Driven Superconducting Circuits Show Josephson Potential Collapse and Inversion

Researchers at the Kavli Institute of Nanoscience, Delft University of Technology, have observed the Josephson potential, a fundamental element in superconducting circuits, collapse to zero and then invert under high drive power. Sercan Deve and Marius Villiers, along with colleagues, engineered a transmon-resonator system designed to eliminate unwanted transitions, enabling access to these drive levels. Through spectroscopy and readout experiments, evidence of the inverted potential confirms the dynamical stabilization of the transmon at its unstable equilibrium point, a behavior directly comparable to an inverted pendulum. This result reveals a new limitation for strongly driven superconducting circuits and opens possibilities for novel control methods and noise-resilient quantum states.

Transmon Circuit Fundamentals & Josephson Energy

Careful circuit engineering, building on established techniques for canceling spurious mixing processes in couplers and qubits, proved essential to achieving this resilience. The transmon itself, a foundational element in many superconducting quantum circuits, relies on a superconducting quantum interference device to introduce the necessary nonlinearity for quantum operations. This nonlinearity’s strength scales with drive amplitude, making precise control at high power a key goal for improving qubit control, readout speed, and coupler performance. However, activating multi-excitation resonances drastically limits the performance of Josephson circuits at high power. The Delft team circumvented this by implementing a native cosine-cosine coupling, eliminating intrinsic multi-excitation resonances and paving the way for higher drive levels. This resilience enables access to drive powers at which a remarkable physical phenomenon is uncovered: the collapse and inversion of the Josephson potential.

Spectroscopic analysis revealed the qubit frequency initially decreasing to zero before recovering, indicating the Josephson potential itself was undergoing a transformation. Readout experiments evidence the inversion through a phase shift in the localization of the transmon’s eigenstates, akin to the autonomous stabilization of a strongly driven pendulum in the upside-down position. This collapse of the Josephson energy represents a previously unrecognised fundamental limitation for both qubit readout and strongly driven Josephson circuits, but also opens avenues for novel control mechanisms and new opportunities for the implementation of a fully protected qubit through driving of a simple transmon circuit.

Drive-Induced Collapse and Inversion of Potential

This achievement reveals a fundamental limitation to strongly driven circuits and opens pathways to novel control mechanisms for quantum states; it is not simply about reaching higher drive powers. The team’s success hinged on engineering a system specifically “free of any detrimental unwanted transitions,” a crucial step allowing them to probe regimes inaccessible with conventional designs. The core of the experiment involved a transmon qubit coupled to a resonator, designed to eliminate spurious circuit modes that can limit performance at high power. This careful design allowed them to apply increasingly strong microwave drives without triggering unwanted transitions, a common obstacle in superconducting quantum circuits.

This isn’t simply a matter of pushing existing circuits to their limits; it reveals a fundamental constraint on strongly driven superconducting systems previously overlooked by designers. The dynamical renormalization of the Josephson potential opens up new opportunities for the implementation of a fully protected qubit, potentially shielding them from environmental noise. The work demonstrates a new level of control over superconducting circuits.

Dynamical Renormalization Enables Novel Quantum Control

The pursuit of ever-stronger control over superconducting qubits often assumes a straightforward relationship between drive power and performance. This result reveals a new limitation of strongly driven superconducting circuits beyond drive-induced transitions. The core of the experiment involved a transmon qubit coupled to a resonator. Crucial to this observation is the implementation of a native cosine-cosine coupling free of intrinsic multi-excitation resonances. Through spectroscopic analysis, a shift emerged: the qubit frequency decreased towards zero, then recovered, indicating the Josephson potential itself was undergoing a transformation. The work demonstrates that a high-frequency flux drive can lower the height of the average Josephson potential, until it collapses, and revives in an inverted fashion.

Through readout experiments, evidence of the inversion was found through a phase shift in the localization of the transmon’s eigenstates, mirroring the stabilization of an upside-down pendulum under strong, rapid shaking. This collapse and inversion is first revealed in a spectroscopy experiment, which shows the qubit frequency going down to zero and recovering for increasing drive power. The dynamical renormalization of the Josephson potential opens new opportunities in quantum information, such as the implementation of a fully protected qubit through driving of a simple transmon circuit. This stabilization occurs because the high-frequency flux drive can fully average it out, and subsequently revive it with opposite sign.