Researchers from Twente build evolutionary circuits
Researchers from Twente have designed a circuit that can be configured as sixteen different logic circuits. The circuit consists of a disorderly network of gold nanoparticles and can evolve to perform Boolean functions.
The article about the circuit was published Monday in the journal Nature Nanotechnology. Wilfred van der Wiel, physicist and professor of nanotechnology at the University of Twente, explains where the idea came from to make such a circuit. “Now we make logic circuits from a lot of transistors and we make those chips according to the von Neumann architecture with blueprints for certain functions. There have already been a number of people who have said: that works very well, but we throw in that process of don’t design away a lot of potential computing power? That’s the price you pay for thinking in those fixed blocks. Nicely predictable, less powerful.”
According to Van der Wiel, the latter is a pity. “Why don’t we get inspiration from the brain? It’s different, not linear, but works very fast because there are so many operations in parallel, something that doesn’t happen with a conventional processor. There it always goes sequentially, via a clock The individual components of the brain are slow, but the parallel operation makes the brain very powerful.”
This gave rise to the question of whether it would not be possible to make something with ‘dead matter’ that mimics that non-linear process. In this case, the researchers did this with gold nanospheres. At a temperature below 5 Kelvin or -268°C, they behave like so-called single-electron transistors. Only one electron at a time can move through a single electron transistor, provided the correct voltage is applied to it.
The latter is important, says Van der Wiel: “If you want to put an extra electron on a piece of metal, in this case only 20 nanometers, then those electrons are, as it were, on each other’s lip and you notice a lot of each other’s repulsion. “Most of the time this system therefore does not allow current to pass through, a phenomenon called the Coulomb blockade. This blockage can be removed by applying a very small amount of voltage to the nanosphere, allowing current to pass through. Then the basal transistor is turned on. in the on position.”
“If you zoom out,” continues Van der Wiel, “you see a very strong, non-linear electrical component. If you bring all those particles together in a disordered network, you still have separate islands through a molecular shell. of only one nanometer thick around the golden spheres. However, electrons can still hop from one to the other. We make a network of all those nonlinear switches. Then we thought: if we do that, we will have a sufficient complex system to build functional circuits?” And indeed, it turned out to be possible.
The network of gold nanoballs is about 200 nanometers in size and the individual balls have a diameter of 20nm. They are all capacitively coupled to each other, so that a lot of cross talk occurs. The latter is normally eliminated as much as possible, but the Twente researchers did not have to worry about this in this design because a ‘genetic algorithm’ is used.
Van der Wiel: “Because we use a genetic search algorithm, we can use all kinds of physics that are available. That way we don’t throw anything overboard. That means that without a design we don’t make a design error, so we are also insensitive to defects. If there are two balls that create a short circuit, then you evolve around it. If you only have enough knobs to turn, we can find that logic circuit.”
This should eventually result in a working circuit. This works as follows: the network is connected to nine electrodes. Two are inputs and there is an output. The remaining electrodes are used as configuration electrodes. “We put simple pulse trains, zero or one, on the inputs, and then we see what comes out at the output. The inputs are fixed. If the desired behavior is not measured at the output, then you switch to the buttons of the six run configuration voltages, after which you see if the output is more similar,” says Van der Wiel. By adjusting the voltage via the six electrodes, the potential landscape of the network of nanoparticles changes. “We don’t know exactly which particle does what. Because there are so many possibilities, we use a genetic algorithm. You will have the best sets of configuration voltages cross-breed and such. You can apply all the tricks of natural evolution in this genetic evolution. “
The desired outcome in the paper of the researchers from the Twente Mesa+ and CTIT institutes was a network that behaved like a logical function, such as an AND, NAND, OR, NOR, XOR or XNOR. So to get here, artificial evolution was used, something that even involves mutations or, as Van der Wiel concludes, “sometimes you have a lucky shot where you arrive at a much more suitable solution, you don’t get stuck in a local optimum. “
Artist impression of the circuit layout. The golden spheres are about 20nm in size. Furthermore, two of the electrodes provide the input of voltage and there is output current. The other six electrodes control the circuit. If you are missing an electrode: there is another one at the bottom.