Low-Temperature Noise Performance of SuperSpec and Other Developments on the Path to Deployment

SuperSpec is a compact on-chip spectrometer operating at mm and sub-mm wavelengths which will enable the construction of sensitive multibeam spectrometers. SuperSpec employs a filter bank architecture, consisting of lithographically patterned niobium superconducting microstrip mm-wave resonators. The power admitted by each resonator is detected by a titanium nitride lumped-element kinetic inductance detector (KID) with resonant frequency from 100 to 200 MHz. We present a characterization of the detector noise performance down to 10 mK measured in a dark setting. We report a device NEP of $2.7\times {10}^{-18}{\text{W Hz}}^{-1/2}$ at 210 mK, which is below the expected photon noise level at high-altitude ground-based observatories. The NEP decreases to a constant value of approximately $7.0\times {10}^{-19}{\text{W Hz}}^{-1/2}$ below 130 mK. The white noise is well modeled by thermal generation–recombination noise (GR noise) down to 130 mK and a noise floor at low temperatures. Moreover, the addition of low-pass coaxial filters further reduces the noise floor to achieve an NEP of $5.7\times {10}^{-19}{\text{W Hz}}^{-1/2}$ below 100 mK. We discuss a photolithographic technique to adjust KID resonances that results in an $f_0$ designed versus measured scatter of $1.7\times {10}^{-5}$, which will allow a significant reduction in resonators lost to clashes in full-scale designs. Finally, we present a demonstration of a new ROACH-2-based readout system operating below 500 MHz and show preliminary data indicating the suitability of this system for future highly multiplexed KID arrays.