Optimization of Millimeter-Wave Optomechanical Torque Sensors

Coupling the small size and high-quality of nanomechanical resonators with low-loss electromagnetic cavities for efficient readout, cavity optomechanical systems have been used to realize exquisite sensors of quantities such as mass, force, and torque. However, owing to their low resonant frequencies, mechanical sensors are often limited by thermal noise. Therefore, reducing this noise and enhancing the coupling strength of a cavity optomechanical sensor is essential for improving its sensitivity. Recent efforts to cool infrared cavity optomechanical torque sensors have been limited by the thermal heating associated with these high energy measurement photons. Here we propose a method to circumvent these detrimental heating effects using a superconducting optomechanical torque sensor operating at mm-wave frequencies. To enhance the sensitivity of these devices, we adjust the geometry of the device to optimize its mechanical resonant frequency and moment of inertia. We perform simulations of various sensor geometries to assess their impact on the device's torque sensitivity and coupling strength. From the simulation results, we identify the optimal configuration that simultaneously minimizes added noise and maximizes coupling strength. Future cavity-optomechanical torque sensors will be fabricated based off of these designs and measured in cryogenic settings, with the ultimate goal of reaching the standard quantum limit of optomechanical torque sensing. This work was supported by the Institute for Quantum Computing and the Electrical Engineering Department at the University of Waterloo; the Transformative Quantum Technologies initiative funded through the Canada First Research Excellence Fund; and the Natural Sciences and Engineering Research Council, Canada.