  We demonstrate a transformational technique for controllably tuning the
electrical properties of fabricated thermally oxidized amorphous aluminum-oxide
tunnel junctions. Using conventional test equipment to apply an alternating
bias to a heated tunnel barrier, giant increases in the room temperature
resistance, greater than 70%, can be achieved. The rate of resistance change is
shown to be strongly temperature-dependent, and is independent of junction size
in the sub-micron regime. In order to measure their tunneling properties at mK
temperatures, we characterized transmon qubit junctions treated with this
alternating-bias assisted annealing (ABAA) technique. The measured frequencies
follow the Ambegaokar-Baratoff relation between the shifted resistance and
critical current. Further, these studies show a reduction of
junction-contributed loss on the order of $\approx 2 \times10^{-6}$, along with
a significant reduction in resonant- and off-resonant-two level system defects
when compared to untreated samples. Imaging with high-resolution TEM shows that
the barrier is still predominantly amorphous with a more uniform distribution
of aluminum coordination across the barrier relative to untreated junctions.
This new approach is expected to be widely applicable to a broad range of
devices that rely on amorphous aluminum oxide, as well as the many other
metal-insulator-metal structures used in modern electronics.
