrated sinφ current-phase
relation (CφR) of Josephson junctions (JJs) fails to fully describe the energy spectra of transmon artificial atoms across various samples and laboratories. While the microscopic theory of JJs contains higher harmonics in the CφR, these have generally been assumed to give insignificant corrections for tunnel JJs, due to the low transparency of the conduction channels. However, this assumption might not be justified given the disordered nature of the commonly used AlOx tunnel barriers. Indeed, a mesoscopic model of tunneling through an inhomogeneous AlOx barrier predicts contributions from higher Josephson harmonics of several %. By including these in the transmon Hamiltonian, we obtain orders of magnitude better agreement between the computed and measured energy spectra. The measurement of Josephson harmonics in the CφR of standard tunnel junctions prompts a reevaluation of current models for superconducting hardware and it offers a highly sensitive probe towards optimizing tunnel barrier uniformity.
arxiv pdf