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 5 K the spectral density follows a 1/f law. Below 5 K, individual defects distort the 1/f shape of the spectrum. The spectral density decreases linearly with temperature between 150 and 1 K and saturates below 0.8 K. At 4.2 K, it is about two orders of magnitude lower than expected from a recent survey [D. J. Van Harlingen , Phys. Rev. B 70, 064510 (2004)]. Due to saturation below 0.8 K the estimated qubit dephasing times at 100 mK are only about two times longer than calculated by Van Harlingen (c) 2006 American Institute of Physics.