Photon-noise-tolerant dispersive readout of a superconducting qubit using a nonlinear Purcell filter

TL;DR

This paper introduces a nonlinear Purcell filter that significantly improves the noise tolerance and measurement rate of superconducting qubit readout, achieving high fidelity with minimal dephasing.

Contribution

The paper proposes and demonstrates a nonlinear Purcell filter that enhances noise suppression and readout speed without degrading qubit coherence.

Findings

Noise tolerance increased by a factor of 3

Measurement rate increased by a factor of 3

Achieved 99.4% readout fidelity

Abstract

Residual noise photons in a readout resonator become a major source of dephasing for a superconducting qubit when the resonator is optimized for a fast, high-fidelity dispersive readout. Here, we propose and demonstrate a nonlinear Purcell filter that suppresses such an undesirable dephasing process without sacrificing the readout performance. When a readout pulse is applied, the filter automatically reduces the effective linewidth of the readout resonator, increasing the sensitivity of the qubit to the input field. The noise tolerance of the device we have fabricated is shown to be enhanced by a factor of 3 relative to a device with a linear filter. The measurement rate is enhanced by another factor of 3 by utilizing the bifurcation of the nonlinear filter. A readout fidelity of 99.4% and a quantum nondemolition fidelity of 99.2% are achieved using a 40-ns readout pulse. The nonlinear Purcell filter will be an effective tool for realizing a fast, high-fidelity readout without compromising the coherence time of the qubit.