  Emission linewidth is an important figure of merit for masers and lasers. We
recently demonstrated a semiconductor double quantum dot (DQD) micromaser where
photons are generated through single electron tunneling events. Charge noise
directly couples to the DQD energy levels, resulting in a maser linewidth that
is more than 100 times larger than the Schawlow-Townes prediction. Here we
demonstrate a linewidth narrowing of more than a factor 10 by locking the DQD
emission to a coherent tone that is injected to the input port of the cavity.
We measure the injection locking range as a function of cavity input power and
show that it is in agreement with the Adler equation. The position and
amplitude of distortion sidebands that appear outside of the injection locking
range are quantitatively examined. Our results show that this unconventional
maser, which is impacted by strong charge noise and electron-phonon coupling,
is well described by standard laser models.
