  An atom in open space can be detected by means of resonant absorption and
reemission of electromagnetic waves, known as resonance fluorescence, which is
a fundamental phenomenon of quantum optics. We report on the observation of
scattering of propagating waves by a single artificial atom. The behavior of
the artificial atom, a superconducting macroscopic two-level system, is in a
quantitative agreement with the predictions of quantum optics for a pointlike
scatterer interacting with the electromagnetic field in one-dimensional open
space. The strong atom-field interaction as revealed in a high degree of
extinction of propagating waves will allow applications of controllable
artificial atoms in quantum optics and photonics.
