We have realized a hybrid optomechanical system by coupling ultracold atoms to a micromechanical membrane. The atoms are trapped in an optical lattice, which is formed by retroreflection of a laser beam from the membrane surface. In this setup, the lattice laser light mediates an optomechanical coupling between membrane vibrations and atomic center-of-mass motion. We observe both the effect of the membrane vibrations onto the atoms as well as the backaction of the atomic motion onto the membrane. By coupling the membrane to laser-cooled atoms, we engineer the dissipation rate of the membrane. Our observations agree quantitatively with a simple model.

We discuss a hybrid quantum system where a dielectric membrane situated inside an optical cavity is coupled to a distant atomic ensemble trapped in an optical lattice. The coupling is mediated by the exchange of sideband photons of the lattice laser, and is enhanced by the cavity finesse as well as the square root of the number of atoms. In addition to observing coherent dynamics between the two systems, one can also switch on a tailored dissipation by laser cooling the atoms, thereby allowing for sympathetic cooling of the membrane. The resulting cooling scheme does not require resolved sideband conditions for the cavity, which relaxes a constraint present in standard optomechanical cavity cooling. We present a quantum mechanical treatment of this modular open system which takes into account the dominant imperfections, and identify optimal operation points for both coherent dynamics and sympathetic cooling. In particular, we find that ground state cooling of a cryogenically pre-cooled membrane is possible for realistic parameters.

Sympathetic cooling with ultracold atoms [1] and atomic ions [2] enables ultralow temperatures in systems where direct laser or evaporative cooling is not possible. It has so far been limited to the cooling of other microscopic particles, with masses up to 90 times larger than that of the coolant atom [3]. Here we use ultracold atoms to sympathetically cool the vibrations of a Si 3 N 4 nanomembrane [4,5], whose mass exceeds that of the atomic ensemble by a factor of 10 10 . The coupling of atomic and membrane vibrations is mediated by laser light over a macroscopic distance [6,7] and enhanced by placing the membrane in an optical cavity [8]. We observe cooling of the membrane vibrations from room temperature to 650 ± 230 mK, exploiting the large atom-membrane cooperativity [9] of our hybrid optomechanical system [10,11]. Our scheme enables ground-state cooling and quantum control of low-frequency oscillators such as nanomembranes or levitated nanoparticles [12], in a regime where purely optomechanical techniques cannot reach the ground state [8,9].The control over micro-and nanomechanical oscillators has recently made impressive progress [13]. First experiments cooled high-frequency mechanical oscillators to the quantum ground state [14][15][16][17] and demonstrated single-phonon control [14] using cryogenic cooling and the techniques of cavity optomechanics [18]. A current challenge is to couple engineered mechanical structures to microscopic quantum systems with good coherence properties, such as atoms [7,19,20], solid-state spin systems [21][22][23], semiconductor quantum dots [24], or superconducting devices [14,25]. The resulting hybrid mechanical systems offer new possibilities for quantum control of mechanical vibrations, precision sensing, and quantum-level signal transduction [10,11,13,18]. Ultracold atoms are an attractive choice for hybrid systems because a well-developed toolbox exists for atomic laser cooling and quantum manipulation [26]. It has been proposed to use hybrid mechanical-atomic systems for sympathetic cooling [6,8,27], creating atom-oscillator entanglement [27,28] and controlling the oscillator on the single-phonon level [29]. However, in the experiments reported so far [7,19,20], the mechanical-atomic coupling was far too weak to realize any of these possibilities.Sympathetic cooling of mechanical oscillators with laser-cooled atoms has received particular interest, as it would allow one to relax the constraints of cavity optomechanical and feedback cooling techniques [8,9]. Up to now, sympathetic cooling with atoms and atomic ions has been used to cool other microscopic particles such as different atoms or molecular ions up to the size of proteins [1][2][3], with applications in cold chemistry and quantum technology [26,30]. In these experiments, the coolant and the target species thermalize through shortrange collisional or electrostatic interactions in a trap. A large difference in their mass reduces the cooling performance, which has prevented extensions to more massive objects.In...