Ultrasensitive nano-optomechanical force sensor operated at dilution temperatures

Cooling down nanomechanical force probes is a generic strategy to enhance their sensitivities through the concomitant reduction of thermal noise and mechanical damping rates. However, heat conduction mechanisms become less efficient at low temperatures, which renders difficult ensure verify proper thermalization. To operate with minimally perturbing measurements, we implement optomechanical readout techniques operating in photon counting regime probe dynamics suspended silicon carbide nanowires dilution refrigerator. Readout vibrations realized sub-picowatt optical powers, where than one collected per oscillation period. We demonstrate thermalization $32\pm2$ mK report on record for scanning sensors, $40\,\rm zN/Hz^{1/2}$ level, sensitivity lateral field gradients fN/m range. This work opens road toward vectorial imaging faint forces minimal excitation levels.