Motional sideband asymmetry of a solid-state mechanical resonator at room temperature

We report the observation of motional sideband asymmetry of a solid-state mechanical oscillator at room temperature. We sideband cool a membrane-in-the-middle system close to the quantum ground state from room temperature, and measure the sideband asymmetry in a dual-homodyne measurement. Sideband thermometry yields a minimum phonon occupation of 9.5. We identify excess heating from the residual thermal intermodulation noise and optical absorption of the cooling laser, which limit the final phonon occupation above the quantum backaction limit.