  We observe matterwave interference of a single cesium atom in free fall. The
interferometer is an absolute sensor of acceleration and we show that this
technique is sensitive to forces at the level of $3.2\times10^{-27}$ N with a
spatial resolution at the micron scale. We observe the build up of the
interference pattern one atom at a time in an interferometer where the mean
path separation extends far beyond the coherence length of the atom. Using the
coherence length of the atom wavepacket as a metric, we directly probe the
velocity distribution and measure the temperature of a single atom in free
fall.
