  We demonstrate a single-atom maser consisting of a semiconductor double
quantum dot (DQD) that is embedded in a high quality factor microwave cavity. A
finite bias drives the DQD out of equilibrium, resulting in sequential single
electron tunneling and masing. We develop a dynamic tuning protocol that allows
us to controllably increase the time-averaged repumping rate of the DQD at a
fixed level detuning, and quantitatively study the transition through the
masing threshold. We further examine the crossover from incoherent to coherent
emission by measuring the photon statistics across the masing transition. The
observed threshold behavior is in agreement with an existing single atom maser
theory when small corrections from lead emission are taken into account.
