Low-frequency Critical Current Fluctuation Measurements in Nb/AlOx/Nb Junctions

We have measured the low frequency critical current noise in Nb/AlO$\mathrm{_{x}}$/Nb Josephson junctions used for qubits in quantum computation circuits. Low frequency current noise measurements were made using a bridge circuit with a SQUID null detector. The current noise spectra density showed a 1/$f$ component at low frequencies for both an unshunted junction biased near 6 mV and a shunted junction biased near $\sim 7$ $\mu$V. In both cases this corresponded to critical current fluctuations with a spectral density at 1 Hz of $2.2 \cdot 10^{-24}$ $\mathrm{A^{2}/Hz}$. Our measured value of critical current fluctuations is roughly two orders of magnitude less than the average of various technologies reported by Van Harlingen et al. (2004).