Highly Sensitive DC SQUID Arrays for the Readout of Optical TES at mK Temperatures

In this article, we present advancements of our submicrometer cross-type <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\text{Nb}/ \text{AlO}_{x}/ \text{Nb}$</tex-math></inline-formula> Josephson junction technology by implementing an additional highly conductive resistive layer, e.g., for thermalization structures. This allows for shifting the typically observed thermal decoupling of shunt resistors of highly sensitive dc superconducting quantum interference devices (SQUIDs) and SQUID arrays down to temperatures in the millikelvin range. We show results on the technological integration, device design of series SQUID arrays, and their characterization at 4.2 K down to 65 mK. Measurements were performed in an adiabatic demagnetization refrigerator. As an application scenario, we have successfully implemented them as readout circuitry for optical transition edge sensors operated at 150 mK requiring both low-noise and large bandwidth of the readout implemented in a flux-locked loop configuration.