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Quantum optomechanics has led to advances in quantum sensing, optical manipulation of mechanical systems, and macroscopic quantum physics. However, previous studies have typically focused on dispersive optomechanical coupling, which modifies the phase of the light field. Here, we discuss recent advances in “imaging-based” quantum optomechanics – where information about the mechanical resonator’s motion is imprinted onto the spatial mode of the optical field, akin to how information encoded in an image. Additionally, we find radiation pressure backaction, a phenomenon not usually discussed in imaging studies, comes from spatially uncorrelated fluctuations of the optical field. First, we examine a simple thought experiment in which the displacement of a membrane resonator can be measured by extracting the amplitude of specific spatial modes. 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. As an experimental demonstration, we measure the angular displacement of the torsion mode of a Si3N4 nanoribbon near the quantum imprecision limit using both an optical lever and a spatial mode demultiplexer. Finally, we discuss the potential for future imaging-based quantum optomechanics experiments, including observing pondermotive squeezing of different spatial modes and quantum back-action evasion in angular displacement measurements. 

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. 

Abstract Squeezed light has long been used to enhance the precision of a single optomechanical sensor. An emerging set of proposals seeks to use arrays of optomechanical sensors to detect weak distributed forces, for applications ranging from gravity-based subterranean imaging to dark matter searches; however, a detailed investigation into the quantum-enhancement of this approach remains outstanding. Here, we propose an array of entanglement-enhanced optomechanical sensors to improve the broadband sensitivity of distributed force sensing. By coherently operating the optomechanical sensor array and distributing squeezing to entangle the optical fields, the array of sensors has a scaling advantage over independent sensors (i.e.,$$\sqrt{M}\to M$$ $\sqrt{M}\to M$, whereMis the number of sensors) due to coherence as well as joint noise suppression due to multi-partite entanglement. As an illustration, we consider entanglement-enhancement of an optomechanical accelerometer array to search for dark matter, and elucidate the challenge of realizing a quantum advantage in this context. 

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.