  The coherent generation of light, from masers to lasers, relies upon the
specific structure of the individual emitters that lead to gain. Devices
operating as lasers in the few-emitter limit provide opportunities for
understanding quantum coherent phenomena, from THz sources to quantum
communication. Here we demonstrate a maser that is driven by single electron
tunneling events. Semiconductor double quantum dots (DQDs) serve as a gain
medium and are placed inside of a high quality factor microwave cavity. We
verify maser action by comparing the statistics of the emitted microwave field
above and below the maser threshold.
