Ever since the first practical computers came on the market in the 1950s, scientists and engineers have been seeking ever more compact data storage technologies, which have gone from giant drums to tiny chips. Now scientists at the Kavli Institute of Nanoscience at the Delft University of Technology (TUDelft) have developed a memory technology that achieves the ultimate physical limit by using individual atoms to represent a single bit of data.

According to IBM, we create 2.5 million terabytes of data every day, which needs to be stored somewhere. This storage media occupies physical space and needs energy both to operate and to keep cool. Traditionally, such storage systems relied on grains of magnetic material or holes burned in plastic, so the way to make a memory medium more compact was to use finer grains or make smaller holes. What the Delft team have done is cut to the chase and used individual atoms to store a piece of information.

The Delft team built a one-kilobyte (8,000 bit) memory on a 96 nm by 126 nm rectangle of copper – which is about 800 times smaller than the width of a human hair – holding a matrix of chlorine atoms. The copper acts as a foundation, while the chlorine atoms reinforce one another to stabilize the matrix. This allowed the scientists to create a medium with a storage density of 500 terabits per square inch (Tbpsi). That's 500 times denser than the best commercial hard disk and lead-scientist Sander Otte says that it could theoretically allow every book ever written to be stored on something the size a postage stamp.

The TUDelft team managed this form of atomic-scale data storage by using a scanning tunneling microscope (STM), which is based on the principle of quantum tunneling. In this, an electron is only "sort of" in any one place at any one time and may be somewhere else at the same time. This allows it to do the seemingly impossible, like being on one side of an impenetrable barrier or gap and then show up on the other – as if it "tunneled" through reality.

A scanning tunneling microscope exploits this by means of a piezoelectric scanning device with a probe attached ending in an atom-sharp needle tip. The tip is given an electric charge and moves across the surface of a sample. If it encounters an atom, the quantum tunneling effect causes electrons to flow between the tip and the atom. The interaction between the probe and the atom is the same that controls chemical reactions. In other words, what makes one atom stick to another. This allows the scanning tunneling microscope to be used as a sort of quantum crane to pick up atoms and move them to exactly where the scientists want them to go.

"You could compare it to a sliding puzzle," says Otte. "Every bit consists of two positions on a surface of copper atoms, and one chlorine atom that we can slide back and forth between these two positions. If the chlorine atom is in the top position, there is a hole beneath it – we call this a one. If the hole is in the top position and the chlorine atom is therefore on the bottom, then the bit is a zero."

But moving atoms about is only half of the system. The other half is organizing the data and making it readable and reliable. The team did this by setting up the memory in eight-byte (64-bit) blocks. These blocks were each given a marker made of a pattern of chlorine atoms in the matrix. Similar to QR codes, these indicate the location of each block of memory. In addition, the markers can show if a block is damaged, so the system can be upscaled without requiring too high a level of perfection in manufacturing.

The data used in the Delft team's memory prototype is the text from physicist Richard Feynman's 1959 lecture "There's Plenty of Room at the Bottom," in which he first raised the possibility of storing information through arranging individual atoms.

Currently, the TUDelft system is in the laboratory phase, though the team hopes to make it more practical in the near future.

"In its current form the memory can operate only in very clean vacuum conditions and at liquid nitrogen temperature (77° K, -321° F, -196° C )," says Otte. "So the actual storage of data on an atomic scale is still some way off. But through this achievement we have certainly come a big step closer."

The research was published in Nature Nanotechnology.

The video below gives an overview of the TUDelft atomic-scale memory system.

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