Tu Delft
Last week, researchers figured out how to make circuits just one atom thick, and this week we’re pushing the physical boundaries of what we can do with data storage. While the ultimate limit will likely be a single atom, a procedure presented in a new paper is slightly less efficient because it requires the space occupied by two atoms. Still, 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 hard to know where to start to describe it all. But since we have to start somewhere, let’s start with the medium itself.
The researchers first evaporated some chlorine and allowed it to settle on a copper surface. Given enough time, a one-atom layer of chlorine will completely cover the copper surface. But if you shorten the process, you get a mix of chlorine atoms and voids on the surface. With a scanning tunneling microscope, which records the electronic state of the surface, you can easily tell the difference between a chlorine atom and the hole where it could be.
The location of the chlorine atoms also remains stable at temperatures above 70K, meaning you can store 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 entire surface and then write bits into it by popping chlorine atoms off. But that’s quite a challenge; 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 save a bit, they just use a neighboring atom and vacancy. If the atom is at the top of their frame of reference, the bit stores a 1. If it is at the bottom, it stores a zero.
On their chlorine-copper medium, the researchers divided the data into blocks of eight bytes, separated by four atoms. The location of each block is indicated by a specific arrangement of atoms on the top left. If there are not enough atoms or holes in a particular block, it can also be marked as bad and not used.
As if this wasn’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 intervention, the system scans the surface of the copper, finds out where every atom and hole is, and then calculates the optimal way to put blocks in place – one that yields the greatest number of blocks with the minimum number of moves. When data is stored, the system just goes through it pushing atoms around until everything in a block has reached its intended value, and then it moves on to the next block. Blocks can be cleared or overwritten later.
A video of the system in action.
This system is not 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 be a maximum of 1 Mbit/second. Still, it all 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 titled “There’s Plenty of Room at the Bottom” (along with proper attribution, of course). There is:
But I’m not afraid to consider the last question of whether we can eventually – in the great future – arrange the atoms as we want; the 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 arrange them so that they are chemically unstable, for example).
Until now we have been content with digging in the ground to find minerals. We heat them and we do things with them on a large scale, and we hope to get a pure substance with just as many impurities and so on. But we must always accept an atomic order that nature gives us. For example, we have nothing to do with a “checkerboard” arrangement, with the impurity atoms spaced exactly 1000 Angstroms apart, or in any other particular pattern.
They then went back and overwrote it with a passage from Darwin’s About the origin of species.
On average, 12 percent of the blocks would be unsuitable for storing data. Even taking those blocks into account, the system can store just under a bit per square nanometer, leading to a density of 500 terabits per square inch. Assuming you’ve stacked layers of copper, you might envision the possibility of storing the entire Library of Congress in a microscopic cube.
Clearly, the need for a scanning tunneling microscope to read the data, as well as a constant supply of liquid nitrogen, significantly reduces the overall storage density of the system. Those limits and the bandwidth issue also limit usability. But the system is still quite impressive, highlighting what can be achieved now that we can control individual atoms.
Nature Nanoscience2016. DOI: 10.1038/NNANO.2016.131 (About DOIs).