Low-frequency noise in Josephson junctions for superconducting qubits

The authors have studied low-frequency resistance fluctuations in shadow-evaporated Al∕AlOx∕Al tunnel junctions. Between 300 and 5K the spectral density follows a 1∕f law. Below 5K, individual defects distort the 1∕f shape of the spectrum. The spectral density decreases linearly with temperature between 150 and 1K and saturates below 0.8K. At 4.2K, it is about two orders of magnitude lower than expected from a recent survey [D. J. Van Harlingen et al., Phys. Rev. B 70, 064510 (2004)]. Due to saturation below 0.8K the estimated qubit dephasing times at 100mK are only about two times longer than calculated by Van Harlingen et al.