  Solid-state superconducting circuits are versatile systems in which quantum
states can be engineered and controlled. Recent progress in this area has
opened up exciting possibilities for exploring fundamental physics as well as
applications in quantum information technology; in a series of experiments it
was shown that such circuits can be exploited to generate quantum optical
phenomena, by designing superconducting elements as artificial atoms that are
coupled coherently to the photon field of a resonator. Here we demonstrate a
lasing effect with a single artificial atom - a Josephson-junction charge qubit
- embedded in a superconducting resonator. We make use of one of the properties
of solid-state artificial atoms, namely that they are strongly and controllably
coupled to the resonator modes. The device is essentially different from
existing lasers and masers; one and the same artificial atom excited by current
injection produces many photons.
