  We propose a nano-scale realization of a phonon laser utilizing
phonon-assisted spin flips in quantum dots to amplify sound. Owing to a long
spin relaxation time, the device can be operated in a strong pumping regime, in
which the population inversion is close to its maximal value allowed under
Fermi statistics. In this regime, the threshold for stimulated emission is
unaffected by spontaneous spin flips. Considering a nanowire with quantum dots
defined along its length, we show that a further improvement arises from
confining the phonons to one dimension, and thus reducing the number of phonon
modes available for spontaneous emission. Our work calls for the development of
nanowire-based, high-finesse phonon resonators.
