Collapse and Inversion of the Josephson Potential in a Strongly Driven Superconducting Circuit

Superconducting circuits, particularly those using Josephson junctions, are essential for quantum information processing. While increasing microwave drive power can improve the efficiency of control and readout operations, it typically triggers unwanted transitions to uncontrolled states, limiting the circuit's dynamic range. This paper investigates whether these limitations can be overcome through circuit engineering to explore the behavior of Josephson circuits under extreme drive conditions.

The authors designed a transmon-resonator system specifically engineered to be free of detrimental multi-excitation resonances. By utilizing a native cosine-cosine coupling and a high-frequency resonator drive, they effectively modulated the Josephson potential. They employed two-tone spectroscopy to track the qubit frequency and performed readout experiments to measure the expectation values of the superconducting phase, allowing them to observe the system's response as the drive power increased.

The study reveals that high-frequency flux driving can cause the Josephson potential to collapse and subsequently invert. This inversion corresponds to the dynamical stabilization of the transmon at its unstable equilibrium point, a phenomenon directly analogous to an inverted pendulum. The researchers confirmed this through spectroscopy, which showed the qubit frequency dropping to zero and recovering, and through readout experiments, which demonstrated a pi-phase shift in the localization of the transmon eigenstates. This result identifies a fundamental limitation of strongly driven circuits while simultaneously suggesting new methods for autonomous stabilization of quantum states.