Quantum control of complex objects in the regime of large size and mass provides opportunities for sensing applications and tests of fundamental physics. The realization of such extreme quantum states of matter remains a major challenge. We demonstrate a quantum interface that combines optical trapping of solids with cavity-mediated light-matter interaction. Precise control over the frequency and position of the trap laser with respect to the optical cavity allowed us to laser-cool an optically trapped nanoparticle into its quantum ground state of motion from room temperature. The particle comprises 108 atoms, similar to current Bose-Einstein condensates, with the density of a solid object. Our cooling technique, in combination with optical trap manipulation, may enable otherwise unachievable superposition states involving large masses.

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...