\section{Acknowledgements}
The authors thank Hendrik Sch\"{u}tz and Ehsan Mansouri for valuable contributions during the initial phase of the experiment.  This work was supported by the EU Horizon 2020 Research and Innovation Program under grant agreement No. 732894 (FET Proactive HOT) and the SNF Cavity Quantum Optomechanics project (grant no. 163387). M.J.B. is supported by MSCA ETN-OMT (grant no. 722923). T.J.K  acknowledges support from ERC AdG (QuREM, grant no. 320966). All samples were fabricated at the Center for MicroNanoTechnology (CMi) at EPFL.
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