Ions in the machine: How simple liquids like water can perform complex calculations

Reservoir computing is a relatively recent idea in computing. Instead of traditional binary programs run on semiconductor chips, the reactions of a nonlinear dynamical system -- the reservoir -- are used to perform much of the calculation. Various nonlinear dynamical systems from quantum processes to optical laser components have been considered as reservoirs. In this study, the researchers looked at the ionic conductance of electrochemical solutions.

"Our simple testing device consists of 90 pairs of planar electrodes with an ionic solution dropped on its surface," explains Professor Megumi Akai-Kasaya, lead author of the study. "The response voltage to the input voltage is then used as the response of the reservoir." This voltage response is due to both the ionic currents that pass through the solution and the electrochemical current. This input-output relationship is both nonlinear and reproducible, which makes it suitable for use in reservoir computing. A unique multiway data acquisition system on the device controls the readout nodes, which enables parallel testing.

The researchers used the device to evaluate two liquids: polyoxometalate molecules in solution and deionized water. The system displayed a "feedforward connection" between nodes, regardless of which sample was used. However, there were differences. "The polyoxometalate solution increased the diversity of the response current, which makes it good at predicting periodic signals," says Professor Akai-Kasaya. "But it turns out that deionized water is best for solving second-order nonlinear problems." The good performance of these solutions demonstrates their potential for more complicated tasks, such as handwriting font recognition, isolated word recognition, and other classification tasks.

The researchers believe that proton or ion transfer with minimal electrochemical reactions over short durations has the potential for development as a more computationally powerful computing system that is low in cost and energy efficient. The simplicity of the proposed system opens up exciting new opportunities for developing computing systems based on electrochemical ion reactions.