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.
Highlights
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
We demonstrate that mesoscopic test samples such as micrometre-scale superconducting disks[21] can be integrated with our cryogenic optomechanical torque-sensing platform, in contrast to other cryogenic optomechanical devices[16,17,18,20], opening the door for mechanical torque spectroscopy[22] of intrinsically quantum systems
One could employ optomechanical back-action cooling (OBC), which has been successful in reducing the phonon occupancy of nanoscale mechanical resonators near to their quantum ground state[18,19,20]
Summary
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. By cooling a cavity-optpomffiffiffiffieffiffichanical torque sensor to 25 mK, we demonstrate a torque sensitivity of 2.9 yNm/ Hz. 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. Only passive cooling is compatible with reducing the thermal noise of an optomechanical torque sensor These architectures are well suited to tests of quantum mechanics and applications of quantum information processing, they are not well suited to integration with external systems one may wish to test. Our cavity optomechanical torque-sensing platform is unique, in that it enables straightforward integration with test samples and operates in a dilution refrigerator with near quantum-limited torque sensitivity. Further improvement demands lowering of the mechanical mode temperature
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