  A large fraction of quantum science and technology requires low-temperature
environments such as those afforded by dilution refrigerators. In these
cryogenic environments, accurate thermometry can be difficult to implement,
expensive, and often requires calibration to an external reference. Here, we
theoretically propose a primary thermometer based on measurement of a hybrid
system consisting of phonons coupled via a magnetostrictive interaction to
magnons. Thermometry is based on a cross-correlation measurement in which the
spectrum of back-action driven motion is used to scale the thermomechanical
motion, providing a direct measurement of the phonon temperature independent of
experimental parameters. Combined with a simple low-temperature compatible
microwave cavity read-out, this primary thermometer is expected to become a
popular thermometer for experiments below 1 K.
