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.

Offset Charge Dependence of Measurement-Induced Transitions in Transmons

  1. Mathieu Féchant,
  2. Marie Frédérique Dumas,
  3. Denis Bénâtre,
  4. Nicolas Gosling,
  5. Philipp Lenhard,
  6. Martin Spiecker,
  7. Wolfgang Wernsdorfer,
  8. Benjamin D'Anjou,
  9. Alexandre Blais,
  10. and Ioan M. Pop
A key challenge in achieving scalable fault tolerance in superconducting quantum processors is readout fidelity, which lags behind one- and two-qubit gate fidelity. A major limitation
in improving qubit readout is measurement-induced transitions, also referred to as qubit ionization, caused by multiphoton qubit-resonator excitation occurring at specific photon numbers. Since ionization can involve highly excited states, it has been predicted that in transmons — the most widely used superconducting qubit — the photon number at which measurement-induced transitions occur is gate charge dependent. This dependence is expected to persist deep in the transmon regime where the qubit frequency is gate charge insensitive. We experimentally confirm this prediction by characterizing measurement-induced transitions with increasing resonator photon population while actively stabilizing the transmon’s gate charge. Furthermore, because highly excited states are involved, achieving quantitative agreement between theory and experiment requires accounting for higher-order harmonics in the transmon Hamiltonian.

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.

Solomon equations for qubit and two-level systems

  1. Martin Spiecker,
  2. Andrei I. Pavlov,
  3. Alexander Shnirman,
  4. and Ioan M. Pop
We model and measure the combined relaxation of a qubit, a.k.a. central spin, coupled to a discrete two-level system (TLS) environment. We present a derivation of the Solomon equations
starting from a general Lindblad equation for the qubit and an arbitrary number of TLSs. If the TLSs are much longer lived than the qubit, the relaxation becomes non-exponential. In the limit of large numbers of TLSs the populations are likely to follow a power law, which we illustrate by measuring the relaxation of a superconducting fluxonium qubit. Moreover, we show that the Solomon equations predict non-Poissonian quantum jump statistics, which we confirm experimentally.

A quantum Szilard engine for two-level systems coupled to a qubit

  1. Martin Spiecker,
  2. Patrick Paluch,
  3. Niv Drucker,
  4. Shlomi Matityahu,
  5. Daria Gusenkova,
  6. Nicolas Gosling,
  7. Simon Günzler,
  8. Dennis Rieger,
  9. Ivan Takmakov,
  10. Francesco Valenti,
  11. Patrick Winkel,
  12. Richard Gebauer,
  13. Oliver Sander,
  14. Gianluigi Catelani,
  15. Alexander Shnirman,
  16. Alexey V. Ustinov,
  17. Wolfgang Wernsdorfer,
  18. Yonatan Cohen,
  19. and Ioan M. Pop
The innate complexity of solid state physics exposes superconducting quantum circuits to interactions with uncontrolled degrees of freedom degrading their coherence. By using a simple
stabilization sequence we show that a superconducting fluxonium qubit is coupled to a two-level system (TLS) environment of unknown origin, with a relatively long energy relaxation time exceeding 50ms. Implementing a quantum Szilard engine with an active feedback control loop allows us to decide whether the qubit heats or cools its TLS environment. The TLSs can be cooled down resulting in a four times lower qubit population, or they can be heated to manifest themselves as a negative temperature environment corresponding to a qubit population of ∼80%. We show that the TLSs and the qubit are each other’s dominant loss mechanism and that the qubit relaxation is independent of the TLS populations. Understanding and mitigating TLS environments is therefore not only crucial to improve qubit lifetimes but also to avoid non-Markovian qubit dynamics.

Operating in a deep underground facility improves the locking of gradiometric fluxonium qubits at the sweet spots

  1. Daria Gusenkova,
  2. Francesco Valenti,
  3. Martin Spiecker,
  4. Simon Günzler,
  5. Patrick Paluch,
  6. Dennis Rieger,
  7. Larisa-Milena Pioraş-Ţimbolmaş,
  8. Liviu P. Zârbo,
  9. Nicola Casali,
  10. Ivan Colantoni,
  11. Angelo Cruciani,
  12. Stefano Pirro,
  13. Laura Cardani,
  14. Alexandru Petrescu,
  15. Wolfgang Wernsdorfer,
  16. Patrick Winkel,
  17. and Ioan M. Pop
We demonstrate flux-bias locking and operation of a gradiometric fluxonium artificial atom using two symmetric granular aluminum (grAl) loops to implement the superinductor. The gradiometric
fluxonium shows two orders of magnitude suppression of sensitivity to homogeneous magnetic fields, which can be an asset for hybrid quantum systems requiring strong magnetic field biasing. By cooling down the device in an external magnetic field while crossing the metal-to-superconductor transition, the gradiometric fluxonium can be locked either at 0 or Φ0/2 effective flux bias, corresponding to an even or odd number of trapped fluxons, respectively. At mK temperatures, the fluxon parity prepared during initialization survives to magnetic field bias exceeding 100Φ0. However, even for states biased in the vicinity of 1Φ0, we observe unexpectedly short fluxon lifetimes of a few hours, which cannot be explained by thermal or quantum phase slips. When operating in a deep-underground cryostat of the Gran Sasso laboratory, the fluxon lifetimes increase to days, indicating that ionizing events activate phase slips in the grAl superinductor.

Quantum non-demolition dispersive readout of a superconducting artificial atom using large photon numbers

  1. Daria Gusenkova,
  2. Martin Spiecker,
  3. Richard Gebauer,
  4. Madita Willsch,
  5. Francesco Valenti,
  6. Nick Karcher,
  7. Lukas Grünhaupt,
  8. Ivan Takmakov,
  9. Patrick Winkel,
  10. Dennis Rieger,
  11. Alexey V. Ustinov,
  12. Nicolas Roch,
  13. Wolfgang Wernsdorfer,
  14. Kristel Michielsen,
  15. Oliver Sander,
  16. and Ioan M. Pop
Reading out the state of superconducting artificial atoms typically relies on dispersive coupling to a readout resonator. For a given system noise temperature, increasing the circulating
photon number n¯ in the resonator enables a shorter measurement time and is therefore expected to reduce readout errors caused by spontaneous atom transitions. However, increasing n¯ is generally observed to also increase these transition rates. Here we present a fluxonium artificial atom in which we measure an overall flat dependence of the transition rates between its first two states as a function of n¯, up to n¯≈200. Despite the fact that we observe the expected decrease of the dispersive shift with increasing readout power, the signal-to-noise ratio continuously improves with increasing n¯. Even without the use of a parametric amplifier, at n¯=74, we measure fidelities of 99% and 93% for feedback-assisted ground and excited state preparation, respectively.

State preparation of a fluxonium qubit with feedback from a custom FPGA-based platform

  1. Richard Gebauer,
  2. Nick Karcher,
  3. Daria Gusenkova,
  4. Martin Spiecker,
  5. Lukas Grünhaupt,
  6. Ivan Takmakov,
  7. Patrick Winkel,
  8. Luca Planat,
  9. Nicolas Roch,
  10. Wolfgang Wernsdorfer,
  11. Alexey V. Ustinov,
  12. Marc Weber,
  13. Martin Weides,
  14. Ioan M. Pop,
  15. and Oliver Sander
We developed a versatile integrated control and readout instrument for experiments with superconducting quantum bits (qubits), based on a field-programmable gate array (FPGA) platform.
Using this platform, we perform measurement-based, closed-loop feedback operations with 428ns platform latency. The feedback capability is instrumental in realizing active reset initialization of the qubit into the ground state in a time much shorter than its energy relaxation time T1. We show experimental results demonstrating reset of a fluxonium qubit with 99.4% fidelity, using a readout-and-drive pulse sequence approximately 1.5μs long. Compared to passive ground state initialization through thermalization, with the time constant given by T1= 80μs, the use of the FPGA-based platform allows us to improve both the fidelity and the time of the qubit initialization by an order of magnitude.