The tiniest guitar ever

As we’re all aware, science rocks! But sometimes, science rocks a little smaller than normal, Adam Murphy found out about a rocking study in Nature Physics, from the Lancaster University's Edward Laird…

Adam - We all love a good guitar riff. Slash, Keith Richards, Rory Gallagher, Pete Townsend; everyone's got a favourite. I'm partial to a bit of Brian May myself. But it seems even quantum physics is getting in on the guitar. I spoke to Edward Laird from Lancaster University about the tiniest guitar that's a little bit different.

Edward - Yes. It is a single-stringed instrument and the string is a carbon nanotube, which is the smallest wire that we can make. It's about 100,000 times thinner than an actual guitar string and it's also much shorter, it's less than one millionth of a metre long. It is like a guitar string because it's suspended between two clamps, one at each end, and we pass an electrical current through it and then we measure the electrical resistance. When it moves that resistance changes. Like a guitar string it vibrates at one particular frequency; because it's so small and so light that frequency is correspondingly much higher than an actual guitar, so it's 231 million hertz, which is a high A. What is new about this particular device is that it oscillates by itself.

Adam - A guitar that plays itself, playing the highest of As. Rock stars around the world would be envious. But why do you want to do that?

Edward - Well there’s two interesting things. The first one is, I would say, the pure scientific motivation of studying the effects of quantum mechanics and of quantum measurements on small moving objects. The theory of quantum mechanics was developed 100 years ago to describe the behaviour of individual particles. In everyday life we don't see any of those things. So what happens in between? And we're now in a position that we can explore this experimentally by taking medium-sized objects - so medium-sized in this case means a million atoms or so, which is about the size of one nanotube - and trying to find out how quantum mechanics makes them behave differently. There is also an application as well which is that you can use small resonators for detecting small forces. Just like in a real guitar string, if you put your finger on it, even if you touch it very lightly, it's very easy to hear the change in frequency or the change in damping that changes the note. If you have our nano guitar it can measure a small force applied to that, and we can detect that by a change in the motion.

Adam - Which means by dragging it across the surface you could get a super powered microscope, or a weighing scale that can weigh atoms. But how on earth can it play itself?

Edward - Ah, okay. So what do you need to set up an oscillation is some kind of feedback effect between the motion and the electrical current.

Adam - Feedback, at least in sound, is where noise from the speaker goes through the mic, which goes through the speaker, which goes through the mic...

Adam - Real guitar players, though - some of them like it. So Jimi Hendrix uses feedback in some of his recordings: he points his guitar at a microphone, and if he sets the parameters properly then the feedback stimulates the guitar string, which stimulates the microphone, which then stimulates the guitar string again and so you amplify the oscillation. What we did was to have a feedback… we set up the feedback loop entirely within the carbon nanotube itself. There is then feedback between the motion and the electrical current that’s flowing through it. And the reason for that feedback is in the details of the way the electrons quantum tunnel down the length of the nanotube. But the effect is exactly the same. The feedback makes the guitar string hum to itself and that's why we call it the self-playing guitar.