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A statically deformable ceiling was used at the Turbulence Dynamic Research facility at the University of Illinois at Urbana-Champaign. This facility was used to impose static spatially varying pressure gradients in a favorable-adverse (FAPG) arrangement. This study focuses on the FPG region of this flowfield which experiences a rapid spatial variation of its pressure gradient over 6.2𝛿. Non-time-resolved particle imaging velocimetry data were captured for 6 pressure gradients and 6 Reynolds numbers. This manuscript focuses on 𝑅𝑒𝜏 = 953 and four pressure gradient cases that have maximum acceleration parameters of 𝐾𝑚𝑎𝑥 × 106 = 0, 2.43, 4.77, and 5.97. The Reynolds stresses and turbulent kinetic energy of the boundary layer were investigated. The results were analyzed for the occurrence of relaminarization and it was concluded that no relaminarization occurred in any of the cases. The Reynolds stresses showed behavior suggestive of an internal layer, including knee points in 𝑢′𝑢′. However, no conclusive evidence was found to support this hypothesis. 

Cavity optomechanics has led to advances in quantum sensing, optical manipulation of mechanical systems, and macroscopic quantum physics. However, previous studies have typically focused on cavity optomechanical coupling to translational degrees of freedom, such as the drum mode of a membrane, which modifies the amplitude and phase of the light field. Here, we discuss recent advances in “imaging-based” cavity optomechanics – where information about the mechanical resonator’s motion is imprinted onto the spatial mode of the optical field. Torsion modes are naturally measured with this coupling and are interesting for applications such as precision torque sensing, tests of gravity, and measurements of angular displacement at and beyond the standard quantum limit. In our experiment, the high-Q torsion mode of a Si3N4 nanoribbon modulates the spatial mode of an optical cavity with degenerate transverse modes. We demonstrate an enhancement of angular sensitivity read out with a split photodetector, and differentiate the “spatial” optomechanical coupling found in our system from traditional dispersive coupling. We discuss the potential for imaging-based quantum optomechanics experiments, including pondermotive squeezing and quantum back-action evasion in an angular displacement measurement. 

Optomechanical systems have been exploited in ultrasensitive measurements of force, acceleration and magnetic fields. The fundamental limits for optomechanical sensing have been extensively studied and now well understood—the intrinsic uncertainties of the bosonic optical and mechanical modes, together with backaction noise arising from interactions between the two, dictate the standard quantum limit. Advanced techniques based on non-classical probes, in situ ponderomotive squeezed light and backaction-evading measurements have been developed to overcome the standard quantum limit for individual optomechanical sensors. An alternative, conceptually simpler approach to enhance optomechanical sensing rests on joint measurements taken by multiple sensors. In this configuration, a pathway to overcome the fundamental limits in joint measurements has not been explored. Here we demonstrate that joint force measurements taken with entangled probes on multiple optomechanical sensors can improve the bandwidth in the thermal-noise-dominant regime or the sensitivity in the shot-noise-dominant regime. Moreover, we quantify the overall performance of entangled probes with the sensitivity–bandwidth product and observe a 25% increase compared with that of classical probes. The demonstrated entanglement-enhanced optomechanical sensors would enable new capabilities for inertial navigation, acoustic imaging and searches for new physics. 

Strained nanomechanical resonators have recently achieved quality factors of 1 billion through the phenomenon of dissipation dilution. Remarkably, the potential of these devices seems unexhausted, exhibiting a scaling law of roughly one order of magnitude (in Q factor) every three years. This paper reviews advances which led to this point, including phononic crystal “soft-clamping,” strain engineering, and a trend towards centimeter-scale devices with extreme aspect ratios. Recent trends include investigation of strained crystalline thin films, fractal-patterned supports, and machine-learning-optimized supports. New possibilities emerging from these advances range from cavity free quantum optomechanics to ultra-sensitive accelerometry. 

Optomechanical accelerometers promise quantum-limited readout, high bandwidth, self-calibration, and radiation-pressure stabilization. We present a simple, scalable platform that enables these benefits with sub-µg sensitivity and 10 kHz bandwidth, based on a pair of vertically integrated SiN membranes.