Can single-magnon counting probe spin-coupled sub-MeV dark matter?
In plain words
In a magnet, spins can ripple together as a wave whose smallest unit is a magnon; dark matter that couples to electron spins could create one. Single magnons have been detected with a quantum bit, but no detector yet has the size and low noise needed for a dark matter search.
Precise statement
Dark matter of mass $m_{\chi} \sim 1\,\mathrm{keV} - 1\,\mathrm{MeV}$ coupling to electron spin (e.g., via an axial-vector or magnetic-dipole interaction) excites gapped magnons of energy 1-100 meV in ordered magnets such as YIG, with rates computed by Trickle, Zhang and Zurek (Phys. Rev. Lett. 124, 201801, 2020). Single-magnon detection by a superconducting qubit was demonstrated with quantum efficiency up to 0.71 in a mm-scale ferrimagnetic crystal (Lachance-Quirion et al. 2020). The question is whether magnon counting can be scaled to target mass of about 1 kg with dark-count rate low enough to cross existing astrophysical bounds on spin-dependent couplings.
What would settle it
A gram-scale or larger magnon detector with measured single-magnon dark-count rate, followed by a published dark matter limit.