Nanomechanics in the quantum regime

Suspended carbon nanotubes, vibrating like tiny guitar strings, are mechanical resonators with low mass, high compliance, and high quality factor, which make them sensitive electromechanical detectors for tiny forces and masses. These same properties are favourable for studying the effects of strong measurement backaction. We make and measure nanotube resonators in this fascinating regime.

Our typical device consists of a clean carbon nanotube, spanned across a trench. A pair of tunnel barriers defines a single-electron transistor, whose conductance is proportional to the displacement. With low coupling, the single-electron transistor is a sensitive transducer of driven mechanical vibrations. At intermediate coupling, electrical backaction damps the vibrations. However, at strong coupling, the resonator can enter a regime where the damping becomes negative; it becomes a self-excited oscillator.

As an example of this behaviour, we recently showed that this electromechanical oscillator has many similarities to a laser, but replacing photons (light) with phonons (sound). The laser’s population inversion provided by the electrical bias, the resonator acts as a phonon cavity. We have demonstrated several laser characteristics, including injection locking and feedback narrowing of the emitted signal.

Through these experiments we will are motivated by the need to create better mechanical sensors and to engineer the interaction of electrons with phonons on the nanoscale.

The phonon laser

Figure: an electromechanical laser analogue. Below: Carbon nanotube electro-mechanical device. When individual electrons (red) tunnel through a suspended nanotube, they excite mechanical motion (blue). Above: Signature of laser behaviour. The signal voltage is proportional to the instantaneous displacement. Below the lasing threshold, the most likely displacement is zero: above threshold, the device oscillates with fixed amplitude.