Scalable Microwave SQUID Multiplexer Readout Architecture for TES-Based THz Security Camera

In this article, we have developed a scalable superconducting quantum interference device (SQUID)-based microwave multiplexer (µMUX) readout architecture adapted for arrays of transition edge sensors (TESs) to be exploited in a terahertz security camera. The camera system combines a scanning optics together with a 128-pixel TES array and aims for security gate operations with a standoff detection distance of up to 25 m. The developed frequency-domain µMUX is used to read out all feed-horn-coupled TESs based on aluminum thermistors, with noise limited by intrinsic TES noise. Both the TESs and µMUXs are operated in a compact cryostat with a base temperature of about 0.9 K. The µMUX has been fabricated in the cross-type $\text{Nb}/ \text{AlO}\_{\rm{x}}/ \text{Nb}$ Josephson junction process developed at the Leibniz Institute of Photonic Technology and incorporates high-quality superconducting thin-film resonators with resonant frequencies in the range of 5–6 GHz. An accordingly implemented field-programmable-gate-array-based readout electronics enables the simultaneous and continuous real-time readout of 128 rf-SQUIDs, including a flux ramp modulation scheme. In operation with the aluminum TES, we achieved a dark noise equivalent power of about $2.5\; \text{fW}/ \text{Hz}^{1/ 2}$ while providing a TES readout rate of 3.75 kHz, necessary for a video frame rate of 25 Hz of the security camera.