  We study dynamics of an artificial two-level atom in an open 1D space by
measuring evolution of its coherent and incoherent emission. States of the atom
-- a superconducting flux qubit coupled to a transmission line -- are fully
controlled by resonant excitation microwave pulses. The coherent emission -- a
direct measure of superposition in the atom -- exhibits decaying oscillations
shifted by $\pi/2$ from oscillations of the incoherent emission, which, in
turn, is proportional to the atomic population. The emission dynamics provides
information about states and properties of the atom. By measuring the coherent
dynamics, we derive two-time correlation function of fluctuations and, using
quantum regression formula, reconstruct the incoherent spectrum of the
resonance fluorescence triplet, which is in a good agreement with the directly
measured one.

Quantum Physics;Mesoscale and Nanoscale Physics;Superconductivity