Interpertation and Origin of the sideband asymmetry of a mechanical oscillator close to the ground state*

Cavity Optomechanics provides an unique way to probe the quantum mechanical motion of a macroscopic oscillator. Motional sideband thermometry has been shown in different measurement schemes to calibrate the phonon bath. In this experiment, we performed the motional sideband asymmetry measurement with multiple tones scheme in the optical domain for the first time. Using a heterdyne detection scheme, the output noise spectrum reveals the correlation between the imprecision and quantum backaction. Besides, we observed an effective Kerr-type optical nonlinearity due to the optical absorption heating. The finite cavity thermal response would produce artifical sideband asymmetry when the cooling tone and the red tone are close. This spurious sideband asymmetry is analyzed with a detailed theoretical model. Our measurement provided a lot of insight towards multiple tones optical measurement in cavity optomechanics, such as back-action evasion measurement and quantum mechanical squeezing.