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.
@article{arxiv.cond-mat/0501283,
title = {Resonant Readout of a Persistent Current Qubit},
author = {Janice C. Lee and William D. Oliver and Terry P. Orlando and Karl K. Berggren},
journal= {arXiv preprint arXiv:cond-mat/0501283},
year = {2016}
}