Achieving coherent quantum control over massive mechanical resonators is acurrent research goal. Nano- and micromechanical devices can be coupled to avariety of systems, for example to single electrons by electrostatic ormagnetic coupling, and to photons by radiation pressure or optical dipoleforces. So far, all such experiments have operated in a regime of weakcoupling, in which reversible energy exchange between the mechanical device andits coupled partner is suppressed by fast decoherence of the individual systemsto their local environments. Controlled quantum experiments are in principlenot possible in such a regime, but instead require strong coupling. So far,this has been demonstrated only between microscopic quantum systems, such asatoms and photons (in the context of cavity quantum electrodynamics) or solidstate qubits and photons. Strong coupling is an essential requirement for thepreparation of mechanical quantum states, such as squeezed or entangled states,and also for using mechanical resonators in the context of quantum informationprocessing, for example, as quantum transducers. Here we report the observationof optomechanical normal mode splitting, which provides unambiguous evidencefor strong coupling of cavity photons to a mechanical resonator. This paves theway towards full quantum optical control of nano- and micromechanical devices.
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