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