Experiments towards strong coupling and ground state cooling in an atom-optomechanical hybrid system

Abstract

Modern experimental quantum physics experienced a rapid development in the past decades. The high degree of control and state of the art experimental techniques motivated the research field of hybrid quantum systems. Hybrid quantum systems combine different hybrid partners that can acquire quantum properties on their own to exploit their individual advantages. Here we report on the latest results at our experiment that aims to realize strong hybrid coupling between a micromechanical Si4N3 trampoline resonator and laser-cooled 87Rb atoms. The coupling is mediated via a coherent light field that is reflected from the resonator and forms an optical 1D lattice potential for the atoms. Due to optical losses on the beam path, the coupling lattice is asymmetrically pumped. This leads to the emergence of atomic density waves in the lattice which has a detrimental effect on the coupling for attractive lattice potentials. We numerically simulated this phenomenon and found that introducing an additional pump-asymmetry-compensation beam can remove the asymmetry-induced instability. As a result we implemented a compensation lattice which allowed us to operate in the previously inaccessible regime of attractive coupling lattice potentials where we reached a maximal cooperativity C_hybrid = (100 ± 25) at room temperature. In a parallel endeavor we successfully implemented a new fiber cavity with F = 785 to significantly improve the coupling between atoms and the mechanical resonator. In this configuration we reached a maximal cooperativity of C_hybrid = (5900 ± 1300) at room temperature. Furthermore we show experimentally that increasing the finesse of the cavity even further to F = 14500 does not improve the coupling strength in good agreement with theoretical predictions such that further improvements by tuning the finesse are not feasible. Lastly we realized a quantum-non-demolition measurement technique to perform rapid state tomography of a mechanical resonator. Here the interaction between the resonator and the light field takes place on time scales much smaller than the mechanical oscillation period, which allows for back-action evading measurements of the resonator state with sub-standard-quantum-limit resolution. We calculated a finesse of F = 14500 to resolve the resonator ground state and manufactured as well as implemented a corresponding new fiber cavity into the experimental setup. We conducted pulsed measurements with cavities of F = 785 and F = 14500. While the medium finesse configuration allowed to resolve the resonator state with an imprecision of 16 zero-point motions, the first experiments using the high finesse configuration suggest that further experimental modifications are necessary to acquire meaningful results.