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In the direct detection of dark matter with masses down to O(keV), the energies im- parted in the detector become vanishingly small. Many novel ideas have been proposed for instrumenting athermal phonon sensors on Fermi-degenerate materials [1], polar crys- tals [2–4], and CVD diamond [5], requiring detector energy thresholds of O(1−100) meV.

Transition-Edge Sensors (TESs) are commonly used in similar applications for their energy variance scaling relation σE2 ∝ V Tc3. However, a baseline energy variance that scales as σE2 ∝ V Tc6 can be achieved by using QETs (Quasiparticle-trap-assisted Elec- trothermal feedback Transition-edge sensors) and decreasing the amount of phonon ab- sorbing surfaces (“low-coverage”) to match the athermal phonon collection bandwidth to the QET sensor bandwidth. This dependence on Tc suggests that huge gains in energy sensitivity can be made with low-Tc devices.

In this talk, I will discuss the R&D progress that I have made towards the develop- ment of these sensors as part of the SPICE/HeRALD and SuperCDMS collaborations. Specifically, I will present the design and testing of a low-Tc, low-surface coverage pro- totype set of detectors that takes advantage of the above QET scaling laws and are expected to have sub-100 meV energy resolutions. Additionally, I will propose an R&D plan to improve the energy collection efficiency of athermal phonon sensors, which would not only benefit the direct dark matter detection community, but the larger quantum sensing community.