On-chip stencil lithography for superconducting qubits

  1. Roudy Hanna,
  2. Sören Ihssen,
  3. Simon Geisert,
  4. Umut Kocak,
  5. Matteo Arfini,
  6. Albert Hertel,
  7. Thomas J. Smart,
  8. Michael Schleenvoigt,
  9. Tobias Schmitt,
  10. Joscha Domnick,
  11. Kaycee Underwood,
  12. Abdur Rehman Jalil,
  13. Jin Hee Bae,
  14. Benjamin Bennemann,
  15. Mathieu Féchant,
  16. Mitchell Field,
  17. Martin Spiecker,
  18. Nicolas Zapata,
  19. Christian Dickel,
  20. Erwin Berenschot,
  21. Niels Tas,
  22. Gary A. Steele,
  23. Detlev Grützmacher,
  24. Ioan M. Pop,
  25. and Peter Schüffelgen
Improvements in circuit design and more recently in materials and surface cleaning have contributed to a rapid development of coherent superconducting qubits. However, organic resists
commonly used for shadow evaporation of Josephson junctions (JJs) pose limitations due to residual contamination, poor thermal stability and compatibility under typical surface-cleaning conditions. To provide an alternative, we developed an inorganic SiO2/Si3N4 on-chip stencil lithography mask for JJ fabrication. The stencil mask is resilient to aggressive cleaning agents and it withstands high temperatures up to 1200\textdegree{}C, thereby opening new avenues for JJ material exploration and interface optimization. To validate the concept, we performed shadow evaporation of Al-based transmon qubits followed by stencil mask lift-off using vapor hydrofluoric acid, which selectively etches SiO2. We demonstrate average $T_1 \approx 75 \pm 11~\SI{}{\micro\second}$ over a 200 MHz frequency range in multiple cool-downs for one device, and $T_1 \approx 44\pm 8~\SI{}{\micro\second}$ for a second device. These results confirm the compatibility of stencil lithography with state-of-the-art superconducting quantum devices and motivate further investigations into materials engineering, film deposition and surface cleaning techniques.

High Impedance Granular Aluminum Ring Resonators

  1. Mahya Khorramshahi,
  2. Martin Spiecker,
  3. Patrick Paluch,
  4. Simon Geisert,
  5. Nicolas Gosling,
  6. Nicolas Zapata,
  7. Lucas Brauch,
  8. Christian Kübel,
  9. Simone Dehm,
  10. Ralph Krupke,
  11. Wolfgang Wernsdorfer,
  12. Ioan M. Pop,
  13. and Thomas Reisinger
Superconducting inductors with impedance surpassing the resistance quantum, i.e., superinductors, are important for quantum technologies because they enable the development of protected
qubits, enhance coupling to systems with small electric dipole moments, and facilitate the study of phase-slip physics. We demonstrate superinductors with densely packed meandered traces of granular aluminum (grAl) with inductances up to 4μH, achieving impedances exceeding 100kΩ in the 4−8GHz range. Ring resonators made with grAl meandered superinductors exhibit quality factors on the order of 105 in the single-photon regime and low non-linearity on the order of tens of Hz. Depending on the grAl resistivity, at 10Hz, we measure frequency noise spectral densities in the range of 102 to 103Hz/Hz‾‾‾√. In some devices, in the single-photon regime, we observe a positive Kerr coefficient of unknown origin. Using more complex fabrication, the devices could be released from the substrate, either freestanding or suspended on a membrane, thereby further improving their impedance by a factor of three.

Simultaneous sweet-spot locking of gradiometric fluxonium qubits

  1. Denis Bénâtre,
  2. Mathieu Féchant,
  3. Nicolas Zapata,
  4. Nicolas Gosling,
  5. Patrick Paluch,
  6. Thomas Reisinger,
  7. and Ioan M. Pop
Efforts to scale up superconducting processors that employ flux-qubits face numerous challenges, among which is the crosstalk created by neighboring flux lines, which are necessary
to bias the qubits at the zero-field and Φ0/2 sweet spots. A solution to this problem is to use symmetric gradiometric loops, which incorporate a flux locking mechanism that, once a fluxon is trapped during cooldown, holds the device at the sweet spot and limits the need for active biasing. We demonstrate this technique by simultaneously locking multiple gradiometric fluxonium qubits in which an aluminum loop retains the trapped fluxon indefinitely. By compensating the inductive asymmetry between the two loops of the design, we are able to lock the effective flux-bias within Φeff=−3×10−4Φ0 from the target, corresponding to only 15 % degradation in T2,E when operated in zero external field. The design strategy demonstrated here reduces integration complexity for flux qubits by minimizing cross-talk and potentially eliminating the need for local flux bias.

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.

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.