Level attraction and idler resonance in a strongly driven Josephson cavity

TL;DR

This paper investigates the nonlinear dynamics of a strongly driven high-Q Josephson microwave cavity, revealing phenomena like level attraction, idler resonance, and exceptional points, which deepen understanding of driven nonlinear quantum circuits.

Contribution

It presents the first detailed study of the susceptibility and mode interactions in a strongly driven Josephson cavity, highlighting level attraction and exceptional points in this regime.

Findings

Observation of cavity susceptibility shift analogous to AC Stark effect

Detection of an idler mode with net gain above bifurcation threshold

Identification of two exceptional points and level attraction in the driven system

Abstract

Nonlinear Josephson circuits play a crucial role in the growing landscape of quantum information and technologies. The typical circuits studied in this field consist of qubits, whose anharmonicity is much larger than their linewidth, and also of parametric amplifiers, which are engineered with linewidths of tens of MHz or more. The regime of small anharmonicity but also narrow linewidth, corresponding to the dynamics of a high-$Q$ Duffing oscillator, has not been extensively explored using Josephson cavities. Here, we use two-tone spectroscopy to study the susceptibility of a strongly driven high-$Q$ Josephson microwave cavity. Under blue-detuned driving, we observe a shift of the cavity susceptibility, analogous to the AC Stark effect in atomic physics. When applying a strong red-detuned drive, we observe the appearance of an additional idler mode above the bifurcation threshold with net external gain. Strong driving of the circuit leads to the appearance of two exceptional points and a level attraction between the quasi-modes of the driven cavity. Our results provide insights on the physics of driven nonlinear Josephson resonators and form a starting point for exploring topological physics in strongly-driven Kerr oscillators.