The hissing sound you hear in the background when you turn up the volume of your music player is called “noise”. Most of this hiss is due to the thermal motion of electrons in the music-player circuitry. Just like molecules in a hot gas, electrons in the circuitry are constantly jiggling about in a random fashion, and this motion this gives rise to an unwanted noise signal.
But there is another type of noise that only comes into play when we have an electrical current flowing. This noise is known as shot noise. Obstacles that generate shot noise in this way are found in many electronic components, such as diodes and some transistors, and electronic engineers take great efforts to try to get rid of the effects of all sources of noise, including shot noise, in their designs.
Now a new study has shown that shot noise can be eliminated at its microscopic origin. And to do so, they have borrowed an idea from an unlikely source – the early days of the steam engine.
Shot noise has its origins in the fact that electrical current is composed of a stream of individual particles – electrons – and that the behaviour of these particles is governed by the strange laws of quantum mechanics.
When an electron encounters an obstacle that you’d think would block its path, quantum mechanics offers the possibility that it can pass through it unhindered. This is called quantum tunnelling, and it makes the seemingly impossible possible. The important thing about quantum tunnelling is that it is a random process — quantum mechanics can tell us with what probability an electron might tunnel, but it can’t tell us whether any particular electron will tunnel or not.
Thus, if a stream of electrons hits an obstacle, some will tunnel and some will not, and this happens in a completely random fashion. If we could listen to the arrival of a stream of electrons tunnelling in this way, it would sound something like the random pitter-patter of raindrops on a flat roof. It is this randomness, as compared with the regimented drip-drip-drip of a tap, that makes up shot noise.
In the 18th century, James Watt was struggling to get his steam engine to run at a constant speed. To solve this problem, he came up with the “centrifugal governor” in 1788, a contraption that consisted of two metal balls rotating on a vertical spindle driven by the steam engine. If the engine ran too fast, the balls would move upwards under the centrifugal force (a force acting on a body moving in a circular path is directed away from the centre around which the body is moving).