Can calorimeters detect single optical phonons with low background?

In plain words

Dark matter between about a hundred and a hundred thousand times lighter than a proton would make a crystal vibrate with only one quantum of vibration energy, a few hundredths of an electron volt. Detectors are not yet sensitive enough to count these single quanta.

Precise statement

Dark matter of mass $m_{\chi} \sim 10\,\mathrm{keV} - 10\,\mathrm{MeV}$ scattering via a dark-photon or hadrophilic mediator excites single optical or acoustic phonons of energy $\omega \sim 10-100\,\mathrm{meV}$ in polar crystals (GaAs, $\mathrm{Al_2O_3}$, $\mathrm{SiO_2}$). The question is whether a phonon sensor (transition-edge sensor, kinetic-inductance detector, superconducting qubit) can reach an energy threshold below 100 meV with dark-count rate below about 1 event/(kg day) at that threshold. An answer is a demonstrated threshold and background, or a proof-of-limit from sensor noise and phonon down-conversion losses.

What would settle it

A calibrated single-phonon-scale threshold measured in a gram-scale crystal underground, with the background rate measured at that threshold.

Status in the literature

Unverified note: it names recent work and was partly written without a live literature search; it awaits a check by an editor.

TESSERACT (Phys. Rev. Lett. 2025) set first limits for 44-87 MeV dark matter with an athermal phonon detector at eV-scale threshold; single-phonon sensitivity is not demonstrated, and the low-energy excess sets the present background.

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