Edward Laird
Professor, Physics Department | Lancaster University

“A coherent mechanical oscillator pumped by a suspended quantum dot”

Suspended carbon nanotubes are mechanical resonators with low mass, high compliance, and high quality factor, which make them sensitive detectors for tiny forces and masses. The most sensitive way to measure such a resonator is by defining a quantum dot in the suspended segment and monitoring the displacement via the current through the dot. However, the force exerted by individual electrons tunnelling through the dot also creates strong electrical backaction. We have measured an extreme limit of this backaction, in which the current excites spontaneous mechanical oscillations.

Our device consists of a vibrating carbon nanotube spanned across a trench. A pair of tunnel barriers defines a quantum dot, whose conductance depends on the displacement. With low coupling, the quantum dot is a sensitive transducer of driven mechanical vibrations. At intermediate coupling, electrical back-action damps the vibrations. However, at strong coupling, the resonator can enter a regime where the damping becomes negative; it becomes a self-excited oscillator.

This electromechanical oscillator has many similarities to a laser, with the population inversion provided by the electrical bias and the resonator acting as a phonon cavity. We characterize the resulting coherence and demonstrate other laser characteristics, including injection locking and classical squeezing.

A coherent nanomechanical oscillator driven by single-electron tunnelling, Y. Wen et al., Nature Physics 16 75 (2020)