Dynamics of an off-resonantly pumped superconducting qubit in a cavity

  1. Raphaël Lescanne,
  2. Lucas Verney,
  3. Quentin Ficheux,
  4. Michel H. Devoret,
  5. Benjamin Huard,
  6. Mazyar Mirrahimi,
  7. and Zaki Leghtas
Strong microwave drives, referred to as pumps, are widely applied to superconducting circuits incorporating Josephson junctions in order to induce couplings between electromagnetic modes. This offers a variety of applications, from quantum-limited amplification, to quantum state and manifold stabilization. These couplings scale with the pump power, therefore, seeking stronger couplings requires a detailed understanding of the behavior of such circuits in the presence of stronger pumps. In this work, we probe the dynamics of a transmon qubit in a 3D cavity, for various pump powers and frequencies. For all pump frequencies, we find a critical pump power above which the transmon is driven into highly excited states, beyond the first seven states which we individually resolve through cavity spectroscopy. This observation is compatible with our theory describing the escape of the transmon state out of its Josephson potential well, into states resembling those of a free particle which does not induce any non-linear couplings.
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