Resonant Readout of a Persistent Current Qubit

TL;DR

This paper presents a novel resonant readout method for a persistent current qubit using a SQUID-based resonant circuit, enabling non-destructive flux state detection and potential nonlinear amplification.

Contribution

It introduces a flux-sensitive resonant circuit with a SQUID in the supercurrent branch for improved qubit readout, avoiding quasi-particle generation.

Findings

Successfully distinguished flux states via frequency shifts.

Characterized the circuit's nonlinear regime for amplification potential.

Demonstrated non-destructive, high-fidelity qubit measurement.

Abstract

We have implemented a resonant circuit that uses a SQUID as a flux-sensitive Josephson inductor for qubit readout. In contrast to the conventional switching current measurement that generates undesired quasi-particles when the SQUID switches to the voltage state, our approach keeps the readout SQUID biased along the supercurrent branch during the measurement. By incorporating the SQUID inductor in a high-Q resonant circuit, we can distinguish the two flux states of a niobium persistent-current (PC) qubit by observing a shift in the resonant frequency of both the magnitude and the phase spectra. The readout circuit was also characterized in the nonlinear regime to investigate its potential use as a nonlinear amplifier.