Autoresonant readout of a high Q resonator coupled to a superconducting quantum bit

The frequency of a nonlinear oscillator changes with oscillation amplitude. When a high-Q, nonlinear oscillator is excited with a frequency chirped drive, the system can respond at either low or high oscillation amplitude depending on whether the drive excitation is below or above a critical value, respectively -- a phenomenon known as autoresonance. We exploit this nonlinear phenomenon to read out the state of a superconducting transmon qubit coupled to a high-Q nonlinear resonator. Because the excitation is non-equilibium, the resonator can be read out faster than its energy decay time. The fidelity for mapping the qubit state onto the oscillator can be as high as $80\%$ and is limited by the $T_1$ lifetime of the qubit, and readily achievable pulse parameters.