Energy calibration of a KID-based phonon detector optimized for sub-GeV dark matter

Detection of sub-GeV dark matter candidates require sub-eV detector thresholds on deposited energy. We provide an update on a gram-scale phonon-mediated KID-based device that was designed for a dark matter search in this mass range at the Northwestern Experimental Underground Site. Currently, the device is demonstrating 6 eV resolution on the energy absorbed by the resonator. We show that TLS noise dominates this energy resolution estimate. After modifying the design to mitigate TLS noise, we project 1.5 eV on energy absorbed by the resonator for an amplifier white-noise dominated device. We also detail a photon-based technique to measure the resolution on energy deposited in the substrate, which we project to be roughly three times greater than the resolution on energy absorbed by the resonator, depending on the phonon collection efficiency of the resonator. This technique deposits photons in the substrate via a 475nm LED laser. Finally, we present a clear path forward to sub-eV thresholds, which includes installation of a quantum-limited superconducting parametric amplifier and adjustments to the material makeup of our resonators.