Probing low-mass dark matter from sub-MeV to sub-GeV with germanium-based quantum phononic spectroscopy

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We present a germanium phonon-to-charge transducer that integrates a slow-phonon phononic-crystal (PnC) region with radio-frequency quantum point-contact (RF-QPC) readout at 4 K, and we evaluate its dark-sector reach. A calibrated signal-collection model — combining geometric guiding, propagation survival, and multiplicity-assisted primary-phonon detection — provides selection-corrected thresholds in the 1 0 − 3 – 1 0 − 2 eV range and a background model informed by nanosecond timing gates and GHz-band power-spectral-density windows. Under standard halo assumptions, a 100 g module achieves projected sensitivity to DM–electron and DM–nucleon scattering at recoil energies below 1 0 − 2 eV , probing cross sections below 1 0 − 43 cm 2 for m χ ∈ [ 0 . 01 , 100 ] MeV / c 2 (with efficiencies and thresholds folded in). We present sensitivities for both heavy-mediator ( F DM = 1 ) and light-mediator ( F DM ∝ 1 / q 2 ) benchmarks, quantify dominant systematics (threshold, phonon quality factor Q , and bulk defect densities), and outline a staged program toward kg ⋅ yr exposures that begins to test models approaching the solar CE ν NS background.