Letter

1. NEC Nano Electronics Research Laboratories, Tsukuba, Ibaraki 305-8501, Japan
   
2. The Institute of Physical and Chemical Research (RIKEN), Wako, Saitama 351-0198, Japan
   
3. CREST-JST, Kawaguchi, Saitama 332-0012, Japan
   
4. VTT Technical Research Center of Finland, Sensors, POB 1000, 02044 VTT, Espoo, Finland

Correspondence to: O. Astafiev^1,2 Correspondence and requests for materials should be addressed to O.A. (Email: astf@zb.jp.nec.com).

Solid-state superconducting circuits^{1, 2, 3} 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^{4, 5, 6, 7, 8} 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 qubit⁹—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.