  We demonstrate ultra-sensitive measurement of fluctuations in a
surface-acoustic-wave~(SAW) resonator using a hybrid quantum system consisting
of the SAW resonator, a microwave (MW) resonator and a superconducting qubit.
The nonlinearity of the driven qubit induces parametric coupling, which
up-converts the excitation in the SAW resonator to that in the MW resonator.
Thermal fluctuations of the SAW resonator near the quantum limit are observed
in the noise spectroscopy in the MW domain.
