Gravity generated by large masses has been observed using a variety of probes
from atomic interferometers to torsional balances. However, gravitational
coupling between small masses has never been observed so far. Here, we
demonstrate sensitive displacement sensing of the Brownian motion of an
optically trapped 7-mg pendulum motion whose natural quality factor is
increased to $10^8$ through dissipation dilution. The sensitivity for an
integration time of one second corresponds to the displacement generated by the
gravitational coupling between the probe and a mm separated 100 mg mass, whose
position is modulated at the pendulum mechanical resonant frequency.
Development of such a sensitive displacement sensor using a mg-scale device
will pave the way for a new class of experiments where gravitational coupling
between small masses in quantum regimes can be achieved.