Characterization of Aluminum Microwave SQUID Multiplexers for CE$\nu$NS Detection
We present the design, fabrication, and characterization of an aluminum-based six-channel microwave SQUID multiplexer ($\mu$MUX) prototype for transition-edge sensor (TES) readout in the RICOCHET experiment. The device consists of aluminum coplanar-waveguide resonators and RF SQUIDs with Dolan-style Al/AlO$_x$/Al Josephson junctions. By measuring the resonator scattering parameters at a range of probe tone frequencies, powers, and flux bias points, we demonstrate agreement between the device response and existing multiplexer models. We also characterize the noise performance in both open-loop and flux-ramping modes. With a high electron mobility transistor (HEMT) amplifier, open-loop measurements yield a flux sensitivity of 1-1.5 $\mu\Phi_0/\sqrt{Hz}$. With flux-ramp modulation, low-frequency 1/f noise is suppressed, and the flux sensitivity is around 3-4 $\mu\Phi_0/\sqrt{Hz}$, corresponding to a current sensitivity of 24-33 $pA/\sqrt{Hz}$ at the input coil. We further demonstrate a reduction in readout noise by incorporating a Josephson traveling-wave parametric amplifier (JTWPA) between the $\mu$MUX and the HEMT. This achieves an open-loop flux sensitivity of 0.3-0.6 $\mu\Phi_0/\sqrt{Hz}$ and an effective system noise temperature below 1 K. These results establish aluminum $\mu$MUX devices as a viable and extensible readout technology for low-noise cryogenic detector arrays.