rf-reflectrometry measurements of a Josephson junction oscillator circuit at milliKelvin temperatures*

We report on rf-reflectometry measurements on a Nb/AlOx/Nb Josephson junction tank circuit. The junction has nominal critical current of 5 $\mu$A and is loaded with an on chip capacitance of 50 pF to suppress the plasma frequency to $f_p \approx 2$ GHz. Measurements were performed at temperature $T \approx 100$ mK in a dilution refrigerator. Reflection data show a clear rf absorption resonance and concomitant phase change about the resonant frequency. We will discuss use of this circuit for state readout \footnote{I. Siddiqi {\it et al.}, Phys.\ Rev.\ Lett.\, {\bf 93} 207002 (2004).} of a phase qubit and as a device for measuring critical current noise in Josephson junctions.