A copper surface covered with chlorine atoms is used to build a rewritable data-storage device with information density as high as 500 terabits per square inch -- yielding the potential to store the contents of the entire US Library of Congress in a 0.1-mm-wide cube.
Single atoms on surfaces offer novel perspectives for information storage. While controlling their location is currently possible, technical limitations arise. In particular, temperatures in the liquid helium range (4 kelvin) are needed for stable configurations and modifying the position of a single atom requires regeneration of the whole surface.
This video explains it well:
Sander Otte and colleagues managed to preserve the positions of more than 8,000 chlorine vacancies (missing atoms) for more than 40 hours at 77 kelvin. By defining a binary alphabet based on the vacancies' positions, they store different texts (for example, part of Richard P. Feynman's lecture "There's plenty of room at the bottom") on the surface, which they can then modify at will bit-by-bit.
Although the slow speed (on the scale of minutes) of single write and read processes means that the device operation needs to be optimised before everyday applications are possible, these results provide a proof-of-principle demonstration of a data-storage device that outperforms state-of-the-art hard drives.
This density is two to three orders of magnitude beyond current hard disk or flash technology. An advance of this size is remarkable, to say the least.