This motion was coupled into a valve which then reduced the flow of steam through the engine, slowing it down. Conversely, if the engine was running too slowly, the balls would drop, the valves would open and the engine would speed up. In this way, Watt was able to stabilise the output of his engine around a constant speed. In doing so he had had come up with an early example of what we would now call feedback control.
James Watt to the rescue
The new experiment focuses on an ultra-small electronics device known as the single-electron transistor, which may one day form the basis of extremely efficient, miniature electronics. These single-electron transistors are somewhat like ordinary transistors, which switch electronic signals, but taken to the extreme limit of miniaturisation such that electrons move through them one at a time. This happens via quantum tunneling, which means the current through a single-electron transistor suffers from the randomness of shot noise.
F.A. Brockhaus, Berlin und Wien
Using sensitive charge measurements, the researchers were able to detect exactly when an electron had tunnelled through the transistor. Based on this electron counting, they then adjusted the voltages of the transistor, following Watt’s recipe for the centrifugal governor: if more electrons than normal had tunnelled, they changed the voltages to reduce the flow; if fewer had tunnelled, the voltages were changed to increase the flow.
In this way, they were able to show that, after a certain time had elapsed, the total number of electrons to have tunnelled through the device could be controlled precisely, with the results being almost entirely free of the randomness of the noisy tunnelling process.
The technique may not make it into your consumer electronics any time soon. The research was carried out at low temperature on a single device so we’d first need to make it work at room temperature and scale up the function. Nevertheless, it does represent an important breakthrough, as it reports the first application of feedback control in electronics that acts at the level of the individual electron.
The results are especially important for the development of future quantum technologies, which look to harness the peculiarities of quantum physics to make devices that vastly outperform our current best. Such machines could be a huge boost in areas including secure communication, code-breaking, precision measurement and quantitative analysis of “big data”. Quantum technologies however require an exquisite degree of control and, as this research shows, tried-and-true feedback techniques with their roots in the steam age may still have an important role to play.