Superconducting Quantum Optics: The coherence properties of superconducting circuits enable them to be developed as qubits in quantum information processing applications. Astafiev et al. (p. 840 ) now show that these macroscopic superconducting devices also behave as artificial atoms and can exhibit quantum optical effects. The ability to fabricate and integrate these superconducting devices in electronic circuitry may help toward developing a fully controlled quantum optics system on a chip.

The control of electrons at the level of the elementary charge $e$ was demonstrated experimentally already in the 1980s. Ever since, the production of an electrical current $ef$, or its integer multiple, at a drive frequency $f$ has been a focus of research for metrological purposes. This review discusses the generic physical phenomena and technical constraints that influence single-electron charge transport and presents a broad variety of proposed realizations. Some of them have already proven experimentally to nearly fulfill the demanding needs, in terms of transfer errors and transfer rate, of quantum metrology of electrical quantities, whereas some others are currently ``just'' wild ideas, still often potentially competitive if technical constraints can be lifted. The important issues of readout of single-electron events and potential error correction schemes based on them are also discussed. Finally, an account is given of the status of single-electron current sources in the bigger framework of electric quantum standards and of the future international SI system of units, and applications and uses of single-electron devices outside the metrological context are briefly discussed.

We present experimental observation of electromagnetically induced transparency (EIT) on a single macroscopic artificial “atom” (superconducting quantum system) coupled to open 1D space of a transmission line. Unlike in an optical media with many atoms, the single-atom EIT in 1D space is revealed in suppression of reflection of electromagnetic waves, rather than absorption. The observed almost 100% modulation of the reflection and transmission of propagating microwaves demonstrates full controllability of individual artificial atoms and a possibility to manipulate the atomic states. The system can be used as a switchable mirror of microwaves and opens a good perspective for its applications in photonic quantum information processing and other fields.