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The optical lever is a precision displacement sensor with broad applications. In principle, it can track the motion of a mechanical oscillator with added noise at the standard quantum limit (SQL); however, demonstrating this performance requires an oscillator with exceptionally high torque sensitivity or, equivalently, zero-point angular displacement spectral density. Here, we describe optical lever measurements on nanoribbons possessing torsion modes with torque sensitivities of and zero-point displacement spectral densities of . By compensating for aberrations and leveraging immunity to classical intensity noise, we realize angular displacement measurements with imprecisions 20 dB below the SQL and demonstrate feedback cooling, using a position-modulated laser beam as a torque actuator, from room temperature to Si₃N₄phonons. Our study signals the potential for a new class of torsional quantum optomechanics. 

Membrane-based cavity optomechanical systems have been widely successful; however, their chip-scale integration remains a significant challenge. Here we present a solution based on metasurface design. Specifically, by non-periodic photonic crystal patterning of a Si₃N₄membrane, we realize a suspended metamirror with a finite focal length, enabling formation of a stable optical cavity with a plane end-mirror. We present simulation, fabrication, and characterization of the metamirror using both free-space and cavity-based measurements, demonstrating reflectivities as high as 99% and cavity finesse as high as 600. The mirror radius of curvature (∼30cm) is inferred from the cavity mode spectrum. In combination with phononic engineering, focusing membrane mirrors offer a route towards high-cooperativity, vertically integrated cavity optomechanical systems with applications ranging from precision force sensing to hybrid quantum transduction. 

Abstract We present Fermi Gamma-ray Burst Monitor (Fermi-GBM) and Swift Burst Alert Telescope (Swift-BAT) searches for gamma-ray/X-ray counterparts to gravitational-wave (GW) candidate events identified during the third observing run of the Advanced LIGO and Advanced Virgo detectors. Using Fermi-GBM onboard triggers and subthreshold gamma-ray burst (GRB) candidates found in the Fermi-GBM ground analyses, the Targeted Search and the Untargeted Search, we investigate whether there are any coincident GRBs associated with the GWs. We also search the Swift-BAT rate data around the GW times to determine whether a GRB counterpart is present. No counterparts are found. Using both the Fermi-GBM Targeted Search and the Swift-BAT search, we calculate flux upper limits and present joint upper limits on the gamma-ray luminosity of each GW. Given these limits, we constrain theoretical models for the emission of gamma rays from binary black hole mergers.