Searching for High Frequency Gravitational Waves Using Phonons

The gravitational wave (GW) spectrum at frequencies above a kHz is a largely unexplored frontier. We show that detectors with sensitivity to single-phonon excitations in crystal targets can search for GWs with frequencies, $\text{THz} \lesssim f \lesssim 100 \, \text{THz}$. Such detectors are already being built to search for light dark matter (DM), and therefore sensitivity to high-frequency GWs will be achieved as a byproduct. With a judicious choice of target materials, a collection of detectors could optimistically achieve sensitivities to monochromatic signals with $h_0 \sim 10^{-23} - 10^{-25}$ over $\text{THz} \lesssim f \lesssim 100 \, \text{THz}$. The detector sensitivity is then compared to the signal strength of candidate high-frequency GW sources, e.g., superradiant annihilation and black hole inspiral, as well as other recent detector proposals.