Cooling of mechanical resonators is currently a popular topic in many fields of physics including ultra-high precision measurements, detection of gravitational waves and the study of the transition between classical and quantum behaviour of a mechanical system. Here we report the observation of self-cooling of a micromirror by radiation pressure inside a high-finesse optical cavity. In essence, changes in intensity in a detuned cavity, as caused by the thermal vibration of the mirror, provide the mechanism for entropy flow from the mirror's oscillatory motion to the low-entropy cavity field. The crucial coupling between radiation and mechanical motion was made possible by producing free-standing micromirrors of low mass (m approximately 400 ng), high reflectance (more than 99.6%) and high mechanical quality (Q approximately 10,000). We observe cooling of the mechanical oscillator by a factor of more than 30; that is, from room temperature to below 10 K. In addition to purely photothermal effects we identify radiation pressure as a relevant mechanism responsible for the cooling. In contrast with earlier experiments, our technique does not need any active feedback. We expect that improvements of our method will permit cooling ratios beyond 1,000 and will thus possibly enable cooling all the way down to the quantum mechanical ground state of the micromirror.

We propose a method to prepare and verify spatial quantum superpositions of a nanometer-sized object separated by distances of the order of its size. This method provides unprecedented bounds for objective collapse models of the wave function by merging techniques and insights from cavity quantum optomechanics and matter-wave interferometry. An analysis and simulation of the experiment is performed taking into account standard sources of decoherence. We provide an operational parameter regime using present-day and planned technology.

The coupling of a levitated submicron particle and an optical cavity field promises access to a unique parameter regime both for macroscopic quantum experiments and for high-precision force sensing. We report a demonstration of such controlled interactions by cavity cooling the center-of-mass motion of an optically trapped submicron particle. This paves the way for a light-matter interface that can enable room-temperature quantum experiments with mesoscopic mechanical systems.optical trapping | quantum optics | cavity optomechanics | nanoparticles | nanomechanics T he ability to trap and to manipulate individual atoms is at the heart of current implementations of quantum simulations (1, 2), quantum computing (3), and long-distance quantum communication (4,5). Controlling the motion of larger particles opens up avenues for quantum science, both for the study of fundamental quantum phenomena in the context of matter wave interference (6), and for unique sensing and transduction applications in the context of quantum optomechanics (7,8). Specifically, it has been suggested that cavity cooling of a single submicron particle in high vacuum allows for the generation of quantum states of motion in a room-temperature environment (9-11), as well as for unprecedented force sensitivity (12, 13). Here, we take steps into this regime. We demonstrate cavity cooling of an optically levitated submicron particle consisting of ∼10 9 atoms (estimated diameter of 340 nm). The particle is trapped at modest vacuum levels of a few millibars in the standingwave field of an optical cavity and is cooled through coherent scattering into the modes of the same cavity (14, 15). We estimate that our cooling rates are sufficient for ground-state cooling, provided that optical trapping at a vacuum level of 10 −7 mbar can be realized in the future, e.g., by using additional active-feedback schemes to stabilize the optical trap in three dimensions (16)(17)(18)(19).Cooling and coherent control of single atoms inside an optical cavity are well-established techniques within atomic quantum optics (20)(21)(22)(23)(24). The main idea of cavity cooling relies on the fact that the presence of an optical cavity can resonantly enhance scattering processes of laser light that deplete the kinetic energy of the atom, specifically those processes where a photon that is scattered from the atom is Doppler shifted to a higher frequency. It was realized early on that such cavity-enhanced scattering processes can be used to achieve laser cooling even of objects without exploitable internal level structure such as molecules and submicron particles (14,15,25,26). For nanoscale objects, cavity cooling has been demonstrated in a series of recent experiments with nanobeams (27-29) and membranes of nanometerscale thickness (e.g., refs. 30 and 31). To guarantee long interaction times with the cavity field, these objects were mechanically clamped, which however introduces additional dissipation and heating through the mechanical support structure. As a consequence, quantum...