A quantum dot is a semiconductor nanostructure (typical size: 10 nm) whose optical properties are similar to those of an atom. Such a nanostructure is usually exploited for quantum optical studies, in particular for generating light pulses containing exactly one photon. Here, it is used to probe the motion of a mechanical oscillator. 

To this end, the quantum dot is directly integrated into the base of the oscillator, a 10 μm high vertical wire. When the latter vibrates, the quantum dot is subjected to an oscillating stress, which modulates the difference between its energy levels and therefore the color of the photons it emits. By measuring precisely these variations in optical spectroscopy, it is possible to resolve the thermal vibrations of different mechanical modes, even at cryogenic temperature. The vibration amplitude is then typically as low as one hundred of femtometers (10^-13 m).
These results pave the way for the development of new sensors. On the other hand, the coupling between the quantum dot and the wire works in both directions: the quantum dot can set the oscillator into motion, an object yet a thousand times larger.