  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.
