Physicists at Tufts University covered a sheet of copper with chlorine atoms, by sliding pairs of atoms back and forth using a scanning tunneling microscope (STM), the bits were arranged to form a byte, enough to encode a single letter upon an atom. The STM method permitted them to source 78 trillion bits per square centimeter, this is hundreds of times denser than state of the art hard drives.
The only problem was limiting the movement of atoms throughout this process. To slow their movement, Tufts dropped the temperature of the copper sheet to -196 degrees, the temperature of liquid nitrogen. Future problems involve solving how best to operate at warmer temperatures like room temperature.