Thanks to the work of the team of my coworkers and my collaborators at Italian CNR, we have observed the quantum of sound produced by a single electron, when moving across the quantum state made available by a single atom encapsulated in the channel of a silicon nanotransistor, called dopant. Dopants are usually implanted in silicon for creating contacts. In a lucky commercial transistor we investigated, one dopant atom randomly escaped from a contact to the middle of the channel during thermal annealing. A single dopant diffused far in the channel is totally irrelevant for room temperature purposes, but it becomes a nanolab by cooling the device to 4.2K. Thanks to the sensitivity of the equipment including a cryogenic amplifier working at 4.2 kelvin, we could measure the emission of single quanta of sound, called phonons, when the electron moves across the state of the dopant atom. Despite noise is usually considered something bad to kill, the novel point of this measurement is that instead of measuring the current across the transistor as usually, we recorded its electronic noise to perform the ultra-sensible spectroscopy of the quantum states. The electron, while passing across the quantum state carried by the dopant atom, emits a quantum of sound, i.e. an elementary vibration of the silicon crystal. In other words, thanks to the quantum state of this single atom, we could record the sound of a single electron clapping.

The research has been published by Applied Physics Express: https://iopscience.iop.org/article/10.35848/1882-0786/abc7cf... (Free PDF preprint is available at: https://arxiv.org/pdf/2007.06462.pdf )