Last week saw researchers figure out how to make circuitry that’s only a single atom thick, and this week we’re pushing the physical limits on what we can do with data storage. While the ultimate limit is probably going to be a single atom, a procedure presented in a new paper is slightly less efficient in that it requires the space occupied by two atoms. Even so, and even after accounting for the equivalent of bad blocks in the storage media, the data density is enough to fit the contents of the Library of Congress within a 100 micrometer cube.

The approach, developed by a team of Dutch and Spanish researchers, has so many ingenious features that it’s difficult to know where to start describing it all. But since we have to start somewhere, we’ll begin with the medium itself.

The researchers first evaporated some chlorine and allowed it to settle on a copper surface. Given enough time, a single-atom layer of chlorine will fully coat the copper surface. But if you cut the process short, you end up with a mix of chlorine atoms and vacant spaces on the surface. With a scanning-tunneling microscope, which registers the electronic state of the surface, you can easily detect the difference between a chlorine atom and the hole where one could be.