  The creation and evolution of nonequilibrium phonons is central in
applications ranging from cosmological particle searches to decoherence
processes in qubits. However, the fundamental understanding of decoherence
pathways for athermal phonon distributions in solid-state systems remains an
open question. Using first-principles calculations, we investigate the primary
decay channels of athermal phonons in two technologically relevant
semiconductors -- Si and GaAs. We quantify the contributions of anharmonic,
isotopic, and interfacial scattering in these materials. From this, we
construct a model to estimate the thermal power in a readout scheme as a
function of time. We discuss the implication of our results on noise
limitations in current phonon sensor designs and strategies for improving
coherence in next-generation phonon sensors.
