Benjamin D'Anjou

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.
arxiv pdf

Probing excited-state dynamics of transmon ionization

  1. Zihao Wang,
  2. Benjamin D'Anjou,
  3. Philippe Gigon,
  4. Alexandre Blais,
  5. and Machiel S. Blok
The fidelity and quantum nondemolition character of the dispersive readout in circuit QED are limited by unwanted transitions to highly excited states at specific photon numbers in
the readout resonator. This observation can be explained by multiphoton resonances between computational states and highly excited states in strongly driven nonlinear systems, analogous to multiphoton ionization in atoms and molecules. In this work, we utilize the multilevel nature of high-EJ/EC transmons to probe the excited-state dynamics induced by strong drives during readout. With up to 10 resolvable states, we quantify the critical photon number of ionization, the resulting state after ionization, and the fraction of the population transferred to highly excited states. Moreover, using pulse-shaping to control the photon number in the readout resonator in the high-power regime, we tune the adiabaticity of the transition and verify that transmon ionization is a Landau-Zener-type transition. Our experimental results agree well with the theoretical prediction from a semiclassical driven transmon model and may guide future exploration of strongly driven nonlinear oscillators.
arxiv pdf