Dynamics of coherent and incoherent emission from an artificial atom in a 1D space

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.