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.

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.

Quantum bath engineering of a high impedance microwave mode through quasiparticle tunneling

  1. Gianluca Aiello,
  2. Mathieu Féchant,
  3. Alexis Morvan,
  4. Julien Basset,
  5. Marco Aprili,
  6. Julien Gabelli,
  7. and Jérôme Estève
We demonstrate a new approach to dissipation engineering in microwave quantum optics. For a single mode, dissipation usually corresponds to quantum jumps, where photons are lost one
by one. Here, we are able to tune the minimal number of lost photons per jump to be two (or more) with a simple dc voltage. As a consequence, different quantum states experience different dissipation. Causality implies that the states must also experience different energy shifts. Our measurements of these Lamb shifts are in good agreement with the predictions of the Kramers-Kronig relations for single quantum states in a regime of highly non-linear bath coupling. This work opens new possibilities for quantum state manipulation in circuit QED, without relying on the Josephson effect.