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.

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.