We demonstrate amplification of a microwave signal by a strongly driven
two-level system in a coplanar waveguide resonator. The effect known from
optics as dressed-state lasing is observedwith a single quantum system formed
by a persistent current (flux) qubit. The transmission through the resonator is
enhanced when the Rabi frequency of the driven qubit is tuned into resonance
with one of the resonator modes. Amplification as well as linewidth narrowing
of a weak probe signal has been observed. The laser emission at the resonator’s
fundamental mode has been studied by measuring the emission spectrum. We
analyzed our system and found an excellent agreement between the experimental
results and the theoretical predictions obtained in the dressed-state model.