Towards ultra-sensitive SQUIDs based on sub-micrometer-sized Josephson Junctions

Abstract

We recently demonstrated a 1${}^{\text{st}}$-order axial gradiometer SQUID system, which is operated in a liquid He dewar with negligible noise contribution. The achieved close to SQUID-limited measured coupled energy sensitivity ${\epsilon }_c$ of $\sim 30h$ corresponds to a white field noise below 180~aT~Hz${}^{-1/2}$. In order to further improve the SQUID noise performance, the junction capacitance was reduced by decreasing its lateral size from $2.5\mu$m to below $1\mu$m. This was realized by extending the fabrication process for submicrometer-sized Josephson Junctions based on the HfTi self-shunted junction technology to an SIS process with AlO${}_{\text{x}}$ as the insulating layer. We achieved energy sensitivities of 4.7$h$ and 20$h$ at 4.2~K for uncoupled and coupled SQUIDs, respectively. We also investigated the temperature dependence of the noise of the uncoupled SQUIDs and reached an energy sensitivity of 0.65$h$ in the white noise regime at 400~mK.

4 pages, 7 figures, ISEC 2019, version containing all the proofs (provided to IEEE TAS prior to publication) which should have been published by IEEE TAS