  We demonstrate that radiation pressure force fields can be measured and
reconstructed with a resolution of $0.3$ fN (at a $99.7\%$ confidence level)
using an ergodic ensemble of overdamped colloidal particles. The outstanding
force resolution level is provided by the large size of the statistical
ensemble built by recording all displacements from all diffusing particles,
regardless of trajectory and time. This is only possible because the noise
driving the particles is thermal, white and stationary, so that the colloidal
system is ergodic, as we carefully verify. Using an ergodic colloidal
dispersion for performing ultra-sensitive measurements of external forces is
not limited to non-conservative optical force fields. Our experiments therefore
give way to interesting opportunities in the context of weak force measurements
in fluids.
