Observation of strong coupling between a micromechanical resonator and an optical cavity field

Abstract

Current experiments aim to achieve coherent quantum control over massive mechanical resonators. The existing approaches exploit a variety of coupling mechanisms of nano- and micromechanical devices to, for example, single electrons via electrostatic or magnetic coupling and to photons via radiation pressure or optical dipole forces. To date, all such experiments have been operating in a regime of weak coupling, in which reversible energy exchange between the mechanics and its coupled partner is suppressed by fast decoherence of the individual systems to their local environments. Controlled quantum experiments are in principle not possible in such a regime but instead require strong coupling, which has until now only been demonstrated between a variety of microscopic quantum systems such as atoms and photons in the context of cavity quantum electrodynamics (cQED), or between solid state qubits and photons. Strong coupling is an essential requirement for the preparation of mechanical quantum states such as squeezed or entangled states and also for utilizing mechanical resonators in the context of quantum information processing, e.g. as quantum transducers. Here we report the observation of optomechanical normal mode splitting, which is unambiguous evidence for strong coupling of cavity photons to a mechanical resonator. By directly measuring optomechanical correlations we also provide a qualitative analysis of the coupling interaction and show that entering the strong coupling regime is accompanied by a breakdown of the rotating-wave approximation. This paves the way towards full quantum optical control of nano- and micromechanical devices.