Measurements of a dc-SQUID phase qubit using rf-reflectometry

We performed measurements of a $Nb/AlOx/Nb$ dc--SQUID phase qubit at 100 mK by monitoring the plasma frequency, $f_p$, of the readout/isolation junction. This qubit contains two Josephson junctions (JJ) separated by a 1.3 nH inductance; one JJ operates as a pure phase qubit, the second JJ isolates and reads out the qubit junction. When driving the isolation junction at 1.5 GHz, near $f_p$, current fluctuations in the SQUID loop cause fluctuations in $f_p$ which appear in the sidebands of the reflected microwave power. At 100 mK we find an effective flux noise, $S_{\Phi}^{0.5}$ of $50\ \mu \Phi_0 / Hz^{0.5}$ at 1 Hz. The measurement bandwidth is about 10 MHz, the upper limit being set by the $Q$ of the readout junction. We will discuss how the measurement performance depends on biasing parameters of each junction.