  Absorption imaging has played a key role in the advancement of science from
van Leeuwenhoek's discovery of red blood cells to modern observations of dust
clouds in stellar nebulas and Bose-Einstein condensates. Here we show the first
absorption imaging of a single atom isolated in vacuum. The optical properties
of atoms are thoroughly understood, so a single atom is an ideal system for
testing the limits of absorption imaging. A single atomic ion was confined in
an RF Paul trap and the absorption imaged at near wavelength resolution with a
phase Fresnel lens. The observed image contrast of 3.1(3)% is the maximum
theoretically allowed for the imaging resolution of our setup. The absorption
of photons by single atoms is of immediate interest for quantum information
processing (QIP). Our results also point out new opportunities in imaging of
light-sensitive samples both in the optical and x-ray regimes.
