(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 ωM. This mirror (which is attached to a fixed support via a spring) is in turn itself a mechanical system, with a vibrational frequency ωM. (b) Device used in the experiment: a patterned silicon nanobeam that supports both localized mechanical and optical resonances. (c) Optical power spectrum S[ω] of light leaving the cavity in the case of a classical mechanical oscillator. The mechanical motion generates equal-area sidebands at frequencies ±ωM from the laser frequency ωL. (d) Same, but in the quantum case. The sidebands no longer have equal weights, as zero-point mechanical motion cannot provide energy. Here, n̅ M is the number of mechanical quanta (due to non-zero temperature), and xzpt is the amplitude of the mechanical resonator’s zero-point motion.