Optimizing Sensitivity of Millimeter-Wave Quantum Optomechanical Torque Sensors

By coupling the small size and high quality of nanomechanical resonators with low-loss electromagnetic cavities for efficient readout, cavity quantum optomechanical systems have enabled exquisitely sensitive measurements of quantities such as mass, force, and torque. However, due to their low resonant frequencies, mechanical sensors are often limited by thermal noise. Therefore, reducing this noise and enhancing the optomechanical coupling strength are crucial for improving sensitivity. Recent attempts to cool infrared cavity optomechanical torque sensors have been constrained by thermal heating associated with high-energy measurement photons.

Here, we propose a method to mitigate these detrimental heating effects using a superconducting optomechanical torque sensor operating in the millimeter-wave frequency regime. To further enhance the device sensitivity, we adjust the sensor’s geometry to optimize its mechanical resonant frequency and moment of inertia.

Finite element method simulations of various geometries are performed to evaluate their impact on torque sensitivity and coupling strength. From these simulations, we identify an optimal configuration that simultaneously minimizes added noise and maximizes coupling strength, with the ultimate goal of reaching the standard quantum limit of optomechanical torque sensing. Achieving this limit would enable the use of such a torque sensor as a precise magnetometer for characterizing magnetic vortices in type-II superconductors, limited only by quantum mechanical noise.