  At the dawn of Quantum Physics, Wigner and Weisskopf obtained a full
analytical description (a \textit{photon portrait}) of the emission of a single
photon by a two-level system, using the basis of frequency modes (Weisskopf and
Wigner, "Zeitschrift f\"ur Physik", 63, 1930). A direct experimental
reconstruction of this portrait demands an accurate measurement of a time
resolved fluorescence spectrum, with high sensitivity to the off-resonant
frequencies and ultrafast dynamics describing the photon creation. In this work
we demonstrate such an experimental technique in a superconducting waveguide
Quantum Electrodynamics (wQED) platform, using single transmon qubit and two
coupled transmon qubits as quantum emitters. In both scenarios, the photon
portraits agree quantitatively with the predictions of the input-output theory
and qualitatively with Wigner-Weisskopf theory. We believe that our technique
allows not only for interesting visualization of fundamental principles, but
may serve as a tool, e.g. to realize multi-dimensional spectroscopy in
waveguide Quantum Electrodynamics.
