Having your data and computing them, too
By Steven Powell, 803-777-1923
A funny thing happened on the way to “writing” 1111 for a team of researchers. It came out 1010.
If you had designed a new kind of computer memory system, tried to write a series of ones in binary code and instead got alternating ones and zeroes, you’d probably be tempted to toss the prototype and start over. But for University of South Carolina physicist Yuriy Pershin and his collaborators, the odd result represents a promising step toward a powerful new kind of computing, called mem-computing.
The collaboration began after a team of experimentalists at Oak Ridge National Laboratory, led by Sergei Kalinin, uncovered some unexpected behavior in ferroelectric materials. They were using a scanning probe microscopy (SPM) tip to write data on the surface of the ferroelectric material.
The method is similar to the way binary data are written on a disk drive. There, an electromagnetic “pen” aligns magnetization of material either north or south on the surface of the disk. One direction of magnetization is defined as representing zero, the other as representing one.
With the ferroelectric material, an SPM tip can apply an electric field that reverses the polarization of a tiny circular area on its surface. What was very unexpected, though, is that in certain situations, attempting to draw a linear series of uniform circles resulted instead in a linear series of circles of alternating size.
At certain diameters and separation distances, even with the SPM tip operating under uniform conditions as it drew each each circle, a consecutive series of circles came out in two different sizes. For example, trying to make uniform circles 125 nanometers apart instead yielded alternating 100- and 25-nm-diameter circles 125 nm apart.
The team defined the size of the tiny circles to represent binary code: the larger circle was one, the smaller was zero. The bits of data on the ferroelectric material, with their boundaries defined by polarization, were thus interacting with each other beyond those boundaries – one bit could sense the presence of a nearby bit. So the researchers were able to create a situation where attempting to write a consecutive series of ones resulted instead in a series of one, zero, one, zero and so on.
“This is nothing more than the NOT operation,” said Pershin, a theoretical physicist on the team that recently published its results in the journal Nature Physics. “Using this observation, we have extended this mechanism to different binary logic gates, like AND and OR.”
The ability to apply rudimentary operations – such as NOT, AND and OR – to data and generate a logical output is the basis of digital computing. According to Pershin, the properties of the ferroelectric material can, in theory, be harnessed as a tool that can do any calculation that the central processing unit (CPU) in a modern computer can do.
Creating a new kind of CPU is one thing, but the fact that the place where information is being stored can also be used to process data is something else altogether. It’s a long-sought-after innovation in computer design.
Modern computers boast incredibly fast CPUs and prodigious memory capacities, but their overall speed is limited by a bottleneck: the CPU is separated from the memory. Much of the time that it takes to complete complex computations is the result of having to wait as data are ferried between memory and CPU.
For years, scientists have sought to create a system in which you can store and process data in the same place. A device that can operate that way is said to be doing memory computing, or mem-computing.
Mem-computing has been theoretically contemplated since at least the early 1970s, but never put into practice because materials simply didn’t behave the way that was needed to make it work. But Pershin, Kalinin and their co-workers have demonstrated that a ferroelectric material (LiNbO3 in their Nature Physics paper) fits the bill.
In the example above, the binary number 1010, each successive digit is NOT the one preceding it. The first bit stored on the surface, one, is processed just next door by NOT logic to give zero. Because the ferroelectric material can be constructed with many architectural designs, having different areas devoted to different kinds of logic, mem-computing is now more than a theoretical possibility.
Moreover, data can be simultaneously “written” in many places on a single sheet of the ferroelectric material – or simultaneously written in many places on many sheets. The system provides the opportunity to do calculations on a massively parallel scale, in contrast to current CPUs, which operate on sets of data one at a time (sequentially).
One particularly appealing aspect of mem-computing is that it mimics the crowning achievement of biology.
“Mem-computing is close to the operation of the brain,” said Pershin. “In the human brain, we do not have any physical separation between computing units and information storage units, and the brain works in parallel. This is what we’re trying to develop in electronics.”
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