Fabry-perot cavity optomechanics with ultrahigh mechanical-Q-factor quartz micropillars at cryogenic temperature

Summary form only given. Recent experiments have confirmed the validity of quantum theory for macroscopic mechanical systems with masses up to several picograms [1,2]. The experimental investigation of macroscopic decoherence mechanisms or possible modifications of quantum mechanics would however strongly benefit from using even larger quantum objects [3]. We have designed an optomechanical system whose effective mass is close to the Planck mass. We expect to cool a mechanical mode at 4 MHz to near its ground state with less than 1 mW of incident laser power to enable quantum optics experiments. The large mass in our system, which is counter-beneficial for the optomechanical coupling strength, is compensated for by the high quality factor of the mechanical oscillator, large optomechanical coupling rates reached through using a high-finesse Fabry-Perot cavity and by a low thermal noise dilution-refrigerator environment.Structures made of monocrystalline quartz have yielded mechanical resonators with material-limited qualitiy factors above 106 at room temperature and up to 109 near 1 Kelvin [4]. Our quartz resonator shown in fig. 1 a) and [5] is expected to allow the coating of a high-reflectivity micromirror at its center surface without decreasing the mechanical quality factor of the compression-expansion mode of its central pillar. This enables us to add a concave mirror along the pillar axis to form an optical Fabry-Perot resonator which is parametrically coupled to the motion of the micropillar. Realizing a high-finesse cavity requires strong focusing of the optical mode on the pillar surface to avoid clipping losses. To form a stable cavity, a second cavity mirror with sub-millimeter radius of curvature is needed, which we fabricate by CO2 laser-photoablation in silica and subsequent dielectric layer deposition. Fig. 1 b) shows the observed clipping loss for larger beam waists at increased cavity length. For some cavity lengths, we have observed coupling between the fundamental and higher-order transverse modes of the cavity and developed a method to counteract the associated decrease of the fundamental mode´s finesse. We have measured the thermal displacement spectra of the 4 MHz-mode both in room-temperature and cryogenic environments by homodyne detecting the cavity reflection at resonance (see fig. 1c). Detuning the laser from the optical resonance resulted in cooling and heating with rates of the order of 500 Hz, which will result in a reduction of the effective mechanical mode temperature by three orders of magnitude in the final experiment. Further work will focus on the optimization of the thermalization of the sample in the cryogenic environment, reducing laser heating and increasing cooling efficiency by working in the resolved-sideband regime, vibration stabilisation of the cryostat and creating samples with simultaneously good optical and mechanical properties, which were so far only achieved separately. Functionalizing also the symmetric backside of the micropillar could enable the transfer of quantum information between two optical modes at different wavelength or between optical and microwave fields.