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

We recently demonstrated a 1$^{\textrm{st}}$-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 $\varepsilon_{c}$ of $\sim$30$\,h$ 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 to below $1\,\mu$m. This was realized by extending the fabrication process for sub-micrometer-sized Josephson Junctions based on the HfTi self-shunted junction technology to an SIS process with AlO$_{\textrm{x}}$ 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.