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Near-field integration of a SiN nanobeam and a SiO$_2$ microcavity for Heisenberg-limited displacement sensing

Placing a nanomechanical object in the evanescent near-field of a high-$Q$ optical microcavity gives access to strong gradient forces and quantum-noise-limited displacement readout, offering an attractive platform for precision sensing technology and basic quantum optics research. Robustly implementing this platform is challenging, however, as it requires separating optically smooth surfaces by $\lesssimλ/10$. Here we describe a fully-integrated evanescent opto-nanomechanical transducer based on a high-stress Si$_3$N$_4$ nanobeam monolithically suspended above a SiO$_2$ microdisk cavity. Employing a novel vertical integration technique based on planarized sacrificial layers, we achieve beam-disk gaps as little as 25 nm while maintaining mechanical $Q\times f>10^{12}$ Hz and intrinsic optical $Q\sim10^7$. The combined low loss, small gap, and parallel-plane geometry result in exceptionally efficient transduction, characterizing by radio-frequency flexural modes with vacuum optomechanical coupling rates of 100 kHz, single-photon cooperativities in excess of unity, and zero-point frequency (displacement) noise amplitudes of 10 kHz (fm)/$\surd$Hz. In conjunction with the high power handling capacity of SiO$_2$ and low extraneous substrate noise, the transducer operates particularly well as a sensor. Deploying it in a 4 K cryostat, we recently demonstrated a displacement imprecision 40 dB below that at the standard quantum limit (SQL) with an imprecision-back-action product $<5\cdot\hbar$. In this report we provide a comprehensive description of device design, fabrication, and characterization, with an emphasis on extending Heisenberg-limited readout to room temperature. Towards this end, we describe a room temperature experiment in which a displacement imprecision 30 dB below that at the SQL and an imprecision-back-action product $<75\cdot\hbar$ is achieved.