  Recently, solid-state mechanical resonators have become a platform for
demonstrating non-classical behavior of systems involving a truly macroscopic
number of particles. Here, we perform the most macroscopic quantum test in a
mechanical resonator to date, which probes the validity of quantum mechanics at
the microgram mass scale. This is done by a direct measurement of the Wigner
function of a high-overtone bulk acoustic wave resonator mode, monitoring the
gradual decay of negativities over tens of microseconds. While the obtained
macroscopicity of $\mu= 11.3$ is on par with state-of-the-art atom
interferometers, future improvements of mode geometry and coherence times could
confirm the quantum superposition principle at unprecedented scales.
