  We report a new method to study two level fluctuators (TLFs) by measuring the
offset charge induced after applying a sudden step voltage to the gate
electrode of a single electron transistor. The offset charge is measured for
more than 20 hours for samples made on three different substrates. We find that
the offset charge drift follows a logarithmic increase over four orders of
magnitude in time and that the logarithmic slope increases linearly with the
step voltage. The charge drift is independent of temperature, ruling out
thermally activated TLFs and demonstrating that the charge fluctuations involve
tunneling. These observations are in agreement with expectations for an
ensemble of TLFs driven out of equilibrium. From our model, we extract the
density of TLFs assuming either a volume density or a surface density.
