5-mg suspended mirror driven by measurement-induced backaction

Quantum mechanics predicts superpositions of position states even for macroscopic objects. Recently, the use of a quasifreely suspended mirror combined with a laser was proposed to prepare such states [H. Müller-Ebhardt, Phys. Rev. Lett. 100, 013601 (2008)PRLTAO0031-900710.1103/PhysRevLett.100.013601]. One of the key milestones towards this goal is the preparation of the mechanical oscillator mainly driven by measurement-induced backaction in the quantum regime. Here we describe the observation of backaction acting on a suspended 5-mg mirror in the classical regime. Furthermore, its quantum component is estimated to be larger than the thermal fluctuating force due to internal damping of the suspension, by a factor of 1.4±0.2 at 325 Hz.