Quantum-limited mechanical force sensor based on cavity optomechanics*

Optomechanical systems have recently reached the quantum limit for measurement of mechanical displacement. The use of high-Q superconducting microwave resonators has been key to this development. Using these advancements, we work towards a resonant mechanical force sensor for scanning probe applications. The detection principle relies on measuring a change of mechanical resonance frequency when a small force is applied to a sharp tip placed on the mechanical resonator. We investigate two separate designs based on capacitive and inductive modulation of a superconductive lumped-element microwave resonance, which couples the mechanical and microwave modes. In both designs the mechanical resonator is a flexural mode of a Si-N plate and the microwave inductance is realized by a meandering nanowire of superconducting NbTiN. For capacitive modulation a mechanical lever, consisting of an inner and outer cantilever that flex in opposition, is used to enhance the coupling. For inductive modulation we investigate a strain-induced change of the kinetic inductance of the superconducting meander.