What would you do with a quantum-limited torque magnetometer?*

In the past decade, cavity optomechanics has demonstrated its awesome potential to bring quantum mechanics into the study of mechanical systems. Yet little work has been done to use cavity optomechanics for sensing applications, such as the study of condensed matter systems. At the University of Alberta, we have focused on harnessing cavity optomechanical detection to measure ever smaller moment of inertia resonators, thereby improving torque sensing. Combining this with low temperature operation, we have reached torque sensitivities on the order of 10^-24 N●m/√Hz, just ten times the device’s standard quantum limit. To date, we have used such cavity optomechanical torque sensors to explore nanomagnetic vortices, collective spin dynamics, and phase-shift keying applications, but potential quantum sensing applications are numerous. Looking forward, it will also be possible to use the toolbox of quantum optics such as single photon detectors and squeezed states, along with cavity optomechanics, to enable new frontiers of quantum sensing and to push beyond the standard quantum limit.