Approaching the standard quantum limit of mechanical torque sensing

Abstract

Reducing the moment of inertia improves the sensitivity of a mechanically based torque sensor, the parallel of reducing the mass of a force sensor, yet the correspondingly small displacements can be difficult to measure. To resolve this, we incorporate cavity optomechanics, which involves co-localizing an optical and mechanical resonance. With the resulting enhanced readout, cavity-optomechanical torque sensors are now limited only by thermal noise. Further progress requires thermalizing such sensors to low temperatures, where sensitivity limitations are instead imposed by quantum noise. Here, by cooling a cavity-optomechanical torque sensor to 25 mK, we demonstrate a torque sensitivity of 2.9 yNm/. At just over a factor of ten above its quantum-limited sensitivity, such cryogenic optomechanical torque sensors will enable both static and dynamic measurements of integrated samples at the level of a few hundred spins.

Ultrasensitive torque detection with an optically levitated nanorotor

“…We measure a record small torque of (1.2 ± 0.5) × 10 −27 Nm and achieve a record high rotational speed exceeding 5 GHz for a nanorotor. The measured torque sensitivity of our system at room temperature is several orders better than that of the state-of-the-art nanofabricated torque sensor at mK temperatures [6]. Our system will be suitable to detect the quantum vacuum friction [7][8][9][10].…”

“…Great effort has been made to improve the torque detection sensitivity by nanofabrication and cryogenic cooling. The most sensitive nanofabricated torque sensor has achieved a remarkable sensitivity of 10 −24 Nm/ √Hz at millikelvin temperatures in a dilution refrigerator [6]. Here we dramatically improve the torque detection sensitivity by developing an ultrasensitive torque sensor with an optically levitated nanorotor in vacuum.…”

Magnetic actuation and feedback cooling of a cavity optomechanical torque sensor

“…1b ), is separated from the evanescent field of a whispering-gallery-mode (WGM) optical cavity by an 87 nm vacuum gap. A platform for the magnetic sample was designed near the end of the torsion arm, amplifying the magnetic actuation of the torsional resonator 18 , yet is sufficiently far from the evanescent field of the WGM such that its optical properties are unaffected ( Q opt = 5.3 × 10 4 ). On this platform we have deposited a ferromagnetic sample, which enables the mechanical motion to be driven, amplified, or dampened, by an alternating (ac) external magnetic field.…”

Recent advances in mechanical torque studies of small-scale magnetism

“…The feedback-cooled modal temperature as determined by fitting to the blue trace is T = 12 K. (Figures from Refs. [109,52], used with permission.) tal mode volume on the order of one cubic wavelength.…”