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.

The location of the chlorine atoms also remains stable at temperatures above 70K, which means you can preserve the data in liquid nitrogen rather than relying on the more expensive liquid helium. The authors found that their data remained stable for at least 44 hours.

It might be possible to coat the whole surface and then write bits into it by popping chlorine atoms off. But doing so is rather challenging; instead, the authors decided to write bits by pushing atoms around, which can be done easily and with an error rate of less than one percent. To store a bit, they simply use a neighboring atom and vacancy. If the atom is at the "top" in their frame of reference, the bit stores a 1. If it's at the bottom, it stores a zero.

On their chlorine-copper medium, the researchers divided the data up into blocks of eight bytes, separated by four atoms. The location of each block is signaled by a specific arrangement of atoms to the upper left. If there are insufficient atoms or holes in a given block, it can also be marked as bad and not be used.

As if this weren't enough, the authors also wrote the equivalent of a disk operating system for their hardware, as shown in the video below. Without any human involvement, the system scans the surface of the copper, figures out where every atom and hole is, and then calculates the optimal way of putting blocks in place—one that gives the greatest number of blocks with the minimal number of moves. When data is stored, the system simply goes through and pushes atoms around until everything in a block is given its intended value, and then it moves on to the next block. Blocks can later be erased or written over.

This system isn't incredibly efficient, as it takes two minutes to write a single block and another minute to read it out. Even with a high-frequency scanning-tunneling microscope, the bandwidth would max out at 1Mbit/second. Still, the whole thing works, as the authors encoded a message that was about a kilobyte long. The data they wrote was part of the text of a Richard Feynman lecture that's called "There's Plenty of Room at the Bottom" (along with the proper attribution, naturally). It reads:

But I am not afraid to consider the final question as to whether, ultimately—in the great future—we can arrange the atoms the way we want; the very atoms, all the way down! What would happen if we could arrange the atoms one by one the way we want them (within reason, of course; you can't put them so that they are chemically unstable, for example). Up to now, we have been content to dig in the ground to find minerals. We heat them and we do things on a large scale with them, and we hope to get a pure substance with just so much impurity, and so on. But we must always accept some atomic arrangement that nature gives us. We haven't got anything, say, with a "checkerboard" arrangement, with the impurity atoms exactly arranged 1,000 angstroms apart, or in some other particular pattern.

They then went back and overwrote it with a passage from Darwin's On the Origin of Species.

On average, 12 percent of the blocks would be unsuitable for storing data. Even accounting for those blocks, however, the system is able to store just under a bit per square nanometer, leading to a density of 500 terabits per square inch. Assuming you stacked layers of copper, you could come up with the ability to store the entire Library of Congress in a microscopic cube.

Obviously, the need for a scanning-tunneling microscope to read the data—as well as a steady supply of liquid nitrogen—drops the overall storage density of the system pretty considerably. Those limits and the bandwidth issue also limit the practicality. But the system is still pretty impressive, and it highlights what can be achieved now that we're able to control individual atoms.

Nature Nanoscience, 2016. DOI: 10.1038/NNANO.2016.131 (About DOIs).