Atomic Scale Memory at a Silicon Surface
  The limits of pushing storage density to the atomic scale are explored with a memory that stores a bit by the presence or absence of one silicon atom. These atoms are positioned at lattice sites along self-assembled tracks with a pitch of 5 atom rows. The writing process involves removal of Si atoms with the tip of a scanning tunneling microscope. The memory can be reformatted by controlled deposition of silicon. The constraints on speed and reliability are compared with data storage in magnetic hard disks and DNA. 
R. Bennewitz, J.N. Crain, A. Kirakosian, J.-L. Lin, J.L. McChesney,   D.Y. Petrovykh, F.J. Himpsel