Figure 1: (a) Canonical optomechanical system: a Fabry-Pérot cavity with a moveable mirror. The position $x$ of the mirror sets the cavity length, and hence the cavity frequency ${\omega }_M$. This mirror (which is attached to a fixed support via a spring) is in turn itself a mechanical system, with a vibrational frequency ${\omega }_M$. (b) Device used in the experiment: a patterned silicon nanobeam that supports both localized mechanical and optical resonances. (c) Optical power spectrum $S[\omega ]$ of light leaving the cavity in the case of a classical mechanical oscillator. The mechanical motion generates equal-area sidebands at frequencies $\pm {\omega }_M$ from the laser frequency ${\omega }_L$. (d) Same, but in the quantum case. The sidebands no longer have equal weights, as zero-point mechanical motion cannot provide energy. Here, ${\overline{n}}_M$ is the number of mechanical quanta (due to non-zero temperature), and $x_{\text{zpt}}$ is the amplitude of the mechanical resonator’s zero-point motion.