Approaching the Standard Quantum Limit of Mechanical Torque Sensing

TL;DR

This paper reports a cryogenic optomechanical torque sensor achieving record sensitivity close to the quantum limit, enabling quantum-level torque measurements on mesoscopic samples.

Contribution

The authors demonstrate a cryogenic optomechanical torsional resonator with unprecedented torque sensitivity, nearing the standard quantum limit, and capable of integrating quantum systems for torque spectroscopy.

Findings

Record torque sensitivity of 2.9 yNm/Hz^{1/2}

Achieved sensitivity within an order of magnitude of the quantum limit

Integrated mesoscopic quantum samples with the sensing platform

Abstract

Mechanical transduction of torque has been key to probing a number of physical phenomena, such as gravity, the angular momentum of light, the Casimir effect, magnetism, and quantum oscillations. Following similar trends as mass and force sensing, mechanical torque sensitivity can be dramatically improved by scaling down the physical dimensions, and therefore moment of inertia, of a torsional spring. Yet now, through precision nanofabrication and sub-wavelength cavity optomechanics, we have reached a point where geometric optimization can only provide marginal improvements to torque sensitivity. Instead, nanoscale optomechanical measurements of torque are overwhelmingly hindered by thermal noise. Here we present cryogenic measurements of a cavity-optomechanical torsional resonator cooled in a dilution refrigerator to a temperature of 25 mK, corresponding to an average phonon occupation of <n> = 35, that demonstrate a record-breaking torque sensitivity of 2.9 yNm/Hz^{1/2}. This a 270-fold improvement over previous optomechanical torque sensors and just over an order of magnitude from its standard quantum limit. Furthermore, we demonstrate that mesoscopic test samples, such as micron-scale superconducting disks, can be integrated with our cryogenic optomechanical torque sensing platform, in contrast to other cryogenic optomechanical devices, opening the door for mechanical torque spectroscopy of intrinsically quantum systems.